v W a *«^-- " Organization, fenfation, fpontaneous motion, and all the operations of life, only exift at the furface of the earth, and in places expofed to the influence of light. Without it nature itfelf would be lifelefs and inanimate. By means of light, the benevolence of the Deity hath filled the furface of the earth with organization, fenfation and intelligence." Elements of C/iemiJlry, by M. Lavoifitr. PHILADELPHIA: PRINTED BY WAY & GROFF, No. 48, North Third-ftrcet. l800. TO JOSEPH GALLEGO, Esq. of RICHMOND, THIS DISSERTATION IS INSCRIBED JS A SMALL, BUT SINCERE, TRIBUTE OF GRATITUDE AND ESTEEM, BY HIS OBLIGED FRIEND, AND HUMBLE SERVANT, J. TRENT. TO Col. WILLIAM HETH, COLLECTOR OF THE DISTRICT ©F BERMUDA HUNDRED, VIRGINIA; AND TO Mr. JOHN RICHARD, Jun. MERCHANT, PHILADELPHIA, THIS INQUIRY IS DEDICATED, AS A MARK OF RESPECT AND ESTEEM, BY THE AUTHOR. AN I N q^u I R Y INTO THE EFFECTS of LIGHT, &c. XJY light I mean that fubtile fluid, which is emitted from luminous bodies and is pofTefTed of very lingular and important properties. Confidered, as to its phyfical or chemical opera- tion, light is certainly one of the moft beneficial fubftances in nature. It not only conduces by its llimulus to organic exiflence, but as the medium of fight, it is the fource of the mofl numerous and pleafurable of our perceptions; and upon its com- binations depend fome of the mofl beautiful and aflonifhing chemical phenomena. No fubject, therefore, can merit our attention more than the properties of this etherial fubftance, by means of which all the beauty and glory of cre- ation are laid open to our view. 8 It is the influence of light in refpiration,* that I wifh to eflablifh in the following inquiry; for which purpofe I will deliver my obfervations and experi- ments under the three following heads. I. The colour of bodies depends upon combined light. II. Light is a conflituent element of oxygen gas. III. The vermillion colour of blood depends, in part, upon the operation of light. * I read a paper on this fubjecl, before the Philadelphia Medical Society in February laft, before I had ever feen or heard of the publication of Mr. Davy, in Beddoes' Medical Contributions. 9 SECTION I. I. THE COLOUR OF BODIES DEPENDS UPON COMBINED LIGHT. PREVIOUS to the difcoveries of the immortal Newton, our conceptions of the nature of light and colours were very imperfect, and to many of the ancients, this curious and important branch of phi- lofophy, appeared to be involved in impenetrable darknefs ; for, in the opinion of Plato, to pry into the myfleries of light, was to encroach upon the prerogatives of Divinity, Notwithflanding its obfcurity, many conjectures were advanced on this fubject. by fome of the older opticians. The celebrated Des Cartes argued very rationally that colour was a modification of light; and after him many others thought the differences of colour depended upon the condenfation and rarefaction of this fubtile etherial fubflance. Their fpeculations, however, though fometimes ingenious and plaufible, were unfupported by ex- periment or any kind of evidence; and it was not till the time of Sir Ifaac Newton, that our ideas of light were by any means jufl or accurate. B IO By a multitude of the clearefl and mofl decilive experiments, this great man has fatisfactorily un- folded many of the mofl important properties of light, and has left behind him a theory of light and colours, by the aljiftance of which, we are enabled to account for many phenomena, which were before inexplicable. His dodrine of the materiality of light and of the different colours depending upon the different refrangibility of the rays of light, though founded on experiment, and fupported by the mofl ingenious reafoning, was flrenuoufly oppofed by cotemporary and fucceeding philofophers, efpecially by the cele- brated Euler, who fuppofed light to confifl in vibra- tions propagated from luminous bodies through a fubtile etherial medium. This hypothefis, though advocated with great zeal by M. Euler, „ completely refuted by the friends of the Newtonian doflrine, efpecially by Mefe M.cheU and Melville, whofe experiments ™ fervauons enablilh moft decidedly that light is a fubftance and not a quality. S * It would not comport with the ufual limits 0f u1augura1e% to enter further into IXi of th.s kind; I muft therefore refer the rL the feveral books on optics. " reader t0 II I believe it is generally agreed among philofophers, that the diverfity of colours, and variety of fhades, under which bodies prefent themfelves to our view, depend upon the operation of light. Many expe- riments have been made, and much ingenious rea- foning advanced in fupport of this opinion. Duhamel* demonflrated long ago, that the co- lour and inward texture of fome bodies are changed in confequence of their being expofed to light. He found that the juice of a certain fhell fifh contracted a fine purple colour, when it was expofed to the light of th- fun, and that the flronger the light, the more fplendid was the colour. Pieces of cloth dipped in this juice, and expofed to the fun, became red, though they were inclofed in glafs; but they fuffered no change, if in the fame expo- fure they were covered by any thing that intercepted light. After this, Beccariusf applied himfelf with dili- gence and fuccefs, to the invefligation of this curi- ous fubjedt, and found by many experiments, that the colour of various fubflances was altered by ex- pofure to light, exclufive of heat or any other circumflance. * Prieftley on vifion. Vol. II. p. 378. f Ibid. Vol. II. p. 379. 12 That neither heat nor air contributed to the pro- duction of thofe changes, was proved by repeating the fame experiments in a much higher temperature in a darkened room, and by expofing the fubflances in an exhaufled receiver; for under thefe circum- flances, the abfence or prefence of light, was ob- ferved to have very great influence on the refults of the experiments. In the firfl, no change took place, whereas in the exhaufled receiver the fubflances underwent the fame change when expofed to light as before. I fhall relate, in the courfe of this inquiry, fome experiments on blood, which corroborate thofe of Duhamel and Beccarius, and I hope they will de- monftrate clearly and decidedly the colouring power of light. The influence of light on organized beings is very remarkable. Animals deprived of it and living in dark places, lofe their colour and become white, as is obfervable in Arctic animals during the long nights, in the countries near the pole. " Worms and grubs, Mr. Dorthes obferves, which live in the earth or in wood, are of a whitifh colour. The birds and flying infects of the night, are likewife diflinguifhable from thofe of the day, by the want of brilliancy of colour j and the difference l3 is equally marked between thofe of the north and ,- of the fouth." Animals which inhabit tropical climates, where the light is conflant and intenfe, ponefs much deep- :er and more brilliant colours, than fuch as live to the north. Wild animals by domeflication, always lofe their dull and obfcure colours, and become much brighter, and more beautiful, in confequence of a more freqent expofure to the light of the fun; and it is remarked, that the colour of our domeftic animals during winter, is lefs bright, than in fum- mer. -■ DoGor Girtanner has obferved, that animals which conceal thcmfelves for the ths greatefl part ■pJFjthe year, in fubterraneous dwellings, lofe their * colour and become white; and that mice kept in a cage, in a dark room, have produced white mice. The influence of light on vegetation, is flill more remarkable; for, by depriving plants of it, they not only lofe their delicate fhades, but ficken and die, and when in green-houfes, the light is admit- ted to them from a particular part, they incline to- wards it, as if to fhew how ejfential this fluid is, to their exiflence. The importance of this fubflance, and its power of producing colour in plants, have been noticed by fevqral authors. " Without the influence of light, fays Chaptal, vegetables would exhibit but H one lifelefs colour; they are deprived of their beau- tiful fhades by the interception of this luminous fluid."* Agreeable to this writer, and M. Four. croy,f vegetables are not only indebted to light, for their colour, but likewife for their fmell, tafle", combuflibility, maturity and the refinous principle: And hence, that aromatic fubflances, refins, vola- tile oils, and thofe colouring matters, of fo much value for their livelinefs and body, are peculiar to fouthern climates, where the light is more pure, conflant and intenfe. The immortal Lavoifier entertained the fame opi- nion of the importance of this fluid with refpect. to vegetative exiflence and its power of imparting colour to vegetables. " Experiments upon vegetatiJm/ he obferves, give reafon to believe, that light com- bines with certain parts of vegetables; and that the green of their leaves, and the various colours of their flowers, are chiefly owing to this combina- tion. This much is certain, that plants which grow in darknefs, are perfectly white, languid and unhealthy, and that to make them recover vigour and to acquire their natural colours, the diredl in- fluence of light is abfolutely necefiary."! * Elements'of Chefniftry. t Elements of Natural Hiftory and of Chemiftry | Elements of Chemiftry. *5 Mr. Davy,§ has proved by experiment, that the colour of vegetables depends upon light, and that by depriving them of this fluid, they become white, though naturally of a deep colour. He found, that red rofe trees, when included from light, be^ fore their flowers began to appear, produced flow, ers almofl white; and that flowers naturally white, if expofed to a concentrated light, became highly coloured. Fruits are alfo indebted to light, for their beautiful colours, and I have often obferved, that that part of them, which is mofl expofed to the fun, is al- ways of the deepeft colour. Deprived of light, vegetables not only lofe their delicate lhades, but alfo their noxious properties. Some poifonous plants, by the abflraction of light, lofe their deleterious qualities, and become agreea- ble to the tafle, and even harmlefs when taken into the flomach. It is in this way, that gardeners pre- pare many vegetables for our tables. The celery, endive and other efculent vegetables, by this treat- ment, lofe their green colour and active qualities, and become white and wholefome articles of diet. In common with the inferior order of beings, man himfelf is indebted to light for his colour, as well as the mofl numerous of his pleafures. § Beddoes* Medical Contributions. i6 In tropical climates, where the light is ftronger and more conflant, the inhabitants are obferved to be of a deeper colour, than thofe who live to the north, under a much lefs powerful fun. In Africa, the natives are black, while in the northern parts of Europe they are white, and between thofe two quarters of the globe, are to be found all the in- termediate gradations of colour. Our own feafons afford us a flriking example of the colouring power of light. During the fcorching heat of fummer, when the light is very intenfe, our fkins often become highly coloured or tanned, as we commonly exprefs it; whereas in win- ter, when the fun is not fo powerful, we lofe the co- lour acquired in fummer, and become much fairer. It is invariably found that fuch parts of our bo- dies as are expofed to light are coloured, while thofe that are covered by our clothes, remain per- fectly white. Ladies, who are lefs expofed to the fun, are fairer than men; and men in confinement "f £ fl°rid ^^ if ^J - dc I have obferved that many of thofe gentlemen, who go to the Eaft-Indies, though fair when they leave this country, acquire very brown complexions before they return; which circumflance, can only be afcnbed to the greater concentration and reflec- *7 tion of the light of the fun from the water, in that warm climate. In what manner colour is produced by the ope- ration of light, I fhall not pretend to explain; it is a fubject. flill involved in very great obfcurity. Whether it is by abftracting oxygen or any other principle from the body, is not afcertained with fufficient certainty to warrant a conclufion. The colour of the fkin depends upon the colour of the rete mucofum; which in the African is black, in the European white, while it is of an intermediate or copper colour in the American. Doctor Beddoes imagined that the black fkin of the African, was produced by the abflradtion of oxygen from the rete mucofum by light, -and his experiments feem to favour this hypothefis; for by expofing the fingers of a negro in the oxiginated muriatic acid, which eafily parts with its fupera- bundant oygen, they foon became white. Doc- tor Girtanner afferts, that white animals and plants are more irritable, than thofe that are coloured; and irritability he proves to depend upon oxygen. By the abflraction of light they lofe their colour, and become much more irritable in confequence of an accumulation of this principle. This theory of the operation of light, explains with eafe and perfpicuity, the diverfity of colour C iS obferved in different countries. The flronger the light and more conflant its application, the greater will be the abftraction of oxygen, and the deeper the colour produced. It is in this way, that the various colours of organized beings are accounted for; but the production of colour in inorganic fub- flances, is not to be explained with the fame eafe by this hypothefis, for great changes ta&e place in fome fubflances expofed to light, in which the prefence of oxygen has never been detected. Befides the lofs of colour, which would inevitably follow the privation of light, our atmofphere would be rendered uninhabitable, from the fupply of pure air being cut off; in the production of which the operation of light is abfolutely neceffary. *9 SECTION It. II. LIGHT IS A CONSTITUENT ELEMENT OF OXYGEN GAS. ACCORDING to M. Lavoifier and other cele- brated chemical philofophers, pure air confifls of oxygen combined with caloric or the matter of heat. This doctrine has obtained very generally among chemifls, and from it is drawn the prefent theory of refpiration and combuflion. However eafily explicable the phenomena of thofe proceffes may appear to be, by this theory of oxygen gas, I fhall attempt to prove that light is one of its principles ; and that it is partly to the action of this etherial fubflance, that fome of the phenomena of refpiration ought to be attributed. That light enters into the compofition of oxygen gas, I infer, from its analyfis and fynthefis. It is well known, that in every rapid decompofition of pure air, light is liberated, and all our experiments to form acids and metallic oxyds, prove it to be one of the elements of pure air. By the combuf- tion of carbone, hydrogene, and other firople 20 fubflances in oxygen gas, a large quantity of the" mofl brilliant light is liberated; to account for which, we mufl either admit that it is a principle of this gas, which is difengaged by its decompofition, or adopt the opinion of Macquer, who fuppofed that the light was emitted from the burning body. This celebrated chemifl attempted to unite the pneumatic theory with that of phlogiflon, and by a doctrine drawn from the two, he certainly ex- plained the phenomena of combuflion with great eafe and perfpicuity; but unfortunately for his theory, no experiments were adduced in fupport of it. The light emitted by the fun, Macquer regards as the matter of fire, and that by admitting it as fixed in bodies, it conflitutes the phlogiflon of Stahl. According to his theory, in every combuf- tion, the oxygen gas difengages the light or phlo- giflon from inflammable bodies, and occupies its place. It is a fufficient refutation of this theory, that no phenomena prove or require its exiflence* whereas, that of light being difengaged from ox- ygen gas, not only folves the phenomena, but is directly drawn from a multitude of experiments. We have no evidence of the exiflence of light in carbone, hydrogene or iron; yet by the com- 21 bullion of thefe fubflances in pure air, to form carbonic acid, water and the calx of iron, a great quantity of vivid light is liberated. The following experiment, made by Mr. Davy, eflablifhes, in the mofl pofitive and unequivocal manner, that light is contained in oxygen gas. " A fmall gun lock, armed with an excellent flint, was fnapped in a veffel filled with oxygen gas. The particles of fleel, feparated by collifion, were the mofl brilliant that can be imagined; and thefc particles, examined by a magnifier, were found to be converted into black oxyd of iron. The fame experiment was made in a veffel filled with carbo- nic acid gas; the iron was fufed, but no light wa* liberated."* This experiment not only proves that light enters into the compofition of pure air, but eflablifhes its exiflence independent of caloric. From light and heat being generally concomitant, many philo- fophersf have fuppofed, that they are caufe and effect; but the above experiment, in conjunction with another, made in an exhaufled receiver, in * Beddoes' Medical Contributions. ■j- " We are unable to determine, whether light be a modification of caloric, or caloric be, on the contrary, a modification of light." Lavoifier's Elements of Chcm'tftry. 22 which no light appeared, yet the metal was fufed, proves that light is not a modification of heat; and goes a great way, in my opinion, to eflablifh the non-exiflence of caloric, or the matter of heat. From many experiments made by Mr. Davy, and thofe of Count Rumford,* on friction, I am difpofed to doubt the materiality of heat. An opi- nion, however, the reverfe of this is advocated by fome of the firfl philofophers in Europe. The mofl powerful arguments have been adduced on both fides of the queflion; but it appears to me that the force of reafoning and certainly the weight of experiment, are againfl the exiflence of caloric. This controverfy between men of fuch diftin- guifhed talents, affords a true, though melancholy proof of the imbecility of the human mind and of the imperfect Hate of fcience. When fuch men as Black, Crawford, Lavoifier and Rumford difpute, who can decide ? It would be preemption in me to advance an opinion. The materiality of heat is certainly queflionable ; and when probability is fhaken, reafon flrongly in- clines to fcepticifm. * Effays Political, ©Economical and Philofophical. Vol. II. 23 Though I entertain doubts on this fubject, I by no means deny, that heat is matter, and that it acts a very important part in the ceconomy of the uni- veHc; on the contrary, I fhall confider it, as one of the elements of pure air. By the experiments of M. Berthollet, light ap- pears to have great affinity with oxygen, and con- tributes with caloric to change it into the flate of gas. This theory of the conflitution of pure air, qua- drates befl with mofl of our experiments, and by it, the phenomena of combuflion and refpiration admit of a more eafy and fatisfactory explanation. The exiflence of light in pure air, is alfo de- monflrated by fynthetical experiments. The at- traction of light for oxygen is fo powerful, that few bodies which contain the latter principle in a loofe flate of combination, can hold it when ex- pofed to light. It is proved by the experiments of Doctors Prieflley, Injenhouz and Mr. Davy, that vegeta- bles, when expofed to folar light, decompofe water and carbonic acid. The hydrogene of the water and carbone of the acid, are attracted by the ve- getable, while oxygen is difengaged in the form of gas, by combining with light. *4 It is by virtue of thefe affinities, that the pure air of our atmofphere is continually renewed. Man and other animals, live only by the afliflance of the vital air which they breathe; whereas vege- tables imbibe and are nourifhed by the air which animals vitiate in their refpiration, and in return difcharge torrents of vital air. Thus we fee by what fimple and ©economical means, the numerous beings on the globe are fupported. The two great clafles of organized bodies depend for their exiflence on the labours of each other, and they are both dependant on light. This is not the only way by which we prove the recompofition of oxygen gas. Many inorganic fub- flances, which contain oxygen, give it out, when in contact with light. A bottle of oxiginated muriatic acid expofed to the fun, parts with its fuperabundant oxygen; but if in the fame expo- fure, the bottle is covered with any thing that will intercept light, the acid fuffers no change; and when heated in a dark place, Chaptal obferves, is even reducible into gas without decompofition. The nitric acid alfo, when expofed to the fun, af- fords oxygen gas; whereas heat alone volatilizes it, without decompofition. The muriate of fiiver and red oxyd of mercury, 25 part with their oxygen in the form of oxygen gas3 when expofed to the light of the fun. M. Lavoifier obferves, when fpeaking of the reduction of red precipitate in a porcelain retort, " As the oxygen gas never appears till the retort becomes red, it feems to prove the principle efla- blifhed by M. Berthollet, that an obfcure heat can never form oxygen gas, and that light is one of its conftituent elements." What a wonderful coincidence of facts, to prove that light is a component part of pure air! The very author, who makes pure air to be a combination of oxygen and caloric, has in his definition of combuftion, acknowledged light to be one of the principles of this air. According to Lavoifier, " Combuftion is the de- compofition of oxygen gas, produced by a com- buflible body. The oxygen which forms the bafe of this gas, is abforbed by, and enters into combi- nation with, the burning body, while heat and light are difengaged." It will be unneceffary for me to multiply in- flances of the compofition and decompofition of oxygen gas, to prove that light is one of its confti- tuent elements. D 26 SECTION III. III. THE VERMILLION COLOUR OF BLOOD DEPENDS, IN PART, UPON THE OPERATION OF LIGHT. —&>*<^— HAVING demonftrated, I hope, in the preced- ing parts of this inquiry, that light is a conflituent principle of oxygen gas, and that mofl colours de- pend upon combined light; I will go on to prove by experiment, the operation of this fubtile fluid on blood, efpecially in contributing to produce its florid colour; from which circumftance, and from the action of light on other bodies, I fhall infer its influence in refpiration. According to the mofl popular theory of refpi- ration, vital air is decompofed in the lungs; the oxygen by combining with the blood occafions its vermillion colour, while the caloric is fet at liberty and produces animal heat. If, agreeable to this doctrine, oxygen gas is de- compofed in the lungs, light mufl be liberated, as well as caloric, as we have made it appear that light is a component part of this gas. It is there- fore, more efpecially our objeft, in this place, to 27 eflablifh by experiments on blood and by deduc- tions from analogy, the effects of this principle in refpiration. Light from its extreme tenuity, is ca- pable of paffing unaltered through the pores of diaphanous bodies; I procured, therefore, very tranfparent phials to experiment with, and when I wifhed to afcertain the effects of light alone, every precaution was ufed to prevent the co-operation of other agents. Experiment i. A vein of the arm was opened, and two phials, holding each two ounces, were filled with blood. Great pains were taken to prevent the contact of air, by applying the phials as near to the vein as poflible, and by corking them as foon as they were filled. One of thefe phials was immediately ex- pofed to the fun, which was mining very bright. The other was put away in a dark clofet. After remaining in this fituation for two hours, they were examined, and the following appearances were exhibited. The blood in the phial expofed to folar light, had loft its dark venous colour and was turn- ed red, but the colour was not half as bright as when in contact with atmofpheric air. That part of the blood which was fhaded, in forhe degree, by refling againfl the window, was not as bright as that which looked directly to the fun, though it was altered ccnfiderably in colour. 28 The phial in the dark clofet fuffered no change. As the blood was placed in the fun, I entertained doubts, whether or not heat might have contri- buted to the change. To afcertain this point, therefore, the following experiment was made. Experiment 2. A vein being opened, the ftream of blood was directed into a two ounce phial, which was held as clofe to the arm as poffible, to avoid the contact of air. In this way three phials were filled and after being well corked, they were difpofed of in the following manner: One was placed in the fun, fhining very bright, about 11 o'clock, A. M. another, after being co- vered with black, was likewife expofed to the fun: but the third was put away in the dark clofet. In two hours they were examined. The blood in the firft phial was turned red, as in the preced- ing experiment; in the fecond, which was expofed as the firft, but covered with black, the blood fuf- fered no perceptible change; which was alfo the cafe with the third. This experiment fatisfied me that light and not heat, was the agent which produced the change, 29 for upon examining the two phials placed ir, C--.,z fun, I found that the one covered with bkc confiderably warmer than the other, yet no a tion took place in the colour of the blood it cj -. tained; whereas the blood in the other, which v ;; not fo much heated, and from which light had no: been excluded, foon acquired a red colour. I can afcribe, therefore, the want of colour in the fecond phial, to nothing elfe but the intercep- tion of light by its black covering. Though this experiment removed my doubts as to the caufe of the change, yet as my friend and fellow-graduate, Mr. Robert Berkeley, who wit- nefled the making of thefe experiments, flill be- lieved in the co-operation of heat, I made the two following experiments, which prove mofl decided- ly, the influence of light exclufive of heat, in co- louring of blood. Experiment 3. Two phials, holding each two ounces, were filled with venous blood, and after being well corked, one of them was immerfed in fnow on the outfidc of a window, on which the fun fhone very bright. About one third of the phial from top to bottom, was expofed to folar light, by fcraping off a part of 3° the fnow; while the reft of it, was completely im- merfed and excluded from light. The other phial was placed as ufual, in the clofet. Upon inflection two hours after, I found that part of the blood on which the light had fallen, was turned red, while the part oppofite to this and fecluded from light, though altered in colour, was not near as bright as that which looked immediate- ly to the fun. The blood in the clofet fuffered no alteration. Here the co-operation of heat could not be fufpect- ed, for it is well known, that all bodies in paffing from the folid to the fluid flate, as was the cafe with the fnow, abforb heat; therefore, the blood in this experiment, inftead of acquiring, muft have loft its own heat, very foon after immerfion. Experiment 4. A vein of the arm was opened, and two phials were filled with blood from it, the ufual precautions being obferved. One covered with black, was placed before a fire, the warmth of which was as much as I could en- dure. The other was expofed to the fun, which fhone with great fplendour. 3* The blood before the fire was examined fre- quently for two hours and better, but no change in colour was obferved to take place; whereas, that expofed to the light of the fun became altered in a very fhort time, and at the expiration of two hours, it was as red as in the preceding experi- ments. The influence of light, exclufive of heat or air, in colouring of blood, is unequivocally demon- ftrated by thefe experiments, which have been re- peated with precifely the fame refults. The blood turned red by light, if expofed in darknefs foon loft its colour and became very black, and was not to be coloured again by re-expofure to light, however intenfe. I alfo remarked, that the blood which had re- mained in the dark clofet for three or four hours, was with difficulty robbed of its black colour, and never contracted fo bright a red as frefh blood, though the mofl concentrated light was applied. What this could have been owing to, I know not; but think it probable, that during its cooling, the blood may have parted with fome principle, or un- dergone fome peculiar change, which indifpofed it to receive a red colour, from the adion of light alone. We know that from blood jufl dr+.vn a 32 confiderable halitus arifes, and after becoming cold, I have found its colour with difficulty changed, even by oxygen gas. To afcertain what would be the effect of expofing blood in the fame atmofphere, but to different de- grees of light, I made the following experiment. Experiment 5. Two fmall china bowls were nearly filled with venous blood and the room was darkened by bring- ing too ihe window fhutters. Through an aperture in i.ie fhutters, a fmall quantity of very brilliant folar light was admitted, which fell upon and com- pletely covered the furface of the blood in one of the bowls. The other bowl was placed, by the fide of this, but no light from the aperture was allowed to fall upon it. In this fituation they continued for two hours, when upon examining them, it was found, that the blood on which the light had fallen, was turned of a mofl brilliant vermillion colour; while that in the bowl which was expofed in the fame atmof- phere, but to a weaker light, had acquired a colour much lefs florid. This experiment unequivocally demonftrates the power of light in augmenting the vermillion colour 33 of biood, and taken in conjunction with the other experiments and obfervations contained in this in- quiry, goes a great way, in my opinion, to eflablifh the influence of this etherial fubftance in refpiration. Let us apply thofe facts to explain fome of the phe- nomena of this important function. Refpiration is confidered, by phyfiologifts, as an operation, by means of which oxygen gas is conti- nually paffing from the gafeous to the concrete ftate; it muft therefore at each inflant abandon the caloric (and light too) which held it in folution and in the ftate of gas. According to this theory, oxygert gas is decom- pofed in the lungs; its bafe combines with, and gives a florid colour to the blood, while its caloric is fet at liberty and produces animal heat. I believe in the decompofition of pure air in refpiration, but cannot agree that all the phenomena of this functioa are produced by oxygen and caloric; nor will the reader find any difficulty in admitting of the co-ope- ration of light, if he will compare the preceding obfervations and experiments, with this doarine, which is univerfally received. Combuftion and re- fpiration are faid to be two complex chemical pro- cefles,whofe phenomena are the fame or very fimilar. Now the liberation of light is admitted in combuf- tion, by the advocates of the prefent theory, yet E 34 they fay nothing of its difengagement in refpiration. If oxygen gas is decompofed in thefe two procefles, certainly the principles of this gas muft be liberated in both. In combuftion, which is a more rapid decompofition of pure air, the difengagement of light is very evident to our fenfes; whereas in refpi- ration it is impoffible to have any fuch evidence, not only in confequence of the concealed fituation of this procefs, but of the flower and more gradual decompofition of pure air in the lungs. We can therefore judge only of the liberation and effects of light in refpiration, by the nature of the air expired and by experiments on blood out of the body. By the refearches of the illuftrious Prieftley and Lavoifier, the compofition of our atmofphere and the nature of the airs expired, have been fully de- veloped. The air we refpire is found to confift of azotic and pure air, in the proportion of 72 parts of the former to 28 of the latter, which undergoes very remarkable changes in the lungs. Doctor Goodwin found, by experiments en him- felf, that the diminution of pure air and the increafe of fixed air,* was confiderable, in a fingle refpi- ration ; while the azotic air fuffered neither increafe nor diminution. Now if agreeable to the doctrine * The air he breathed contained 2 parts in ico of fixed air. 35 of Goodwin and Girtanner, the increafe of fixed air, is owing to the oxygen of the decompofed pure air, uniting with the carbone of the blood, it muft of neceffity follow that light is liberated, for we have proved by experiment,* that light does not enter into the compofition of carbonic acid ; nor have we any evidence of its being contained in azotic gas or in water, which is the other product of refpiration. A late ingenious writer, Mr. Davy, denies the decompofition of vital air in refpiration, and afferts that the liberated carbonic acid and water are con- ftituent principles of venous blood, which are dif- placed by the vital air; for which the venous blood has., a greater elective attraction, than for its con- flituent elements, water and carbonic acid. Mr. Davy denies the exiflence of caloric altoge- ther, and makes oxygen gas to confift of light and oxygen, to which he has given the name of phof- oxygen. As the reader may not have feen this gentleman's paper, I will give a fhort view of his theory. " Refpiration, he obferves, is a chemical proccfs, the combination of phofoxygen, with the venous blood in the lungs, and the liberation of carbonic acid and aqueous gas from it. From the combina- • Vid. p. ii. 3« tlon and decompofition arifes an increafe of repul- five motion, which, combined with that produced by the other chemical proceflfes taking place in the fyftem, and that generated by the reciprocal action of the folids and fluids, is the caufe of animal heat; a heat which the other fyftems have fuppofed to arife chiefly from the decompofition of oxygen gas, oxygen and caloric."* Mr. Davy fuppofes, that the florid colour of blood is produced by phofoxygen combining with it, in its entire ftate, and urges the following objections to the decompofition of pure air in refpiration. Carbone, hydrogene and iron, he fays, never' decompofe oxygen gas at fo low a temperature as 9 8° and that it is never decompofed by thofe fubflances, without combuftion, flame and great heat. This is afluming more, I apprehend, than daily obfervation and experience authorize, for we know that the combinations of oxygen are decompofed by many fubflances in a temperature much lower than that of the human body. Several of the metals are calcined by expofing them in a moift atmofphere, without any perceptible liberation of light or heat. In fermentation and putrefaaion, large quantities of carbonic acid are formed and extricated in a temperature much inferior to that of the lungs and without combuftion or flame. His objections, there, * Beddoes' Medical Contributions. 37 fore, to the decompofition of oxygen gas, in ref- piration are not founded. But admitting for a mo- ment, that oxygen gas cannot be decompofed in fo low a heat as 98°, in our laboratories^ are we from thence to infer, that the lungs have not the power of decompofing it ? Do not vegetables decompofe water and carbonic acid, with the utmoft facility, in the ordinary temperature of our atmofphere? Have we any experiments that prove the principles of water and carbonic acid to be in a weaker ftate of combination, than thofe of oxygen gas ? Since then vegetables can decompofe fome of the ftrongeft com- binations of oxygen, why deny a fimilar faculty to a more perfea order of organized beings? Is it wife in us to eftimate the powers of the animal cecono- my, by thofe of an imperfed art ? Certainly not; for the more we inveftigate the ftruaure and ope- rations of animated beings, the more are we ftruck with the faculties they poffefs, and convinced of the wifdom of their Creator! I have no doubt of the decompofition of oxygen gas in refpiration, but cannot afcribe all the pheno- mena of this important fundion to the exclufive adion of oxygen and caloric, as mofl chemical phi- lofophers have done. 1 conclude, therefore, and think my conclufion is warranted by the preceding obfervations and expe- riments, that light is a constituent element of oxygen 3» gas, and that it is to the difengagement* and opera- tion of this fubftance in refpiration, that fome of its phenomena ought, in part, to be attributed, more efpecially the vermillion colour of pulmonary blood. There is no knowing what difcoveries may be made in this important fubjea, by a more extenfive chemiftry. By the refearches of Prieftley, Lavoifier and other celebrated chemifts, many of the elements are already known to us, and it is to be hoped that by a continuance of their labours, the nature of thofe fubtile fluids will be difcovered, which at pre- fent elude the perception of our ienfes and are only known to us by their direful effeas. ' May not the deAroid fluid be produced in this way ? Is electricity any thing more than " light in a condenfed ftate ?" FINIS. MUMist. WZ. 3/70 ItOO *■-! r •*tsi *«r:; '-•V*#J* :.^X*: v *V ** »»i *«■ ■%♦ ...Kf - - v-w -. ** "41 *-■ ^ ■;