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Frofessor Joshua Lederberg
The University of Wisconsin, Madison 6

Dear Professor Lederberg,

I thank you for your letter of Febr. 20 .I received it here in
Goteborg,when I was laid up with pneumonia from which I am now recovering,
My work in the Pacific ceased end of last Oct when I returned to Sweden

via Rome.

I am much interested in your views and sh.1l do my best to

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give you the information required. The volume of air pas:..ed through my =

filtra varied between several 100 and a couple of thousands df cubi
metres , mbstly from £ 600 to 1 500 cu.metres.

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The figure given in my letter to Nature of 0,572 mg of cosmic dust per -
1 000 m? of sir passed through the filters is based on the assumption, +

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that the proportion offmickel in the cosmic dust is on an average 2,5 oy

accerding to Watsons estimate.However to ascribe this average dust-content

to the whole air-column from the surface up to the 100 km level is
naturally an assumption which may or may not be justified,According to a
letter I have received from Frof. Opik of the Armagh Observatory of N

Ireland this value should be reduced to one {hird according to the distri-
bution with height which he ascribes to the cosmic dust in the upper layers

Further assumptions entér into ghe calculus ofthe" flux" viz, the rate of
descent of the cosmic dust through the atmosphere. Assuming the"Verweils-
zeit"in the atmpsphere to be 2 years,like that of the Krakatoa dust of
1885-85,the weight of cosmic dust settling through 1 mefhorigzonta}
surface should be 28,86 mg/m year. Howevergtaking a rate of descent for

the dust 18 times higher, as Bowen does ( Verweilszeit only 40 days)
the flux is accordingly increased in the same pYoportion,i.e. to 505,8 ng
per n@ and year.My personal opinions that the true value of the flux will
be much nearer to the former,lower value than to the latter. I hope that
the observations now being continued may settle this point.
4s I have left the continued sampling in the han@of Frof Waltef Steiger
Geophysics Dept University of Hawaii, may I suggest that yOu send your
desiderata to him ,quoting this my legter to you.I think he wikl do his
best to meet your requirements.
8EOI xog
Yop, cieHabout the sampling of cismic dust for microbiological

studies are certainly interesting.I am afraid the technique I have been
using offers no guarantees against contamination with terrestrial spores
For that purpose dust»esampling at still higher altitudes might be
advisable,There are two localities sapitable for such purpose , onethe
Observatory on the Jungfraujoch in Switzerland,the other the Andean
observatory at Chacaltaya in Bolivia »Director Prof Ismael Escobar, altitu
17 060 feet. I bele€ve airsampling at either, or aerial proton wan
might give important results. The prevalence of "aerial p ton " at
lower altitudes would involve serious complications from your point of
view.

arrhenius’"pan-spermi" hf psshedis hypothesis was much disc¥ssed
in my youth . I believe the sterilizing effects of ultraviolet light
and of cosmic rays outside our atmosphere is a main argument against
the survival of ev. viable spores through interplanetary space, but
on this subject I would not at present hazard any opinion.

A reprint from a paper by myself and a collaborator now apparing
in Pacific Science will be sent to you as soon as published.

With my best wishes for your your interesting research
I am
yours sincerely