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OCR Page 1 of 2April 28th, 1930.
Dr. Louis Schwartz,
Surgeon, U.S.P.H.S.,
Room 505 Broadway,
New York City.
Dear Sir:
Mr. Lee has handed ne your letter of April 24th, request-
ing information relative to the composition of Undark.
Undark consists of a mixture of a specially prepared
crystaline zinc sulphide, containing minute amounts of copper,
manganese and cadmium, and radioactive salts in the form of radium,
mesothorium or 8. nixture of the two. For the past three or four
years the only radioactive material used has been radiun. Prior
to that time we used either radium alone or a mixture of radium
and mesothorium. The amount of radioactive material used, expressed
in terms of micrograms of radium element per gram of zine sulphide,
may vary from seven to thirty in the grados of material most widely
and commonly used.
Under separate cover we are sending you one gram of our
grade "L" Undark.
Very teuly yours,
Vice President.
HhBarker:RH
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