JANUARY 4, 1901. ] 
DIFFRACTION GRATINGS FOR GRATUITOUS 
DISTRIBUTION. 
To THE EDITOR OF SCIENCE: Two or three 
years ago while engaged in some experimental 
work on the reproduction of diffraction grat- 
ings by photography, I devised a method of 
copying a Rowland 14,000 line to the inch 
grating, and silvering the copy, transforming 
it into a reflecting grating. The original was an 
excellent glass grating kindly loaned to us by 
the Johns Hopkins University. I was unable 
to get satisfactory copies with the bichromated 
gelatine but succeeded very well with albumin. 
I found this difficulty, however. To reproduce 
so fine a spacing it was necessary to use an ex- 
ceedingly thin film, so thin, in fact, that the re- 
tardation of the light waves in traversing the 
‘bars’ was insufficient to give spectra of any 
brillianey. By thickening the film I was able 
to get a brilliant grating occasionally, but 
usually the lines ran together. It then occurred 
to me to silver the gratings, for the retardation 
by reflection is four times the retardation by 
transmission. The thin film failures, which I 
had thrown into a drawer as scrap plate-glass, 
were accordingly immersed in a chemical silver- 
ing solution, and when washed and dried were 
found to give brilliant spectra. One of these 
was exhibited by Professor Boys at a conversa- 
zione of the Royal Society about two years ago. 
Having about thirty of these gratings, which, 
while not as perfect as an original Rowland 
grating, being made on ordinary plate glass, are 
nevertheless suited for the ordinary purposes of 
the laboratory, I am desirous of placing them 
where they will do the most good. There must 
be among our many universities some physical 
laboratories which are not fortunate enough to 
have a good diffraction grating. I shall be very 
glad to distribute these copies to laboratories 
desiring them, as long as the supply holds out. 
I shall be glad if applicants will state whether 
the laboratory possesses a good spectrometer, 
and also the number of students engaged in the 
pursuit of physics. Some of the gratings are 
very good indeed, others quite poor as to gen- 
eral appearance, but all will give tolerably 
good spectra, and can*be used for wave-length 
determinations. They will show the nickel 
line between the sodium lines in the solar 
SCIENCE. 30) 
spectrum very distinctly. Applications from 
high-schools will not be considered. Failure to 
receive any reply must be taken as evidence 
that the supply has been exhausted. 
R. W. Woop. 
UNIVERSITY OF WISCONSIN. 
NOTES ON INORGANIC CHEMISTRY. 
ARGON AND ITS COMPANIONS. 
On November 15 a paper was read before 
the Royal Society by Professor William Ramsay 
and Dr. Morris W. Travers on ‘Argon and its 
Companions,’ which was a continuation of the 
previous papers of the same authors on the inert 
gases of the atmosphere. In the early summer 
of 1898 the discovery of neon and krypton was 
announced, and later a heavier atmospheric gas 
was found, to which the name xenon was given. 
At that time krypton and xenon were not 
obtained in a condition pure enough for the 
investigation of their physical constants. 
The present paper deals chiefly with these 
three gases, which have been isolated and 
studied. By the evaporation of a large amount 
of liquid air a mixture of argon, krypton and 
xenon was obtained, the former largely pre- 
dominating. This mixture was liquefied by 
liquid air and the three separated by fractional 
distillation, many times repeated. At the tem- 
perature of boiling air krypton has consid- 
erable vapor-tension, while that of xenon is 
scarcely appreciable. Neon was isolated from 
the first portion of gas escaping from boiling 
air. This consisted chiefly of nitrogen, which 
was then liquefied and a part of the liquid 
evaporated by passing through it a current of 
air. This gas, after the removal of the oxygen 
by hot copper, contained most of the helium 
and neon present in theair. After purification 
from nitrogen in the usual manner, the helium 
and neon were separated from the argon pres- 
ent by fractional distillation. Toseparate these 
gases was very difficult, but was finally accom- 
plished by condensing the neon by means of 
boiling hydrogen. In this way pure neon was 
obtained. 
A determination of the ratio of the specific 
heats of these gases showed that they are all 
monatomic. A number of the physical prop- 
