i879-j 
Proceedings of Societies . 507 
may practically be considered to be effected. The authors have 
little doubt that the main theories upon which they insist, con- 
firmed as they are by experiments made or faCts obtained, under 
very great variety of circumstances, may be accepted as, at any 
rate, close approximations to the truth. It is satisfactory to find 
that the laws which rule the tensions and temperatures of gases 
under ordinary circumstances do not lose their physical signifi- 
cance, but are still approximately applicable, at the high temper- 
atures and pressures they have been considering. At all events, 
it appears certain that the rules and tables they have laid down, 
as based on their analyses, experiments, and calculations, may 
for all practical purposes be accepted as correCt, and may, 
bearing in mind the restrictions to which they refer in this 
memoir, be applied to nearly every question of Internal 
Ballistics. 
June 19. — “ On the Production of Coloured SpeCtra by Light,” 
by Capt. Abney, R.E., F.R.S. Last year I incidentally men- 
tioned in a note to the Royal Society (“ Proceedings,” vol. xxviii., 
p. 291) that the production of natural colours by the agency of 
light, examples of which were shown by Becquerel, was probably 
caused by the oxidation of silver compounds employed. I have 
ventured to return to the subject, in order to show that the 
colours are so produced and are not due to interference. I have 
sent, for the Society’s inspection, pictures of the solar speCtrum 
on silver plates, and also on compounds of silver held in situ 
by collodion. It will be observed that the speCtrum has imprinted 
itself in approximately its natural colours ; that on the silver 
plates it is more brilliant than on the collodion film, but that in 
the latter the colours are seen by transmitted as well as by 
reflected light. I reserve for the present the exaCt details of the 
production of these pictures, but may say that they are produced 
by oxidation of silver compounds when placed in the speCtrum, 
an exposure of two minutes being amply sufficient with a. wide 
slit to impress the colours. The colouring-matter seems to be 
due to a mixture of two different sizes of molecules of the same 
chemical composition, one of which absorbs at the blue end and 
the other at the red end of the speCtrum, and the sizes of these 
molecules are unalterable whilst exposed to the same wave- 
lengths as those by which they were produced. I believe it pos- 
sible and probable that the colours may be preserved unchanged 
when exposed to ordinary daylight. 
Meteorological Society, June 18. — Mr. C. Greaves, F.G.S., 
President, in the chair. 
“ Thermometer Exposure — Wall versus Stevenson’s Screens,” 
by William Marriott, F.M.S. It being the practice of some 
observers to expose their thermometers on walls facing north, it 
seemed a suitable objeCt of inquiry whether instruments so 
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