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POPULAR SCIENCE REVIEW. 
constitution has not been made known ; it appears to have been prepared 
in such a manner that, by the intervention of a layer of a material which 
is soluble in water, the attenuated upper film of collodion, or other similar 
substance, may be separated during immersion from the body of paper, 
which itself merely aids in giving strength to the tissue whilst undergoing 
the manipulation necessary to the production of the photograph. 
With reference to the forthcoming Exhibition of the Photographic 
Society the council have determined upon trying to make good the 
deficiency of past years by charging a small rent for space to all who are 
not associated as members of the Society. A sub-committee has already 
been appointed for the purpose of securing a room or gallery for the 
purposes of the exhibition, and for carrying out the general arrangements 
connected with the undertaking. Intimation has already been given 
to the effect that the council will again distribute a series of prize 
medals for the best examples in the several branches of photo- 
graphic art. 
PHYSICS. 
The Molecular Mobility of Gases. — Under this title we have a most 
valuable communication by Professor Graham, in the “ Comptes Rendus ” 
for July 27. The author was led to his results by a new examination of 
the diffusion of gases depending on this molecular mobility. When the 
air or other mixture of gases is placed on one side of a graphite plate, a 
vacuum being maintained on the other, the passage of the gases may be 
effected in three different ways : — 1. The gases may enter the vacuum by 
passing through a single minute aperture, such as that made by a fine 
needle in platinum-foil. The rapidity of the various gases then depends 
on their specific gravity. In this case the gas in its entirety partakes of 
the movements of effusion or efflux, whilst only the molecules are affected 
in the case of diffusion. 2. If the efflux aperture be in a very thick plate, 
so as really to form a sort of tube, the effusion rates of the gases are dis- 
turbed. If, however, the length of the tube be 4,000 times its diameter, a 
constant proportion between the effusion rates of the different gases is 
observed. The gases now pass by capillary transpiration. “ The rates of 
transpiration are not governed by specific gravity, and are indeed singularly 
unlike the rates of effusion.” “ The transpiration ratios of gases appear to be 
in constant relation with no other known property of the same gases, and 
they form a class of phenomena remarkably isolated from all else at 
present known concerning gases.” 3. A plate of artificial graphite, which 
appears to be practically impenetrable to gas by either of the modes above 
described, nevertheless admits of the passage of gases by the aid of mole- 
cular or diffusive movements. The author found that the times of 
passage of various gases were quite different from anything observed in 
the previous instances. They bore a close relation, however, to the 
square roots of the densities of the respective gases. This was more 
precisely determined by causing the gases to pass into a Torricellian 
