1904-5.] Dr Aitken on Evaporation of Musk. 895 
As the extremely small amount of musk that can be detected 
is likely long to retain its classical interest, and as there is a 
point connected with the subject on which there still seems to be 
some dispute, I have thought it worth while to see if the doubt 
could not be cleared up by some experimental methods. There 
seems to be a difference of opinion as to the state in which the 
musk exists in the air after it leaves its visible form. While 
some consider that it passes off as a gas or vapour, others think it 
goes off in solid particles. It is somewhat difficult to understand 
how this idea that musk passes into the air as a solid could ever 
come to be so strongly held, as everyone has seen snow and ice 
evaporating during frosty weather, and have never supposed the 
particles leaving the ice to be different from those leaving water. 
There seem to be a number of ways in which we might test 
whether musk exists in the air in the form of solid particles, or 
as a gas. First, we might try the cloudy condensation test. If 
the musk is in solid particles, these particles will become nuclei of 
cloudy condensation in supersaturated air, and thus make their 
presence visible. Second, we might pass the musk-laden air 
through tightly-packed cotton-wool. If the musk passes through 
the wool, we may conclude it does so in the form of a gas, as 
cotton-wool keeps back all particles floating in the air. Third, we 
might try diffusion. If the musk is gaseous, it will diffuse 
through the air and not require air-currents to carry it, which 
would be necessary for solid particles. Diffusion through porous 
vessels, tightly-packed cotton-wool, etc., might also be tried, as 
we would hardly expect solid particles to diffuse through these. 
Fourthly, we might try gravitation. If the musk is in solid 
particles, it will settle at the bottom of the enclosure, as do the 
finest dust-particles in air. 
Before going further, it may be as well to refer to a point which 
requires attention. It might be asked, May not the subdivision 
of solids go on so far that they may be reduced to the dimensions 
of molecules? And if so, How would they differ from gaseous 
molecules, and how would they conduct themselves when sus- 
pended in a gas? Would they take up movements similar to 
those of the surrounding gaseous molecules? If so, they might 
diffuse through the gas, and both diffusion and gravitation tests 
