THEEMO-DYNAMICS. 33 



in the reasoning which has led finally to the adoption of 

 this theory. I mentioned some few weeks ago, in a 

 Lecture on the Radiation of Heat, what a striking proof 

 the theory of radiation and of the transmission of heat by 

 undulations through ether gives of the idea that heat in a 

 body consists in the motion of its molecules ; because in 

 order that these vibrations which are transmitted through 

 the ether should be originated, it is necessary that some- 

 thing, such as the molecules of the body, should be in 

 agitation to communicate these vibrations. The most strik- 

 ing evidence of the truth of this theory is that in connection 

 with gases which was first disclosed by the discoveries of 

 Dr. Graham when he enunciated the laws of the diffusion 

 of gases. Graham found that all gases have a tendency to 

 permeate each other, that is that one gas tries to pass 

 through and permeate the substance of another, and even 

 where the two gases are separated by some partition, say 

 of plaster of Paris, the gases on the two sides of the 

 plaster of Paris have a tendency to pass through. Moreover, 

 Graham found that the velocity with which these gases pass 

 through the plaster of Paris is different, and depends on 

 their density; the less dense gas passing through the most 

 quickly. 



Suppose I take a jar and fill it with a light gas such as 

 coal-gas; then if I place the jar full of gas over the 

 plaster of Paris partition we shall have common air below 

 the partition and coal-gas above. The consequence of this 

 will be that the coal-gas will tend to pass most quickly 

 through and the air will pass slowly out through the 

 plaster of Paris, and so you will notice a depression on the 

 level of the liquid in the tube. To perform this experiment 

 I will simply fill this jar with coal-gas and then place it 

 over the plaster of Paris partition, and you will see by the 

 descent of the liquid the rapidity with which the coal gas 

 penetrates into the cell. The tube with its funnel-shaped 

 end is now nearly full of gas, and on removing the jar the 

 air will penetrate into the tube and the gas will pass out 

 again. The gas will pass out quicker than the air passes 

 in, and consequently the pressure will be diminished, and 

 you will see the liquid in the tube rising. The first great 

 steps towards the proof of the theory of heat lay in the 

 experimental determination of the fact, that wherever 



VOL. II. D 



