40 ENERGY OF SUBSTANCE IN SOLUTION 



This brings us to the study of gaseous diffusion. Before enter- 

 ing on this subject it is necessary to keep in mind Dalton's Law 

 of Partial Pressures, which may be stated as follows : In a mixture 

 of gases each gas exerts the same pressure as it would exert if 

 it were alone present in the volume occupied by the mixture. 

 The pressure of each gas is called its partial pressure. In other 

 words, if several gases are brought together, each of them will 

 be distributed through the whole space as if the other gases 

 were entirely absent. 



Gaseous diffusion takes place, and as a result there ensues a 

 perfectly uniform mixture of the gases no matter what their 

 original proportions were, and irrespective of 

 the masses of their respective molecules. This 

 diffusion takes place independent of gravity. 

 A heavy gas will diffuse upwards into a lighter 

 one. 



Now, suppose we separate the two gases, A 

 and B, by a thin porous septum such as a 

 plug of plaster of Paris. It was observed by 

 Graham that the pressure of the gases did not 

 remain the same on both sides of the mem- 

 brane. The pressure was greatest on the side 

 of the heavier gas. The molecules of lighter 

 gas B would bombard the septum far oftener 

 than those of the heavier gas A, and therefore 

 there would be a greater chance of some of 

 them hitting the pores and getting through 

 and so raising the pressure in A. 



That this is so may be very easily demon- 

 _ st rated by a simple piece of apparatus (Fig. 4). 

 *~ -^riT~-"- : C is an unglazed earthenware cell such as is 

 4> ~ simple diffusi - usually employed to hold the zinc rod in a 

 Leclanche battery. Into its mouth is fixed 

 a rubber stopper carrying a glass tube, the lower end of 

 which passes just through one of the holes in a similar rubber 

 stopper of the bottle B. The glass tube A passes right 

 through the other hole of this stopper and goes to the 

 foot of B. B is filled with coloured water. Both stoppers 

 must he thoroughly air-tight. If now a wide-mouthed bottle 

 containing coal gas is inverted so as to enclose C, the coloured 

 fluid will rise in A showing increase in pressure in C. If the gas 

 used were heavier than air, negative diffusion would take place, 



