182 Prof. Graham on the Diffusion of Liquids. 



spective of quantity dissolved. In the two solid crystalline 

 hydrates, pyrophosphate of soda and sulphate of soda, we see 

 the same ten equivalents of water associated with both salts, 

 but obviously united with unequal degrees of force, the one 

 hydrate being persistent in dry air and the other highly efflo- 

 rescent. So also in the solutions of two salts which are equally 

 soluble in point of quantity, the intensity of the attraction 

 between the salt and the water may be very different, as ex- 

 emplified in the large but feeble solubility in water of such 

 bodies as the iodide of starch or the sulphindylate of potash, 

 compared with the solubility of hydrochloric acid or of the 

 acetate of potash, which last two substances are capable of 

 precipitating the two former, by displacing them in solution. 

 Witness also the unequal action of animal charcoal in with- 

 drawing different salts from solution, although the salts are 

 equally soluble; and the unequal effect upon the boiling-po'i t 

 of water produced by dissolving in it the same weight of x i- 

 rious salts. Besides being said to be small or great, the s< la- 

 bility of a substance has also therefore to be described as we ak 

 or strong. 



The gradations of intensity observed in the solvent force 

 are particularly referred to, because the inquiry may arise 

 how far these gradations are dependent upon unequal diffusi- 

 bility; whether indeed rapidity of diffusion is not a measure 

 of the force in question. 



I have only further to premise, that two views may be taken 

 of the physical agency by which gaseous diffusion itself is 

 effected, which are equally tenable, being both entirely suffi- 

 cient to explain the phsenomena. 



On one theory, that of Dr. Dalton, the diffusibility of a gas 

 is referred immediately to its elasticity. The same spring or 

 self-repulsion of its particles which sends a gas into a vacuum, 

 is supposed to propel it through and among the particles of 

 a different gas. 



The existence of an attraction of the particles of one gas 

 for the particles of all other gases is assumed in the other 

 theory. This attraction does not occasion any diminution of 

 volume of gases on mixing, because it is an attraction residing 

 on the surfaces of the gaseous molecules. It is of the same 

 intensity for all gases, hence its effect in bringing about inter- 

 mixture is dependent upon the weight of the molecules of the 

 gases to be moved by it; and the velocity of diffusion of a gas 

 comes to have the same relation to its density on this hypo- 

 thesis as upon the other*. 



* Both of the molecular theories of the diffusion of gases were first pub- 

 licly explained, and at the same time ably discussed, with the reference to 



