62 PRINCIPLES OF CHEMISTRY 



various other substances. Consequently, in solution there acts a 

 peculiar force, as in actual chemical combinations, and solution is de- 

 termined by a peculiar kind of movement (by the chemical energy of a 

 substance) which is proper to the substance dissolved and to the sol- 

 vent. 



Graham made a series of experiments similar to those above 

 described, and he showed that the rate of diffusion* 1 in water is very 

 variable that is, a uniform distribution (under perfect rest, and with 

 such an arrangement of the strata of the solutions that uniformity 

 takes place in opposition to gravity) of a substance in the water dis- 

 solving it is attained in different periods of time with different solutions. 

 Graham compared diffusive capacity with volatility. There are sub- 

 stances which diffuse easily, and there are others which diffuse with 

 difficulty, just as there are more or less volatile substances. Seven 

 hundred cubic centimetres of water was poured into a jar, and by means 

 of a syphon (or a pipette) 100 cub. centimetres of a solution containing 10 

 grams of a substance was cautiously poured in so as to occupy the lower 

 portion of the jar. After the lapse of several days, successive layers of 

 50 cubic centimetres were taken from the top downwards, and the quan- 

 tity of substance dissolved in the different layers determined. Thus, 

 common table salt, after fourteen days, gave the following amounts (in 

 milligrams) in the respective layers, beginning from the top : 104. 120, 

 126, 198, 267, 340, 429, 535, 654, 766, 881, 991, 1,090, 1,187, and 2,266 

 in the remainder ; whilst albumin in the same time gave, in the first 

 seven layers, a very small amount, and beginning from the eighth layer, 

 10, 15, 47, 113, 343, 855, 1,892, and in. the remainder 6,725 milli- 

 grams. Thus, the diffusive power of a solution depends on time and 

 on the nature of the substance dissolved, which fact may serve, not only 

 for the explanation of the process of solution, but also in distinguishing 

 one substance from another. Graham showed that substances which 

 rapidly diffuse through liquids are able to rapidly pass through mem- 

 branes and crystallise, whilst substances which diffuse slowly and do not 

 crystallise are colloids, that is, resemble glue, and penetrate through 



17 The researches of Graham, Fick, Nernst, and others showed that the quantity of a 

 dissolved substance which is transmitted (rises) from one stratum of liquid to another in 

 a vertical cylindrical vessel is not only proportional to the time and to the sectional area 

 of the cylinder, but also to the amount and nature of the substance dissolved in a stratum 

 of liquid, so that each substance has its corresponding co-efficient of diffusion. The cause 

 of the diffusion of solutions must be considered as essentially the same as the cause of 

 the diffusion of gases that is, as dependent on movements which are proper to their 

 molecules ; but here most probably those purely chemical, although feebly-developed, 

 forces, which incline the substances dissolved to the formation of definite compounds, 

 also play their part. 



