64 I'KIXCII'LES OF CMIK3IJSTKY 



If it be desired to increase the rate of solution, recourse must 

 be had to stirring, shaking, or some such mechanical movement, 

 obliging the solution formed round the given substance to rise up- 

 wards if the solution be heavier than water. But if once a uniform 

 solution is formed, it will remain uniform if the temperature be 

 uniform, no matter how heavy the dissolved substance is, or how long 

 the solution be left at rest, which fact again shows the presence of a 

 force holding together the particles of the body dissolved and of the 

 solvent. 19 



naturally passes through into the water until the solution attains the same strength on 

 both sides of the membrane. By replacing the outside water with fresh water, a fresh 

 quantity of the crystalloid may be separated from the dialyser. While a crystalloid is 

 passing through the membrane, a colloid remains almost entirely in the dialyser, and 

 therefore a mixed solution of these two kinds of substances may be separated from each 

 other by a dialyser. The study of the properties of colloids, and of the phenomena of 

 their passage through membranes, should elucidate much respecting the phenomena 

 which are accomplished in organisms. 



19 The formation of solutions may be considered in two aspects, from a physical and from 

 a chemical point of view, and it is more evident in solutions than in any other department 

 of chemistry that these provinces of natural science are allied together in a most intimate 

 manner. On one hand solutions form a particular aspect of a physico-mechanical inter- 

 penetration of homogeneous substances, and a juxtaposition of the molecules of the sub- 

 stance dissolved and of the solvent, similar to the juxtaposition which is exhibited in 

 homogeneous substances. From this point of view this diffusion of solutions is exactly 

 similar to the diffusion of gases, with only this difference, that the nature and store of 

 energy is different in gases from what it is in liquids, and th,t in liquids there is consider- 

 able friction whilst in gases there is comparatively little. The penetration of a dissolved 

 substance into water is likened to evaporation, and solution to the formation of vapour. 

 This resemblance was clearly expressed even by Graham. In recent years the Dutch 

 chemist, Van't Hoff , has developed this view of solutions in great detail, having shown (in 

 a memoir in the Transactions of the Swedish Academy of Science, Part 21, No. 17, 

 ' Lois de 1'equilibre chimique dans Petat dilue, gazeux au dissous,' 1886), that for dilute 

 solutions the osmotic pressure follows the same laws (of Boyle, Mariotte, Gay-Lussac. 

 and Avogadro-Gerhardt) as for gases. The osmotic pressure of a substance dissolved in 

 water is determined by means of membranes which allow the passage of water, but not 

 of a substance dissolved in it, through them. This property is found in animal proto- 

 plasmic membranes and in porous substances covered with an amorphous precipitate 

 such as is obtained by the action of copper sulphate on potassium ferrocyanide (Pffeifer 

 Traube). If, for instance, a one p.c. solution of sugar be placed in such a vessel, 

 which is then closed and placed in water, then the water passes through the walls 

 of the vessel and increases the pressure by 50 mm. of the barometric column. If the 

 pressure be artificially increased inside the vessel, then the water will be expelled 

 through the walls. The osmotic pressure of dilute sohitions determined in this manner 

 (from observations made by Pffeifer and De Vries) was shown to follow the same laws 

 as those of the pressure of gases ; for instance, by doubling or increasing the quantity of 

 a salt (in a given volume) n times, the pressure is doubled or increases n times. One of 

 the extreme consequences of the resemblance of osmotic pressure to gaseous pressure 

 is that the concentration of a uniform solution varies in parts which are heated or cooled. 

 Soret (1881) indeed observed that a solution of copper sulphate containing 17 parts of 

 the salt at 20 only contained 14 parts after heating the upper portion of the tube to 

 80 for a long period of time. This aspect of solution, which is now being very carefully 

 and fully worked out, may be called the physical side. Its other aspect is purely 

 chemical, for solution does not take place between any two substances, but requires a 



