COLLOIDS. 31 



cally combined in definite proportions, but the quantity continually 

 changes with the temperature and the vapor pressure. On the other 

 hand, the imbibition stands in close relation to the osmotic pressure 

 which is evident, if we define the osmotic pressure of a substance as 

 its ability to attract water. The relation between imbibition and 

 osmotic pressure is still closer in those cases when the substance finally 

 is dissolved in water. 



If a hydrogel is placed in a salt solution instead of in pure water, 

 the imbibition phenomena essentially change. This was first studied 

 by HoFMEiSTER, 1 using gelatin plates. The process is rather com- 

 plicated, as salt is taken up by one side of the gelatin plate and water 

 by the other, and the taking up of water is influenced by the quantity 

 of salt taken up. It has also been found that when gelatin plates are 

 treated with solutions of increasing concentration of the same salt, 

 the taking up of salt increases at first with the salt concentration, then 

 becomes slower, and attempts to reach a maximum and then remains 

 almost stationary. As long as the taking up of salt increases, the quan- 

 tity of water passing into the gelatin also increases; when the salt fails 

 to pass then the water also ceases to pass. It has also been found that 

 the maximum of salt absorption for sulphate, tartrate and citrate can 

 be attained with much lower molecular concentrations than with chloride, 

 nitrate and bromide. From this it follows that the sulphate, tartrate 

 and citrate have a retarding action upon imbibition within certain limits 

 of concentration, while the chloride, nitrate and bromide have an 

 accelerating action. 



PAULI 2 has investigated the influence of salt solutions upon the solid- 

 ification and melting-point of gelatin. If the salts are arranged in the 

 order of their ability to lower the solidification point of gelatin we 

 come to the series sulphate, citrate, tartrate, acetate (water), chloride, 

 chlorate, nitrate, bromide, iodide. This series corresponds well with 

 that of HOFMEISTER. 



Acids and alkalies exert a special influence upon gelatin, as they 

 both, in very dilute solutions, strongly accelerate imbibition (SriRO, 3 

 Wo. OsTWALD 4 ). From the previously mentioned investigations of 

 LILLIE, on the osmotic tension of gelatin solutions, it was found that the 

 addition of acids and alkalies increased it (page 17). 



Since GRAHAM'S fundamental experiments it was believed that col- 

 loidal sols could not diffuse into gels while crystalloids^ could pass just 



1 Arch. f. exp. Pathol. u. Pharm., 28, 210 (1891). 



2 Pfliiger's Arch., 71, 333 (1898). 



3 Hofmeister's Beitrage, 5, 276 (1904). 



4 Pfliiger's Arch., 108, 563 (1905). 



