120 LECTURE VII. 



through animal or vegetable membranes. They belong to that class of 

 bodies designated by .Graham * as " colloids." If the colloid be liquid, 

 we call it a sol; while if it be solid, we designate it as a gel. Liquid and 

 solid gelatin represent these two phases. If the colloid is distributed 

 throughout water, in appearance practically dissolved, we call it in the 

 hydrosol condition. We are acquainted with many such colloidal sub- 

 stances among the inorganic compounds. Silicic acid is a good example 

 of this. If a solution of sodium silicate is treated with an excess of 

 hydrochloric acid, the silicic acid set free, remains apparently in solution. 

 If this is then transferred to a dialyzer, the excess of hydrochloric acid and 

 the sodium chloride produced in the reaction diffuses into the liquid in 

 this case distilled water which is placed on the other side of the mem- 

 brane. The silicic acid, on the other hand, remains behind, in the form 

 of a tough, viscous mass, which can be coagulated by introducing a few 

 bubbles of carbon dioxide gas. " Albumin solutions " act in an analogous 

 manner. If we transfer blood serum, which occurs as a pale yellow, 

 clear liquid, to a dialyzer, a floculent precipitate quickly separates out. 

 This is the globin of the serum, which separates, the salt which had held 

 it in " solution," having been withdrawn. 



A question widely discussed, is this : Is the colloid occurring in the sol 

 form to be considered as an actual solution, or as a suspension? It is 

 variously answered. As the albumin solutions conduct the electric cur- 

 rent, the products in '* solution " appear as both anions and cations, it 

 has been assumed that an actual solution exists. 2 Nevertheless, there is 

 no sharp dividing line between a real and an apparent solution. We are 

 acquainted with all possible intermediate stages. 3 



As the colloids lose many of their characteristic properties by various 

 agencies, so, also, the albumins are easily deprived of their colloidal nature. 

 The process is called a " coagulation." It is irreversible. As we gener- 

 ally deal with the coagulated products in our investigations of the 

 albumins, we shall devote a little space to discussing the ordinary 

 methods employed in effecting coagulation. One of the most important 

 characteristics of albumin solutions is that of coagulating on heating. 

 One of the most important factors in the phenomenon of coagulation 

 is the amount of salt held in solution. We can heat an albumin 

 solution, which has been very carefully freed from salt by dialysis, and it 

 will not coagulate. If salt is then carefully added, albumin separates 



1 T. Graham: Philosophical Trans. 151, Part 1, 183, (1861). 



a J. Sjoqvist: Skand. Arch. Physiol. 5, 277 (1895). S. Bugarsky and L. Liebermann: 

 Pfliiger's Arch. 72, 51 (1898). 



3 We have suspensions, colloidal solutions, and true solutions. It is easy to dis- 

 tinguish between the end members of the series, but no sharp distinction is drawn 

 between these three classes. TRANSLATORS. 



