50 INTRODUCTION 



a charge as to increase the solution tension of the colloid, the 

 solubility of the colloid is increased, but if it is of opposite 

 charge to that of the colloid, the surface tension is no longer 

 counterbalanced by the solution tension, and the bulk of the 

 molecule is reduced, while its weight remains the same ; hence it 

 falls out of the solution. A similar effect may be produced by 

 the union of several molecules by a polyvalent ion their total 

 surfaces will then be much reduced as compared with what it 

 was when they were separate, and so the surface energy is no 

 longer sufficient to keep them in solution. In general, precipi- 

 tation of colloids results from the reduction of the surface in 

 proportion to the mass, because of an aggregation of the particles ; 

 this may be brought about by changing the surface electrical 

 conditions, by uniting the molecules chemically, or by reducing 

 the amount of the solvent. 



The Structure of Colloids and of Protoplasm. Two 

 very different sorts of substances are usually included under 

 the term colloid, because they show the essential features of 

 colloids in most respects ; but as in many other respects they 

 are quite unlike each other, it may be well to distinguish between 

 them in some way. As a type of one class we may take gela- 

 tin ; of the other, such a substance as colloidal arsenic sulphide. 

 Gelatin solutions form gels upon cooling or evaporation, and 

 redissolve when heated or when more solvent is added. Arse- 

 nic sulphide does not form gels upon cooling, and when solidi- 

 fied in any way, does not redissolve. In addition, the gelatin 

 type is very viscous, and is not coagulated by the presence of 

 salts unless these are added in large amounts ; while the other 

 type does not render the fluid in which it is dissolved appreci- 

 ably more viscid, and it forms a precipitate immediately if 

 minute amounts of electrolytes are introduced. As the former 

 type resembles in many details the true solutions, while the 

 latter approaches more closely to the suspensions, it has been 

 proposed to distinguish them by the terms " colloidal solution " 

 and " colloidal suspension. " l Of the two types, the colloidal 

 solutions are by far the more important in biological considera- 

 tions, since the colloidal suspensions are usually prepared arti- 

 ficially and seldom occur in nature, e. g., Bredig's colloidal 

 suspensions of the noble metals. 



The colloidal solutions of proteids, which constitute the 

 chief part of every cell, are of two types one, such as albu- 

 min, forms a coagulum when heated, which, under ordinary con- 

 ditions is not reversible ; that is, it does not again go into 

 1 Noyes, American Chemical Journal, 1905 (27), 85. 



