THE CHEMISTRY AND PHYSICS OF THE CELL 43 



them it becomes visible because of the reflection of the light 

 waves on the surfaces of solid particles that are suspended in 

 the fluid, as a ray of sunlight becomes visible in a dusty room 

 (TyndalPs phenomenon l ). Hence this solution, which even 

 with the microscope appears as perfectly homogeneous as a salt 

 solution, is in reality full of solid particles. Finally, still other 

 solutions of arsenic sulphide may be obtained in which the 

 particles are so fine as to diffuse like an ordinary solution of a 

 crystalloid. 



In a similar manner various other colloids may be found to 

 show different characters, some agreeing with the properties of 

 the typical suspensions, and some with the properties of the 

 true solutions. They stand in an intermediary position, differ- 

 ing quantitatively in one way or another from the true solu- 

 tions, but yet approaching them closely and sometimes almost 

 indistinguishably resembling them. For the most part, how- 

 ever, the colloids show characteristics decided enough to entitle 

 them to separate classification, and to make any confusion with 

 the crystalloids impossible. 



The Characteristics of Colloids. The chief properties 

 of the colloids are, then, as follows : 



Amorphous Form. This, like almost all other "colloidal 

 properties/ 7 is not absolute, for in egg-albumin, hemoglobin, 

 and various globulins we have proteids which in every respect 

 are typical colloids, yet they form crystals readily and abun- 

 dantly. Oxyhemoglobin, the molecular weight of which is 

 calculated at about 14,000, exhibits TyndalPs phenomenon, 

 and will not pass through a very fine porcelain filter, and there- 

 fore resembles the colloids decidedly, yet it forms beautiful 

 crystals. The very fact that crystals are formed, Spiro points 

 out, is proof that when in solution the individual molecules must 

 have been free and separate, for otherwise they could scarcely 

 unite in the definite spatial relations necessary to produce 

 crystalline forms. 2 



1 The so-called ultra-microscopic method of studying minute particles, devised 

 by Siedentopf and Zsigmondy, depends upon the same phenomenon. In this 

 method the particles are illuminated in the microscopic field by an intense ray 

 of light, and the reflection of light causes the particles to appear as minute 

 luminous points. Particles as small as 0.005 // can be detected in this way, and 

 ordinary colloidal solutions of albumin appear filled with moving particles. 



2 This indicates that in colloidal solutions the molecules may be free, and 

 not necessarily aggregates. This is perhaps only true for the substances of 

 very great molecular dimensions, such as the proteids ; the colloidal solutions 

 of substances with smaller molecules having the molecules united in large 

 groups. On this basis the essential difference between colloidal and true solu- 

 tions is merely one of the size of the free particles. 



