THE COLLOIDS 83 



elements. Absorption, secretion, growth, pharma- 

 cological and perhaps pathological processes as well 

 as immunity, are connected with such changes in 

 state, a discovery of the nature of which is the 

 raison d'etre of bio-chemistry. 



The colloidal constitution of living matter is also 

 connected with the problems of bio-chemistry, and 

 especially the electrical and chemical reactions which 

 occur in protoplasm. Colloids exist in two forms : 

 a liquid form called by Graham the sols, and a solid 

 or jelly-like form or gels. The colloids consist of 

 proteioidal and lipoidal materials. They possess a 

 high molecular weight, they do not pass through 

 animal membranes, and their osmotic pressure is so 

 low that they diffuse only with the greatest difficulty. 

 Neither do they possess electrical conductivity, 

 although they for the most part move through an 

 electric current and have an electrical reaction. 



A colloid consists of very fine particles of matter 

 in a state of suspension. These particles vary in 

 number, size and electrical charge ; and, like the 

 very minutest particles of atmospheric dust, a.ppear 

 to be unaffected by gravity. They are, however, 

 rendered visible by illumination ; and they remain 

 suspended owing to the development in them of an 

 electro-statical condition induced by friction of their 

 surfaces against each other in their molecular move- 

 ments. Colloids may be organic or inorganic. An 

 illustration may throw some light upon their con- 

 stitution. If a clean metal plate is put into water 

 it assumes a weak negative electric charge, while 

 the surrounding liquid becomes electro-positive. 

 Traces of the metal gradually drop off the plate 



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