16 ANIMAL BIOLOGY 



in the colloidal state — a condition of matter that chemists have 

 long been familiar with in the inorganic world. A colloid has 

 been described as matter divided into particles larger than one 

 molecule and suspended in a medium of different matter. There- 

 fore butter and cream are each colloids: the former consisting of 

 water finely divided and suspended in oil, and the latter essentially 

 of finely divided oil in water. But protoplasm is a stupendously 

 more complex colloidal system. It comprises not two, but very 

 many substances, some in simple and others in highly complex 

 molecular form, so finely divided that they are invisible with the 

 ordinary microscope. 



Now colloidal systems in general are characterized by tremen- 

 dous surface activity — the result of energy relations between the 

 contact surfaces of the particles of the different component sub- 

 stances. This being so, and protoplasm being a colloid composed 

 of very many different kinds of materials, the total surface area 

 between suspended substances and suspending media is very great, 

 and thus affords the requisite conditions for an exceedingly in- 

 tricate system of energy relations. And when we add to this the 

 fact that at such surfaces chemical changes, some involving 

 changes in electrical potential, occur; and also that mechanical 

 changes are induced by precipitation, coagulation, and constant 

 redistribution between the suspending media and the substances 

 in suspension, we begin to get at least a glimpse of the exceedingly 

 intricate and delicate energy-transforming system that protoplasm 

 really is. To work out these intricacies is one of the imposing tasks 

 still before the biologist, chemist, and physicist. 



But the statement that protoplasm is a colloidal system — 

 roughly speaking, a rather fluid sort of jelly — leaves the reader 

 without any clear conception of the appearance of protoplasm. 

 As a matter of fact it is as difficult to describe the appearance of 

 protoplasm as it is to define it. Protoplasm must be seen under 

 the microscope to be appreciated. With a moderate magnification, 

 it presents a fairly characteristic picture, appearing like a trans- 

 lucent, colorless, viscid fluid containing many minute granules as 

 well as clear spaces or vacuoles. If it is examined in water it ex- 

 hibits no tendency to mix with the surrounding medium, though 

 investigations show that osmotic interchanges are constantly 

 going on. For this reason it is impossible to consider proto- 

 plasm except in connection with its surroundings, whatever 



