14 GENERAL BIOLOGY 



Physical Structure of Protoplasm. Proteins 

 are not soluble in water in the usual sense, that 

 is, they do not make a clear solution as do 

 sugar and salt. They do absorb a great quantity 

 of water, however, and swell up enormously. In 

 the presence of large amounts of water they may 

 become very finely divided and form permanent 

 aqueous suspensions, which differ from true solu- 

 tions in that they will not diffuse through vegetable 

 parchment or animal membranes. Such substances 

 are usually known as " colloids," in contrast to 

 ** crystalloids " or substances which do" diffuse 

 through such membranes. Another characteristic 

 of colloids is their property of coagulating or " set- 

 ting," a familiar example of which may be observed 

 in the hardening of the white of an egg in the pro- 

 cess of boiling. Such coagulation may be produced 

 by heat, electric currents, dehydration, and chemical 

 reagents. Certain classes of colloids, like the egg 

 albumen, are unalterable when once coagulated, and 

 are known as irreversible. Others may be brought 

 back to the fluid state any number of times. Such 

 substances, of which gelatin is an example, are called 

 reversible. The essential difference between " solu- 

 tions " of colloids and of crystalloids consists in 

 the fact that the particles of the former are larger 

 and are enveloped in a film of water, whereas true 

 solution involves the separation of the crystalloid 

 into its molecules or even into its ions, in which con- 

 dition the particles obey the law of gases and the 

 solution exerts pressure (osmotic pressure) in all 



