THE CELL; THE COLLOIDAL CONDITION 15 



to have thought that the distinction was one of nature rather 

 than of condition. He also appears to have believed that colloids 

 were always organic. < 



The Colloidal State. — Since Graham's day much work has been 

 done on colloids and it is now clearly established that " colloid' ' 

 means a state of matter and not a peculiar kind of substance. Egg 

 albumin and hemoglobin may be obtained in both the colloid 

 and the crystalloid state. Gallic acid gives a colloidal solution 

 in water and a crystalloidal solution in glacial acetic acid. Col- 

 loids are substances in which the particles of the material are 

 in a finely subdivided state varying in diameter from about 1 /x 

 to 1 nux. (A micron or ju. is one-thousandth of a millimeter; a m/z, 

 or millimicron, is one-thousandth of a micron or a millionth of a 

 millimeter.) Hence a colloid is matter in a particular state of sub- 

 division and is not the name for an especial kind of matter. 



If one started with a piece of silver and divided it up into 

 smaller and smaller pieces, he would finally get pieces so small that, 

 when stirred up in water, some time would be required before 

 they settled out. But such pieces are not colloidal; they are merely 

 forming mechanical suspensions. If these pieces are divided still 

 finer there will come a time when they are too small to settle out 

 when stirred up and yet they are much larger than molecules of 

 silver. Such a mixture of silver and water is a colloidal solu- 

 tion. 



Phases and Colloidal Mixtures. — In such a colloidal mixture 

 a solid is suspended in a liquid, and there is said to be present a 

 solid and a liquid phase. The liquid, in this case, is continuous 

 and the solid is dispersed in the liquid. The liquid is then spoken 

 of as the continuous phase and the solid as the dispersed phase. 

 It is thus obvious that with the three states of matter — solid, 

 liquid, and gas — there are the following eight possibilities for col- 

 loidal mixtures : 



1. Continuous phase a gas. 



a. dispersed phase a solid, — smoke or very fine dust. 



b. " " liquid, — fog or mist. 



2. Continuous phase a liquid. 



a. dispersed phase a solid, — suspensoids. 



b. " " liquid, — emulsoids, milk. 



c. " " gas, — foam. 



3. Continuous phase a solid. 



a. dispersed phase a solid, — certain alloys, ruby glass. 



b. " " liquid, — opals and pearls. 



c. " " gas, — pumice stone. 



