ANIMAL LIFE AND ANIMAL PRODUCTS. FOODS 441 



ing beam of strong light is sent through the liquid horizontally 

 (Fig. 107), and the illuminated place is viewed 

 from above through a microscope. When the 

 room is dark, and the only light comes from 

 the horizontal beam, a colloidal solution 

 shows minute points of light. A true solu- 

 tion such as one of common salt or of 

 alcohol remains perfectly dark. The points 

 of light are produced by myriads of suspended 

 particles, which although extremely minute, are of far larger than 

 molecular dimensions. Solutions of soap, gelatine, and many 

 dyes, blood serum, and innumerable other liquids contain such 

 suspended particles. 



The particles of a colloid (like starch), when viewed in this way, 

 show a continual unordered, zig-zag movement (Brownian 

 movement), which is more rapid the smaller the particles, and is 

 due to the impacts of the molecules of the solvent. 



Other Properties of Colloidal Suspensions. The freezing- 

 point, boiling-point and vapor pressure of a liquid containing a 

 colloid in suspension are all practically identical with those of the 

 pure solvent. This is quite different from what we have seen 

 to be the case with true solutions (p. 117), and indicates that the 

 fraction of the " solute " particles present is substantially zero. 



The unit particles of suspended material, indeed, are complex 

 aggregates so much larger than the ultimate molecules into which 

 true solutes are broken up that the number of them present, com- 

 pared with the number of solvent molecules, is entirely negligible. 



When a " solution " of a colloid is placed in a " diffusion- 

 shell " (test-tube shaped) of parchment, surrounded by pure 

 water, none of the colloid escapes through the minute pores of the 

 shell. Ordinary, non-colloidal solutes do escape, more or less 

 rapidly (salt rapidly, sugar slowly, see p. 407). In this way a 

 mixture of colloid and non-colloid (say, starch and salt) can be 



