Double Refraction in Liquids. 497 



merely to the heating, as steady cooking lessens the double 

 refraction. A solution having - 5 grams in a litre gave, when 

 fresh, a retardation of '000308 X at 27°. After being heated 

 in a water-bath for ten hours it gave only "000246 \ under 

 similar circumstances. 



The colour which a gelatine solution assumes after standing 

 for several hours also points to the existence of masses at 

 least comparable in size with the wave-length of light. A 

 dilute solution, when first made, appears to the eye as clear 

 as water. After a few hours it becomes opalescent ; slightly 

 green when seen by diffused light, and red by transmitted 

 light. 



A solution of gum arabic, which was transparent when first 

 made, became so dispersive that, while light passed through 

 it readily, no image could be seen if the object was more 

 than 5 centimetres away, even when one looked directly 

 toward the sun. 



The fact that a dilute solution, if not distorted too much, 

 will sustain a strain as if it were an elastic solid, has been 

 alluded to already; also a rebound and oscillation when the 

 strain is released. 



A 20-per-cent. solution is tough, like rubber ; yet the 

 vapour-pressure over it is little less than over pure water. 



These facts lead to the conclusion that, even when the con- 

 centration is very small, the gelatine does not remain in 

 solution in the true sense, but forms solid elastic masses. The 

 term " solid solution " would scarcely be applicable, since it 

 has been used to cover amalgams and some other mixtures. 

 That the concentration may be very small is shown by the 

 fact that when there was only 0*1 gram in a litre, a retardation 

 of '000168 X was found after the solution had stood in the chest 

 for twenty-four hours. 



The relation of the speed to the amount of double refraction 

 iudicates that these solid masses will endure a strain up to a 

 certain point ; but beyond that point breaking down takes 

 place. This point is given by the maxima in the curves. 

 When rupture occurs, the double refraction diminishes as the 

 velocity of rotation increases. This idea of the increased 

 breaking down of the gelatinous structure seems to be borne 

 out by the fact that the depolarization increases with the speed. 

 The structure must be coarse-grained enough to diffuse light 

 if our suppositions with regard to the polarizing effect be true. 

 It is also significant that the depolarization disappears abruptly 

 at 33°, the melting-point of jelly. 



While the difference in the diffusibility of crystalloids and 

 colloids might be explained by assuming a very high molecular 



