76 COLLOIDS IN BIOLOGY AND MEDICINE 



hydrosols are greenish brown, gold hydrosols are red but become blue 

 and finally brown when electrolytes are added. In the ultramicro- 

 scope the individual particles are not of a uniform color. For in- 

 stance, collargol has blue, red, violet and green particles, the particles 

 of a red gold solution are chiefly green, those of blue solutions range 

 from yellow to reddish brown. 



Theoretically, submicrons of the same size should have the same 

 color so that the variety of color in the ultramicroscope indicates 

 variation in the size of the particles. As a matter of fact, as we have 

 said, the smaller submicrons of finely divided red gold hydrosol are 

 almost all green though there are very small brown submicrons. 

 There is no entirely acceptable explanation for the color variation of 

 submicrons of identical size. 



The number of particles visible in the ultramicroscope is, in the 

 case of hydrophile colloids, usually far less numerous than might 

 have been expected from their other properties. This is the result 

 of their inferior reflecting power. If a piece of swollen gelatin is 

 immersed in water it becomes invisible, because no light is reflected 

 to the eye. On this account the ultramicroscope is not suitable for 

 determining the number or size of^ the particles of hydrophile 

 colloids. 



We may here recall an observation of G. QuiNCKE* 3 which is 

 perhaps destined to be of great importance for many biological 

 questions but which deserves attention, even from a purely physical 

 standpoint. G. QUINCKE observed that in the induced clarification 

 of mastic, gamboge, kaolin and india ink suspensions, the flocks 

 usually separated on the dark side; in spontaneous clarification of 

 kaolin turbidity, however, they settled on the light side. A turbid 

 solution of tannate of glue settled out, mostly on the side towards 

 the light. He described this phenomenon as positive and negative 

 photodromy. This fact is suggestive of many of the phenomena 

 which H. SiEDENTOPF* 1 observed in his light reactions in the ultra- 

 microscope. 



Jellies, especially those of higher concentration, on deformation 

 by compression or traction, show double refraction (KUNDT). Nega- 

 tive refraction was observed in the case of gum arabic, collodion and 

 gelatin, positive in the case of tragacanth and cherry gum. The 

 same kinds of jellies when dried showed respectively the same kinds 

 of refraction. If gelatin poor in water is brought in contact with 

 gelatin rich in water, so that there is a mutual interchange of water, 

 both become doubly refractive. (M. W. BEIJERINCK.) On re- 

 peated swelling and shrinking of jellies, the positive double refraction 

 passes through an isotropic condition into a negative. (QUINCKE.) 



