342 PHYSICAL PROPERTIES 



is afforded by potassium caseinate in 75 per cent alcohol (Cf. 

 Chap. XI). 



Certain authors, following Graham, regard all colloidal solu- 

 tions, including protein solutions, as being "supersaturated." 

 The above considerations dispose of this view also. Linebarger 

 (64) has also shown, by direct experimentation on the rate of coagu- 

 lation, that solutions of proteins cannot be correctly regarded as 

 supersaturated. Very numerous observations, detailed in previous 

 chapters, will occur to the reader which also conflict with this view, 

 for example, the constancy of the proportion between base and 

 casein at "saturation" of the former with the latter. 



8. The Opalescence of Protein Solutions ; the Tyndall Effect. 

 — The majority of protein solutions are decidedly opalescent, 

 i.e., they contain particles of sufficient size to scatter and reflect a 

 proportion of transmitted light. Even when the opalescence is 

 very small, when the solution is viewed at right angles to the 

 direction of a pencil of light traversing it, a certain degree of 

 scattering (the Tyndall effect) is observed. Upon this fact many 

 investigators have based the view that proteins in solution form 

 heterogeneous and not molecularly dispersed systems. 



It has, however, been pointed out by Konovalov (53) that the 

 dust particles, which render it so difficult to obtain even the purest 

 water in an "optically void" condition, may act as nuclei, under 

 conditions in which the expenditure of energy necessary for an 

 alteration in concentration is very small (i.e., when the osmotic 

 pressure of the dissolved substance is very small), attracting 

 layers of the dissolved substance and so producing opalescence. 

 In this way it is possible that the observed opalescence in such 

 solutions is due only to a minute proportion of the dissolved sub- 

 stance condensed by capillary action around accidental insoluble 

 contaminations. Arrhenius (3) has advanced a similar view. "It 

 may, however, be urged that the presence of some submicroscopic 

 particles does not prove at all that the whole of the quantity of 

 protein present, or even a considerable part of it, is in this state 

 of pseudo-solution." 



It is, however, not at all inconceivable that the opalescence of 

 many colloidal solutions may be due to the grossness of the colloid 

 molecules themselves,* but it is unlikely that this is the case in 

 solutions of proteins. It has been shown by Lobry de Bruyn (16) 



* C. O. Weber (128), p. 75. 



