162 Dr. W. KamsdeiL Separation of Solids in the [June % 



accumulation of alcohol to the exclusion of the suspended or dissolve] 

 solid. 



18. Various hitherto obscure phenomena find their explanation in 

 the facts observed, e.g. : 



i. The ready formation of a " skin " on hot milk exposed to 

 evaporation is explained by (a) The presence of a delicate skin 

 or pellicle on the free surface even of cold milk or of caseinogen 

 solutions not exposed to evaporation ; (b) The presence of a 

 similar pellicle at the inter-faces between caseinogen solutions 

 and pure neutral olive oil or butter fat. 



The existence of a proteid " haptogen-membrane " around the cream- 

 globules of milk cannot any longer be doubted, and their role 

 in contributing to the ready formation of a thick skin on 

 hot milk, as first demonstrated by Hertz and Jamison,* finds 

 a complete explanation. The apparently contradictory obser- 

 vations of Rettger,f demonstrating the possibility of obtaining 

 very delicate skins by heating caseinogen and other solutions, 

 although free from fat globules, appear to be due to the de- 

 hydration and thickening by evaporation of the surface pellicles 

 present on such solutions even in the cold. 



ii. The homogeneous " Grenz-membran " described by Biitschli (and 

 the optical homogeneity of thin films referred to by Hardy) in 

 various coagulated or dehydrated colloids, is the above-described 

 membrane of solid colloid formed at an air or other appropriate 

 surface. 



iii. The high pressure required to force solutions of saponin and 

 albumin through capillary tubes when bubbles of air are present 

 is largely due, among other factors, to the presence of solid mem- 

 branes around the air-bubbles, and the increased resistance to 

 deformation thus brought about. As Plateau showed, the resist- 

 ance offered is enormously greater than that of water contain- 

 ing similar air bubbles. (Cf. air-embolism in the blood-capillaries 

 of a mammal.) 



iv. The failure of proteids and other colloids in solution to pass 

 through fine filters without considerable loss is largely due 

 to the formation of surface membranes and mechanical coagula 

 upon air, grease, or other suitable surfaces in the pores of the 

 filter. 



19. The following table indicates some of the substances whose 

 aqueous solutions or suspensions have shown evidence of the accumula- 

 tion of solid or highly viscous matter on their free surfaces, either by 



* Vide ' Journ. of Physiol.,' London, 1901, vol. 27, p. 26. 

 t Tide ' Amer. Journ. of Phjsiol.,' Mny, 1902. 



