TRANSPARENT SOAP. 231 



the first instance, and properly scented, forms the best kind of 

 transparent soap ; but the greater part of that extensively adver- 

 tised and sold is made by substituting sugar for glycerine, a change 

 greatly to the disadvantage of the user, as sugar has a very irritat- 

 ing effect upon some skins. 



Another kind of transparent soap of much worse quality is also 

 largely sold, often under the name of " Glycerine Soap," 

 made by the "cold process" (Expt. 275), using castor oil 

 and other oils and fatty matters, with a considerable excess of 

 caustic soda and a liberal addition of sugar, the effect of which is 

 to produce a jelly-like product, about half toffee and half a 

 corrosive soap, which has a very bad effect upon sensitive skins, 

 but is very attractive in appearance, especially when pleasantly 

 scented and prettily tinted by means of a little orange or red 

 dyestuff. Nimium ne crede colori ! 



6, Physical Adhesion and Allied Phenomena of 

 Surface Action. 



CHAPTER XVI. 



CONDENSATION OF GASES UPON THE SURFACE OF SOLIDS. 



In the previous pages a number of experiments have been 

 described, illustrating the physical absorption of gases by liquids 

 (no chemical action taking place), forming what are ordinarily 

 termed solutions, and somewhat analogous actions taking place 

 between certain kinds of solids and gases (Chapter VII.), often 

 referred to as occlusion. In this latter kind of action the gas 

 penetrates into the interior of the solid and becomes pretty uni- 

 formly distributed throughout the whole mass ; so that if the 

 outside portions were filed off or otherwise detached, and examined 

 separately from the inner part of the solid, no very great difference 

 would be noticeable between the two as regards the quantity of 

 gas taken up by a given weight of solid. 



True occlusion of this kind is exhibited by comparatively few 

 solids, and then only in relation to particular gases, at least to an 

 easily measurable extent ; but a somewhat analogous, though 

 different kind of action takes place between all solids and all gases 

 to a greater or less degree (under such conditions that chemical 

 action does not modify the effect), the chief difference being that 



