THE CHEMISTRY OF CLE AXING. 109 



factiirer to make a so-called soap often containing less than twenty 

 per cent of true soap. 



Having got our soap, the next point is to try and gain an idea of 

 the way in which it acts as a detergent. Supposing we are fortu- 

 nate enough to have a sample of pure neutral soap, we find that, on 

 dissolving some of it in water, it undergoes a partial decomposi- 

 tion into alkali and fatty acid, this action being called the hy- 

 drolysis of soap. The small quantity of alkali so set free attacks 

 the fatty matter which glues the dirt to the skin, and by dissolv- 

 ing it loosens and enables the water to wash off the particles of 

 dirt. If this were the only action, however, soap would have no 

 advantage over soda, a solution of which would equally well per- 

 form this part of the operation. As the soap decomposes and the 

 alkali removes the grease and dirt, the fatty acid liberated simul- 

 taneously from the soap comes in contact with the newly cleansed 

 skin, and not only softens and smooths it, but also neutralizes any 

 trace of free alkali, and so prevents irritation and reddening of 

 the cuticle. 



These are probably the main actions by which soap cleanses, 

 but other causes also play a subsidiary part. We know that a 

 solution of soap causes a lather when agitated, this being due to 

 the cohesive power given to the particles of which the liquid is 

 built up by the presence of the soap a phenomenon which also 

 enables us to blow bubbles with the soap solution on account of 

 the strength of the fine film of liquid a property which is not 

 found in water alone. 



The power of cohesion which the soap solution possesses is in 

 all probability an important factor in removing the particles of 

 dirt from the skin at the moment that they are loosened by the 

 action of the alkali. Prof. W. Stanley Jevons suggested yet a 

 fourth way in which the soap solution might act : when finely 

 divided clay is suspended in water, the microscope reveals the 

 fact that the minute particles are in rapid movement, and hence 

 settle but slowly in the liquid. This movement he christened 

 pedetic action, and he observed that the addition of soap or 

 silicate of soda often used in soap to the liquid enormously 

 increased this agitation of the particles, which would tend to 

 aid the breaking away of the dirt particles the moment they were 

 set free. 



Many soaps, even among the varieties intended for the toilet, 

 contain a considerable excess of free alkali, which, being greater 

 than the liberated fatty acids can neutralize, causes most painful 

 irritation of the skin, as is testified by the smarting which an- 

 noys the chin after the use of certain shaving soaps ; and every 

 lady knows that an alkaline soap, when used for washing the 

 hair, renders it harsh and brittle, and destroys the gloss ; but a 



