Page:Popular Science Monthly Volume 46.djvu/119

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facturer 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 fortunate enough to have a sample of pure neutral soap, we find that, on dissolving some of it in water, it undergoes a partial decomposition into alkali and fatty acid, this action being called the hydrolysis of soap. The small quantity of alkali so set free attacks the fatty matter which glues the dirt to the skin, and by dissolving 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 perform this part of the operation. As the soap decomposes and the alkali removes the grease and dirt, the fatty acid liberated simultaneously 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 annoys 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