THE CHEMISTRY OF CLEANING. 107 



color ; and now on mixing the solution rendered red by tlie acid, 

 and the second one turned green by the alkaline base, we once 

 more obtain the original violet color, and on examining the solu- 

 tion can find no trace of either acid or alkali, but can distinguish 

 the presence of a compound called sodic sulphate, which can be 

 obtained in the crystalline form by concentrating the solution, 

 and such a compound formed by the union of an acid and a base 

 we are in the habit of calling a salt. During the combination of 

 the sulphuric acid and sodic hydrate to form sodic sulphate, we 

 also had water being formed, which, like the neutral salt, had no 

 action upon our colored solution. If we had carefully weighed 

 our sulphuric acid and the sodic hydrate, we should have found 

 that it is only in certain definite proportions that they unite to 

 give a solution without effect on the vegetable coloring matter. 



One of Chevreul's greatest discoveries was that in tallow 

 the fat of oxen or sheep you had a salt of organic origin, from 

 which, by decomposing the tallow with heated steam, you could 

 obtain the sweet viscous liquid "glycerin,'' which played the 

 part of base in the compound, and two acidulous compounds 

 one a lustrous white wax, called stearic acid, and the other an oil 

 called oleic acid. 



Now a salt can have its base replaced by another base. If I 

 take two solutions, the one containing sulphate of copper and the 

 other chloride of iron, and add to each sodic hydrate, decomposi- 

 tion takes place in each case, sodic sulphate is left in solution, and 

 the hydrates of copper and iron being insoluble in water, separate 

 out as precipitates. In the same way, if we add sodic hydrate to 

 tallow, glycerin separates out, and two salts sodic oleate and 

 sodic stearate are formed, a process which we call saponification, 

 as the two sodium salts are " soaps." 



It is not necessary to use tallow ; any vegetable or animal fat 

 or oil will give reactions of a similar character, and it may be 

 broadly stated that soap is formed by the action of sodic or potas- 

 sic hydrate upon fats or oils which contain fatty acids. 



It is only potassic and sodic hydrates which can be used for 

 ordinary soap-making, as the soaps formed by the combination of 

 other metallic hydrates with the fatty acids are insoluble in water, 

 and therefore useless for detergent purposes. The soap formed 

 by using sodic hydrate has the property of setting hard, and all 

 the ordinary forms of washing-soap contain sodium as the base. 

 The potash soaps are far softer, and do not set ; the soft soap used 

 for scrubbing and cleansing in many manufacturing processes, 

 and also a few toilet creams and shaving pastes, being of this 

 character. 



It would occupy far too much time, and would, moreover, be 

 outside the scope of this lecture, to go into the details of the 



