522 PRINCIPLES OF CHEMISTRY 



The chemical reactions of sodium hydroxide serve as a type for those- 

 of a whole class of alkalis that is, of soluble basic hydroxides which 

 may be obtained from water and a metallic oxide, and whose compo- 

 sition is expressed as a compound of the metal, M, and hydroxyl, OH ; 

 MOH. The solution of sodium hydroxide is a very caustic liquid 

 that is to say, it acts in a destructive way on most substances, for 

 instance, on most organic tissues hence caustic soda, like all soluble 

 alkalis, is a poisonous substance ; acids, for instance, hydrochloric, 

 forming common salt, serve as antidotes. The action of caustic soda 

 on bones, fat, starch, and similar vegetable and animal substances 

 explains its action on organisms. Thus bones, when plunged into a 

 weak solution of caustic soda, fall to powder, 34 and evolve a smell of 

 ammonia, which is due to the caustic soda changing the gelatinous- 

 organic substance (it contains carbon, hydrogen, nitrogen, oxygen, and 

 sulphur, like albumin) of the bones, dissolving it and in part destroy- 

 ing it, whence ammonia is disengaged. Fats, tallow, and oils become 



ethereal salts in the same way that it acts on the majority of metallic salts namely, it 

 liberates alcohol, and forms the sodium salt of that acid which was in the ethereal salt. 

 The reaction takes place in the following way : 



RQO + NaHO = NaQO + RHO 

 Ethereal Caustic Sodium Alcohol, 



salt. soda. salt. 



Such a decomposition is termed saponification, because similar reactions were known 

 very long ago for the ethereal salts corresponding with glycerin, C 3 H 5 (OH)3 (Chapter 

 IX.), found in animals and plants, and composing what are called fats or oils. Caustic 

 soda, acting on fat and oil, forms glycerin, and sodium salts of those acids which were 

 in union with the glycerin in the fat, as Chevreul showed at the beginning of this century. 

 The sodium salts of the fatty acids are known in practice as soaps. That is to say, that 

 soap is made from fat and caustic soda, glycerin being separated and a sodium salt. 

 formed. As glycerin is usually found in union with certain acids, so also are the sodium 

 salts of certain acids found in soap. The greater part of the acids found in conjunction 

 with glycerin in fats are the solid, palmitic and stearic acids, C 16 H 32 O 2 and C 18 H 56 Oo, and 

 the liquid, oleic acid, C 18 H 34 O 2 ; hence soap principally contains a mixture of the sodium 

 salts of these acids. In preparing soap the fatty substances are mixed with a solution of 

 caustic soda until an emulsion is formed ; the proper quantity of caustic soda is then 

 added in order to produce saponification when heated, the soap being separated from 

 the solution either by means of an excess of caustic soda or else by common salt, which 

 displaces the soap from the aqueous solution (salt water does not dissolve soap, neither 

 does it form a lather). The water, acting on the soap, partly decomposes it (because the 

 acids of the soap are feeble), and the alkali set free acts during the application of soap. 

 Hence it may be replaced by a very feeble alkali. Strong solutions of alkali corrode the 

 skin and tissues. They are not formed from soap, because the reaction is reversible,, 

 and the alkali is only set free by the excess of water. Thus we see how the teaching of 

 Berthollet renders it possible to understand many phenomena which occur during every- 

 day experience. 



34 On this is founded the process of Henkoff and Engelhardt for treating bones. The- 

 bones are mixed with ashes, lime, and water : it is true in this case that more 

 potassium hydroxide than sodium hydroxide is formed, but their action is almost 

 identical. 



