170 SOAPS AND PROTEINS 



between the specific gravity of mixed ^lycerids and the type of 

 fatty acids found in the glycerids. In general, however, the 

 lower the specific gravity of a fat the more likely it is to be found 

 rich in the higher members of any fatty acid series. 



The Baponification value of an oil. fat or wax which indicates 

 the number of milligrams of potassium hydroxid required for the 

 complete saponification of one gram of the oil, fat or wax is indica- 

 tive of the amount of potassium hydroxid required to neutralize 

 completely the fatty acids in that oil, fat or wax. Other things 

 being equal, a high saponification value therefore means a high 

 content of the lower fatty acids. 



The REICHERT-MEISSL value which shows the number of 

 cubic centimeters of decinormal potash required to neu- 

 tralize that portion of the volatile fatty acids which is obtained 

 from 2.5 grams of a fat after distillation by the REICHERT process, 

 represents, obviously, the proportion of volatile fatty acid con- 

 tained in any mixed fat most likely to yield soaps lying within 

 the range of those commonly used for washing purposes. 



The iodin value of a fat or fatty acid is a measure of the pro- 

 portion of unsaturated fatty acids contained therein. It may be 

 used as an index, in comparative studies, to the probable pro- 

 portion of the unsaturated fatty acids present in any oil, fat or 

 wax to those of the saturated fatty acids and, by inference, of 

 the proportion of the soaps of these two series of fatty acids with 

 their varying physico-chemical or colloid-chemical constants pro- 

 ducible from the original material. 



4. Hot and Cold Process Soap Manufacture 



As familiarly known, soap manufacture may be carried on 

 by either (a) the cold process or (6) the hot process. So far 

 as the chemistry is concerned the two are supposed to yield the 

 same result. In either case, a weighed amount of fat or oil has 

 added to it the amount of caustic soda which analysis has shown 

 the fat to require for conversion into neutral soap. What is 

 obtained in the end is a neutral soap holding a certain amount 

 of water plus the glycerin split off in the process of the conversion 

 of the fat to soap. 



It is a matter of practical experience that while the process 

 of soap manufacture in the cold is the simplest, the results thus 



