1915] on Science and Industrial Problems 315 



Such, then, is the wording of the problem submitted to practica 

 chemists for decision as to whether it was a workable process or not. 



It must be remembered that in investigating the experimental 

 conditions under which any chemical reaction takes place, failure, 

 either partial or complete, may often be encountered, and it is there- 

 fore essential to possess a ready means of detecting, not only whether 

 complete success has been obtained, but also to what extent, if any, 

 the desired reaction has actually taken place. The case under dis- 

 cussion provides an apt example of w^hat is meant by these words. As 

 already mentioned, the manufacturer, as a practical person, is not con- 

 cerned with providing an illustration of the meaning of the chemical 

 word saturation, but requires a method for converting an oil into a 

 workable solid fat. 



It might, therefore, be argued that the hardness of the resulting 

 product would be a sufficiently good criterion of success. It is, 

 however, quite possible to cause an oil to absorb a certain amount of 

 hydrogen without complete solidification, and hence if the reaction i& 

 to be followed, scientifically speaking, by the experimental chemist, 

 some much more delicate or exact method must be employed for 

 determining the course of the reaction, and the one actually selected 

 is known as the iodine number or iodine value. 



It has already been mentioned that oleic and other similar acids, 

 capable of absorbing hydrogen, are called unsaturated, but hydrogen 

 is not the only substance which can be absorbed by these acids, and 

 among others may be mentioned iodine. Just as oleic acid absorbs 

 two atoms of hydrogen to become stearic acid — 



^IS^SiOo + Ho = Cisll3,j02 

 so it will absorb two atoms of iodine to become di-iodostearic acid — 



If, therefore, after determining the iodine value or iodine absorbed 

 by an oil, hydrogen is allowed to act on the latter in presence of 

 reduced nickel and during the course of the reaction, samples of the 

 product are withdrawn and their iodine values determined, it is found 

 that the amount of iodine absorbed becomes less and less, because 

 some of the unsaturated oil has been converted into the saturated 

 hard fat. I i ence we get at any one moment and under any specified 

 set of conditions an exact and quantitative measurement of the 

 amount of hydrogen absorbed. 



The results of such a series of experiments are conveniently ex- 

 pressed in the form of a curve, for example as shown in Fig. 1, 

 illustrating the behaviour of whale oil under successful conditions, 

 i.e. employing a definite temperature, purified hydrogen, and a suit- 

 able proportion of nickel prepared in an approved manner. The time 



