22 PROXIMATE CONSTITUENTS OF PLANTS 



then cooled and weighed, and from the increase in 

 weight the percentage of fat in the sample determined. 

 In the case of linseed oil, however, the final heating in 

 the steam oven must be carried out in an atmosphere 

 of hydrogen or coal-gas, or the following alternative 

 method, equally applicable to any substance taken, may 

 be employed. 



In the alternative method the actual fat or oil 

 extracted is not directly weighed, but is determined 

 indirectly by drying the cartridge and its contents in a 

 steam oven, and estimating the loss in weight of the 

 substance due to the action of the ether. 



The above standard method is used for the determination of 

 any of the natural fats and oils, and also of the resins present in 

 such plant products as hops (see p. 53). 



13. Determination of Iodine Value of Linseed Oil. 



Various of the natural oils, notably linseed, poppy seed, 

 sunflower seed, soya bean, cotton seed, and colza (rape seed) 

 oils, containing the glycerides of various unsaturated organic 

 acids, show the ordinary properties possessed by unsaturated 

 bodies containing double or triple bonds. For example, they 

 readily combine with the oxygen of the air, giving in the case of 

 the drying oils, hard resinous products ; they also combine with 

 chlorine, bromine, or iodine to give halogen addition products, 

 the number of halogen atoms taken up depending upon the 

 degree of unsaturation. 



By determining the amount of halogen taken up by a certain 

 quantity of a given oil, the identity of the oil, and also the 

 presence or absence of impurities, can be established, from a 

 knowledge of the amount of halogen absorbed by the oil in the 

 pure condition. 



Hiibl's method for the determination of the " iodine value " of 

 natural fats, oils, and waxes is based upon the above consideration, 

 and the estimation of this number is of the utmost importance in 

 the technology of oils. 



The " iodine value " is the number of grams of iodine absorbed 

 by 100 grams of the oil in question, and it is found that the value 



