The ethereal solution, freed by the above process from paraffin, leaves on evaporation a crystalline mass which is cholesterin, retaining still a small quantity of fat matters. In treating the crystals with alcoholic potash these fat bodies are saponified and the residue is taken up with ether. The cholesterin is obtained in fine needles melting at from 170° to 176°. It presents all the reactions of cholesterin, especially the characteristic reaction with chloroform and sulfuric acid.
368. Absorption of Oxygen.—Among oils a distinction is made between those which oxidize readily and those which are of a more stable composition. Linseed oil, for instance, in presence of certain metallic oxids, absorbs oxygen readily and is a type of the drying oils, while olive oil represents the opposite type.
The method of determining the quantity of oxygen absorbed is due to Livache and is carried out as follows:[335]
Precipitated metallic lead (by zinc) is mixed in a flat dish, with the oil to be tested, in the proportions of one gram of lead to three-quarters of a gram of oil, and exposed to the air and light of the workroom. The dish is weighed from time to time until there is no longer any increase in weight.
Instead of lead, finely divided copper has been used by Krug in this laboratory, but the percentage of absorption of oxygen is not so high with copper as with lead. Krug found the quantities of oxygen absorbed, after nine days, by the samples treated with copper and lead respectively to be the following:
| Copper, per cent oxygen absorbed. | Lead, per cent oxygen absorbed. | |
| Olive oil | 1.69 | 2.03 |
| Cottonseed oil | 4.25 | 5.30 |
| Peanut oil | 2.74 | 3.87 |
| Linseed oil | 5.55 | 7.32 |
Livache found that linseed oil absorbed about twice as much oxygen as indicated by the data just given.
369. Elaidin Reactions.—In discriminating between oils and fats having a preponderance of olein and others with a smaller proportion of that glycerid, the conversion of the olein into its isomer elaidin is of diagnostic value. The following will be found a convenient method of applying this test:[336]
About ten cubic centimeters of the oil are placed in a test tube together with half that quantity of nitric acid and one gram of mercury. The mixture is shaken until the mercury dissolves when the mass is allowed to remain at rest for twenty minutes. At the end of this time it is again shaken and placed aside. In from one to three hours the reaction is complete. Olive, peanut and lard oils give very hard elaidins. The depth to which a plunger of given weight and dimensions sinks into an elaidin mixture at a given temperature, has been used as a measure of the percentage of olein contained in the sample of oil, but it is evident that such a determination is only roughly approximate. Copper may be used instead of mercury for the generation of the oxids of nitrogen, but it is not so effective. The vapors of nitric oxids may also be conducted directly into the oil from a convenient generator. The reaction may also be accomplished by shaking the oil with nitric acid and adding, a drop at a time, a solution of potassium nitrite.