Fats of the third order, the stearic group, are called “saturated” because they have taken up all the hydrogen they can hold.  Fats of the other two groups are called “unsaturated.”  The first, which have the least hydrogen, are the most eager for more.  If hydrogen is not handy they will take up other things, for instance oxygen.  Linseed oil, which consists largely, as the name implies, of linoleic acid, will absorb oxygen on exposure to the air and become hard.  That is why it is used in painting.  Such oils are called “drying” oils, although the hardening process is not really drying, since they contain no water, but is oxidation.  The “semi-drying oils,” those that will harden somewhat on exposure to the air, include the oils of cottonseed, corn, sesame, soy bean and castor bean.  Olive oil and peanut oil are “non-drying” and contain oleic compounds (olein).  The hard fats, such as stearin, palmitin and margarin, are mostly of animal origin, tallow and lard, though coconut and palm oil contain a large proportion of such saturated compounds.

Though the chemist talks of the fatty “acids,” nobody else would call them so because they are not sour.  But they do behave like the acids in forming salts with bases.  The alkali salts of the fatty acids are known to us as soaps.  In the natural fats they exist not as free acids but as salts of an organic base, glycerin, as I explained in a previous chapter.  The natural fats and oils consist of complex mixtures of the glycerin compounds of these acids (known as olein, stearin, etc.), as well as various others of a similar sort.  If you will set a bottle of salad oil in the ice-box you will see it separate into two parts.  The white, crystalline solid that separates out is largely stearin.  The part that remains liquid is largely olein.  You might separate them by filtering it cold and if then you tried to sell the two products you would find that the hard fat would bring a higher price than the oil, either for food or soap.  If you tried to keep them you would find that the hard fat kept neutral and “sweet” longer than the other.  You may remember that the perfumes (as well as their odorous opposites) were mostly unsaturated compounds.  So we find that it is the free and unsaturated fatty acids that cause butter and oil to become rank and rancid.

Obviously, then, we could make money if we could turn soft, unsaturated fats like olein into hard, saturated fats like stearin.  Referring to the symbols we see that all that is needed to effect the change is to get the former to unite with hydrogen.  This requires a little coaxing.  The coaxer is called a catalyst.  A catalyst, as I have previously explained, is a substance that by its mere presence causes the union of two other substances that might otherwise remain separate.  For that reason the catalyst is referred to as “a chemical parson.”  Finely divided metals have a strong catalytic action.  Platinum sponge is excellent but too expensive.