Fish oils are also employed in the manufacture of such closely-related, although happily diverse, substances as soap and margarine. All animal and vegetable fats and oils are essentially compounds of glycerine, with one or other of three acids: palmitic, stearic and oleic. Palmitic and stearic acids and their compounds are solids at the ordinary temperature, whereas oleic acid and its compounds are liquid. This difference appears to be connected in some way with the molecular structure of these substances. When oleic acid is heated with hydrogen gas under pressure, in the presence of finely-divided nickel, it absorbs hydrogen and is transformed into stearic acid. Oleic acid, therefore, is said to be unsaturated with respect to hydrogen, whereas stearic acid is called a saturated acid. This process, whereby a liquid oil is transformed into a solid fat, is called hydrogenation, or hardening. Both the margarine industry and the soap industry require large quantities of hard fats. Originally the soap industry absorbed the available supplies of hard animal fats such as beef suet, hog’s lard, and mutton suet. The margarine industry depended upon these same supplies of animal fats, and the rapid growth in the production of margarine during recent years has seriously diminished the supply of hard fats necessary for the manufacture of soap. The hydrogenation of whale oil and various fish oils has now made it possible to supply this demand, and has also made possible the industrial utilization of substances, such as fish oils, for which formerly comparatively little use could be found. Hardened whale oil melts at 40° to 50°C., and is a white solid entirely devoid of taste or smell. It is used for making soap, and as a lard substitute for cooking purposes. =Fish Glue.= Fish glue is the most important liquid glue on the market. The bulk of the fish glue manufactured to-day is made from the waste and offal that are discarded by the curers. This waste consists of heads, bones, viscera and skins. The best glue is obtained from the skins of non-oily, demersal fish, for example cod, haddock, soles, plaice and hake.