If the fat existing in the animal body is contained in cellular tissue, its separation may be simply effected by placing the incised tissue in hot water.  The cells burst and the fat collects itself on the surface of the water.  If vegetable substances contain fat in large quantity, as, for example, seeds, it may be obtained by expression.  The dried seeds are bruised and expressed between either cold or hot metallic plates.  Olives are laid in heaps before expression; when they begin to ferment, they can be completely expressed.  If animal and vegetable substances contain only a little fat, it must be extracted by ether.

In the pure condition the fats are mostly odorless and tasteless; when they possess an odor, it arises mostly from the presence of small quantities of volatile fatty acids, as butyric acid, capric acid, &c.; which becomes free through the decomposition of their oxide of glycyl combinations.  This ensues by the presence of water and air through a kind of fermentation, and as it appears, by the presence of a nitrogenous substance.  The fats are insoluble in water, and, with the exception of castor oil, are taken up by cold alcohol in very small quantities, however, more in proportion as they contain oleine.  In boiling alcohol they are dissolved, but are, for the most part, again separated on cooling, particularly those rich in stearine.  All fats are taken up by ether but those containing stearine in the smallest quantity.

Their specific gravities fluctuate between .91 and .93.  When heated, fats assume a dark color, and boil between 482 deg. and 572 deg.  Fahr., but the boiling-point continuously rises, while an uninterrupted decomposition proceeds.  From oxide of glycyl ensues acroline; oleic acid affords a fatty acid, and among the decomposition products of fats containing stearine and margarine are found pure margaric acid, and, at the same time, some hydro-carbons are formed.  When exposed quickly to a high temperature, fats are completely decomposed. (Oil gas.) In closed vessels the pure fats undergo no change, but, placed in thin layers in the air, the fats containing oleine and oline rapidly absorb oxygen under the strong evolution of heat, which will inflame porous bodies, as cotton wool.  The purer the fats are the more quickly their oxidation results.  When the fats contain slimy materials, these latter can be destroyed with a little oxide of lead and water. (Preparation for the application of varnishes.) The action of nitric acid, nitrous acid, chlorine, sulphuric acid, &c., on fats is the same as that of these bodies on the fatty acids.  The fatty oils dissolve sulphur in the heat which is again partly precipitated on cooling.  When sulphur is heated with fatty oils, namely, with linseed oil, it dissolves by degrees, and a thick dark mass is formed, the so-called balsam of sulphur.  By raising the heat, a violent reaction ensues under the evolution of sulphuretted hydrogen, and, at the same time, an oil resembling oil of garlic volatilizes.  This oil begins to boil at 160 deg.  Fahr., but its boiling-point rises continually.