THE process of frying follows next in natural order to those of roasting and grilling. A little reflection will show that in frying the heat is not communicated to the food by radiation from a heated surface at some distance, but by direct contact with the heating medium, which is the hot fat commonly, but erroneously, described as ‘boiling fat.’ As I am writing for intelligent readers who desire to understand the philosophy of the common processes of cookery, so far as they are understandable, this fallacy concerning boiling fat should be pushed aside at once. Generally speaking, ordinary animal fats are not boilable under the pressure of our atmosphere (one of the constituent fatty acids of butter, butyric acid, is an exception; it boils at 314° Fahr.). Before reaching their boiling-point, _i.e._ the temperature at which they pass completely into the state of vapour, their constituents are more or less dissociated or separated by the repulsive agency of the heat, new compounds being in many cases formed by recombinations of their elements. When water is heated to 212° it is converted completely into a gas, which gas, on cooling below 212°, returns to the fluid state without any loss. In like manner if we raise an essential oil, such as turpentine, to 320°, or oil of peppermint to 340°, or orange-peel oil to 345°, or patchouli to 489°, and other such oils to certain other temperatures, they pass into a state of vapour, and these vapours, when cooled, recondense into their original form of liquid oil without alteration. Hence they are called ‘volatile oils,’ while the greasy oils which cannot thus be distilled (in which class animal fats are included) are called ‘fixed oils.’ A very simple practical means of distinguishing these is the following: make a spot of the oil to be tested on clean blotting-paper. Heat this by holding it above a spirit-lamp flame, or by toasting before a fire. If the oil is volatile the spot disappears; if fixed, it remains as a spot of grease until the heat is raised high enough to char the paper, of which charring (a result of the dissociation above-named) the oil partakes.