The Chemistry of Cookery

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. But the practical cook may say, ‘This is wrong, for the fat in my frying-pan does boil. I see it boil, and I hear it boil.’ The reply to this is, that the lard, or dripping, or butter that you put into your frying-pan is oil mixed with water, and that it is not the oil but the water that you see boiling. To prove this, take some fresh lard, as usually supplied, and heat it in any convenient vessel, raising the temperature gradually. Presently it will begin to splutter. If you try it with a thermometer you will find that this spluttering-point agrees with the boiling-point of water, and if you use a retort you may condense and collect the splutter-matter, and prove it to be water. So long as the spluttering continues the temperature of the