Page:Popular Science Monthly Volume 24.djvu/111

 nearly to blackness, and all their powers of offensiveness vanish, as such offense is due to slow decomposition of the original organic compounds, which now exist no longer, and the remaining caramel or carbon cinders being quite inoffensive or no further decomposable by atmospheric agency.

In the more violent factory process of purification by sulphuric acid the similar action which occurs is due to the powerful affinity of this acid for water; this may be strikingly shown by adding to thick sirupsyrup [sic] or pounded sugar about its own bulk of oil of vitriol, when a marvelous commotion occurs, and a magnified black cinder is produced by the separation of the water from the sugar.

The following simple practical formula may be reduced from these data. When a considerable quantity of much-used frying fat is accumulated, heat it to about 300° Fahr., as indicated by the crackling of water when sprinkled on it, or, better still, by a properly constructed kitchen thermometer graduated to about 400° Fahr. Then pour the melted fat on hot water. This must be done carefully, as a large quantity of fat at 400° poured upon a small quantity of boiling water will illustrate the fact that water when suddenly heated is an explosive compound. The quantity of water should exceed that of the fat, and the pouring be done gradually. Then agitate the fat and water together, and, if the operator is sufficiently skillful and intelligent, the purification may be carried further by carefully boiling the water under the fat, and allowing its steam to pass through; but this is a little dangerous, on account of the possibility of what the practical chemist calls "bumping," or the sudden formation of a big bubble of steam that would kick a good deal of the superabundant fat into the fire.

Whether this supplementary boiling is carried out or not, the fat and the water should be left together to cool gradually, when a dark layer of carbonized impurities will be found resting on the surface of the water, and adhering to the bottom of the cake of fat. This may be peeled off and put into the waste grease-pot, to be further refined with the next operation. Ultimately the worst of it will sink to the bottom of the water. Then it is of no further value, and will be found to be a mere cinder.

Regarding the fat used in frying as a medium for conveying heat, freedom from any special flavor of its own is a primary desideratum. Olive-oil of the best quality is almost absolutely tasteless, and, having as high a boiling-point as animal fats, it is the best of all frying media. In this country there is a prejudice against the use of such oil. I have noticed at some of those humble but most useful establishments where poor people are supplied with penny or twopenny portions of good fish, better cooked than in the majority of "eligible villa residences," that