818 THE POPULAR SCIENCE MONTHLY. 



The foregoing paper was written several months ago, and since 

 that time important contributions to the subject have been made by 

 Geddes, Ilamann, Jickeli, Entz, and especially by Brandt. The latter 

 has published a summary of the whole matter, enriching it by observa- 

 tions of his own made at the zoological station at Naples. Brandt 

 attaches less physiological significance to symbiosis than does Geddes, 

 from whom he still differs in considering even Hydra and Spongilla 

 as symbiotic and not "vegetating " animals. At present the views of 

 Brandt, as laid down in his last paper (see "Mitt, aus zool. Stat. 

 Neapel," iv, ii, 1883), seem likely to prevail ; and in that article the 

 student will find a complete literature and a full discussion of the 

 whole subject. 



THE CHEMISTEY OF COOKEBY. 



By W. MATTIEU WILLIAMS. 

 XIII. 



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 these papers are intended 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, bxityric acid, is an exception ; it boils at 314 Fahr.). Before 

 their boiling-point, i. e., the temperature at which they pass completely 

 into the state of vapor, is reached, their constituents are more or less 

 dissociated or separated by the repulsive agency of the heat, new com- 

 pounds being in many cases formed by recombinations of their ele- 

 ments. 



When water is heated to 212 it is converted completely into a gas, 

 which gas returns to the fluid state without any loss on cooling below 

 212. 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 various other temperatures, 

 they pass into a state of vapor, and these vapors, when cooled, recon- 

 dense into their original form of liquid oil without alteration. Hence 

 they are called " volatile oils," while the greasy oils which can not 



