loo THE POPULAR SCIENCE MONTHLY. 



rial for the soap-maker, and grease for lubricating machinery. Un- 

 savory stories have been told about the manufacture of butter from 

 Thames mud or the nodules of fat that are gathered therefrom by the 

 mud-larks, but they are all false. It may be possible to purify fatty 

 matter from the foulest of admixtures, and do this so completely as to 

 produce a soft, tasteless fat, i. e., a butter substitute, but such a curi- 

 osity would cost more than half a crown per pound, and therefore the 

 market is safe, especially as the degree of purification required for 

 soap-making and machinery-grease costs but little, and the demand 

 for such fat is very great. 



These methods of purification are not available in the kitchen, as 

 oil of vitriol is a vicious compound. During the siege of Paris some 

 of the Academicians devoted themselves very earnestly to the subject 

 of the purification of fat in order to produce what they termed " siege- 

 butter " from the refuse of slaughter-houses, etc., and edible salad oils 

 from crude colza oil, the rancid fish oils used by the leather-dresser, 

 etc. Those who are specially interested in the subject may find some 

 curious papers in the " Comptes Rendus " of that period. In vol. Ixxi, 

 page 36, M. Boillot describes his method of mixing kitchen-stu£F and 

 other refuse fat with lime-water, agitating the mixture when heated, 

 and then neutralizing with an acid. The product thus obtained is 

 described as admirably adapted for culinary operations, and the method 

 is applicable to the purpose here under consideration. 



Further on in the same volume is a "Note on Suets and Alimentary 

 Fats " by M. Dubrunfaut, who tells us that the most tainted of ali- 

 mentary fats and rancid oils may be deprived of their bad odors by 

 "appropriate frying." His method is to raise the temperature of the 

 fat to 140° to 150° Centigrade (284° to 302° Fahr.) in a frying-pan ; 

 then cautiously sprinkle upon it small quantities of water. The steam 

 carries off the volatile fatty acids producing the rancidity in such as 

 fish-oils, and also the neutral offensive fatty matters that are decom- 

 posed by the heat. In another paper by M. Fua this method is ap- 

 plied to the removal of cellular tissue of crude fats from slaughter- 

 houses. It is really nothing more than the old farm-house proceeding 

 of "rendering" lard, by frying the membranous fat until the mem- 

 branous matter is browned and aggregated into small nodules, which 

 constitute the " scratchings " — a delicacy greatly relished by our Brit- 

 , ish plowboys at pig-killing time, but rather too rich in pork-fat to 

 supply a suitable meal for people of sedentary vocations. 



The action of heat thus applied and long continued is similar to 

 that of the strong sulphuric acid. The impurities of the fat are organic 

 matters more easily decomposable than the fat itself, or, otherwise 

 stated, they are dissociated into carbon and water at about 300° Fahr., 

 which is a lower temperature than that required for the dissociation 

 of the pure oil or fat (see No. 13 of this series). By maintaining this 

 temperature, these compounds become first caramelized, then carbon- 



