THE CHEMISTRY OF COOKERY. 819 



thus be distilled (in which animal fats are included) are called " fixed 

 oils." 



A very simple practical means of distinguishing these is the follow- 

 in^ : 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 fry- 

 ing-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 melted fat, i. e., the oil, 

 remains about the same, the water-vapor carrying away the heat. 

 "When all the water is driven off, the liquid becomes quiescent, in spite 

 of its temperature, rising from 212 to near 400, then a smoky vapor 

 comes off and the oil becomes darker ; this vapor is not vapor of lard, 

 but vapor of separated and recombined constituents of the lard, which 

 is now suffering dissociation, the volatile products passing off while 

 the non-volatile carbon ( i. e., lard-charcoal) remains behind, coloring 

 the liquid. If the heating be continued, a residuum of this carbon, in 

 the form of soft coke or charcoal, will be all that remains in the heated 

 vessel. 



We may now understand what happens when something humid 

 say a sole is put into a frying-pan which contains fat heated above 

 212. Water, when suddenly heated above its boiling-point, is a pow- 

 erful explosive, and may be very dangerous, simply because it expands 

 to 1,728 times its original bulk when converted into steam. Steam- 

 engine boilers and the boilers of kitchen-stoves sometimes explode 

 simply by becoming red-hot while dry, and then receiving a little 

 water which suddenly expands to steam. 



The noise and spluttering that are started immediately the sole is 

 immersed in the hot fat, are due to the explosions of a multitude of 

 small bubbles formed by the confinement of the suddenly expanding 

 steam in the viscous fat, from which it releases itself with a certain 

 degree of violence. It is evident that, to effect this amount of erup- 

 tive violence, the temperature must be considerably above the boiling- 

 point of the exploding water. If it were only just at the boiling-point, 

 the water would boil quietly. 



