254 CHOPS. XI. 



The method used in the determination of the remaining item, 

 "the nitrogen-free extract," in these analyses is least satisfac- 

 tory of all being simply to take the difference between the 

 sum of the percentages of the other constituents and 100. 



In the form just described thousands of analyses have 

 been published, and though the results are of considerable 

 value they cannot be considered satisfactory. 



Water. The assumption that only water is expelled by 

 heating a food to a temperature of about 100 is certainly 

 not warranted, as many organic compounds undergo change 

 below this temperature ; moreover, many fats- and oils absorb 

 oxygen and consequently increase in weight w r hen exposed to 

 air. The latter objection can be overcome by drying in a cur- 

 rent of hydrogen. 



Ash. In this case, as in all vegetable and animal matter, 

 the ash left on incineration has not the composition of the 

 inorganic compounds present in the plant or animal. This ob- 

 jection, however, is not a very important one. 



Fat. Since this is really the matter soluble in ether, it is 

 better described as " ether extract." In the case of many sub- 

 stances, chorophyll, resins, waxes, and organic acids, e.g., 

 lactic acid, are dissolved by ether and are included in the 

 "ether extract." In some instances the amounts of these 

 non-fatty substances may be considerable in proportion to the 

 true fat. 



Nitrogenous matter. However accurate the determination of 

 nitrogen may be, the figures given under this head can never 

 be very trustworthy. In the first place all the nitrogen in a 

 substance is not usually present as albuminoids, but may be 

 partly as amides, ammonium salts, or nitrates. Then, too, all 

 albuminoids do not contain the same proportion of nitrogen* 

 and therefore multiplying by 6 '25 does not give a correct 

 measure of their amount. A method of distinguishing between 

 the nitrogen existing as true albuminoids and that as amides, 

 &c., has already been described. I 



Crude fibre. The method used in the estimation of this 

 material is obviously based upon an assumption that from a 

 material freed from fat by ether extraction dilute sulphuric acid 



* See p. 216. t See p. 219. 



