530 REPOBT— 1880. 



tie nitrogenous constituents which the food supplied. And when allowance was 

 made for the different condition of the nitrogenous constituents, and for the greater 

 or less amount of the non-nitrogenous ones which would probably be indigestible 

 and effete, the indications were still more remarkable and conclusive. 



In very many cases the animals were slaughtered, and carefully examined as to 

 whether the tendency of development had been more that of growth in frame and 

 flesh, or in fatness. Here, again, the e\'ideuce was clear — that the tendency to growth 

 in frame and flesh was favoured by a high proportion of nitrogen in the food, and 

 that to the production of fat by a high proportion of digestible non-nitrogenous 

 constituents. 



In a few cases the actual amount of fat in the animals in the lean, and in the fat 

 condition, was determined ; and the results admitted of no doubt whatever that a 

 very large proportion of the stored-up fat could not have been derived from the 

 fatty matter of the food, and must have been produced within the body. 



So decisive and consistent were the very numerous and very varied results in 

 regard to these points, that we had no hesitation in concluding — not only that much 

 of the fat stored up was produced within the body, but that the source of much, 

 at any rate, of the produced fat must have been the non-nitrogenous constituents of 

 the food — in other words, the carho-hydrates. 



As already stated, however, as the question whether the source of the produced 

 fat was the proteine compounds or the carbo-hydrates was not then prominent, we 

 had not so arranged the experiments as to obtain the largest possible increase in fat 

 with the smallest possible supply of nitrogenous compounds in the food ; nor did we 

 then even calculate whether or not there was sufficient nitrogenous matter consumed 

 to be the eom-ce of the whole of the fat produced. 



This question, indeed, excited very little interest, until, at a meeting of the 

 Congress of Agricultural Chemists held at Munich in 1865 (at which I happened 

 to be present), Professor Voit, from the results of experiments made in Pettenkofer's 

 respiration apparatus with dogs fed on flesh, announced his conclusion that fat 

 must have been produced from the nitrogenous substance, and that this was probably 

 the chief, if not the only, source of the fat, even of herbivora — an opinion which he 

 subsequently urged much more positively. 



In the discussion which followed the reading of Professor Voit's paper, Baron 

 Liebig forcibly called in question his conclusions ; maintaining not only that it was 

 inadmissible to form conclusions on such a point in regard to herbivora, from the 

 results of experiments made with carnivora, but also that direct quantitative results 

 obtained with herbivorous animals had afforded apparently conclusive evidence in 

 favour of the opposite view. 



Voit's paper excited considerable controversy, in which Mr. Lawes and myself 

 joined. We maintained that experiments to determine such a question should be 

 made, not with carnivora or omnivora fed on flesh, but with herbivora fed on their 

 appropriate fattening food, and on such herbivora as common experience showed 

 to be pre-eminently fat-producers. We pointed out ^ that the pig comprised, for a 

 given live-weight, a comparatively small proportion of alimentary organs and con- 

 tents ; that, compared with that of the ruminants, his food was of a high character, 

 yielding, for a given weight of it, much more total increase, much more fat, and 

 much less necessarily effete matter ; that, in proportion to his weight, he consumes 

 a larger amount of food, and yields a larger amount, both of total increase and of 

 fat, within a given time ; and, lastly, that he contains a larger proportion of fat, 

 both in a given live weight and in his increase whilst fattening. 



It is ob^dous that, with these characteristics, there is much less probable range 

 of error in calculating the amoimt and the composition of the increase in live-weight 

 in relation to the amount and composition of the food consumed, than in the case 

 of the ruminants ; and that, therefore, the pig is veiy much more appropriate for 

 the purpose of experiments to determine the sources in its food of the fat it pro- 

 duces. 



Accordingly, we calculated a number of our early experiments made with pigs, 

 to determine whether or not the nitrogenous substance they consumed was suffi- 



> ' On the Sources of the Fat of the Animal Body,' Phil. Mag., December 1866. 



