408 NLTRITION. 



mixed food, until we have been able to find the suitable amount 

 and the most correct proportion for each individual case; but this 

 mode of testing could only be adopted at the expense of trouble 

 and time, and would throw little or no light on the question. We 

 must, therefore, look about us for some guide in this method of 

 inquiry, and this we shall find in the investigation and quantitative 

 determination of the excretions of the animal body. If the latter 

 actually afford a standard for the metamorphosis of animal matter, 

 and if we are able, from their quantity and composition, to 

 judge of the true loss which the animal body experiences during 

 the activity of its organs, they ought also to give us the quantity 

 and quality of those substances, which the organism requires for 

 the restoration of its effete matters. This last method of inquiry, 

 which is based upon the proposition that the requirements of 

 nutrition are regulated by the amount of the loss in the body ? 

 appears at first sight to be so simple that one might almost wonder 

 why, after such labours as those of Boussingault, Liebig, Valentin, 

 Barral and other distinguished investigators, this problem has not 

 yet been completely solved; but the inquiry is here met by 

 numerous difficulties which have hitherto prevented any exact deter- 

 mination even for the simplest relations, partly on account of the 

 constant fluctuations in vital activity and partly from those external 

 influences which do not admit of calculation but which materially 

 affect inquiries of this nature. In order to simplify the obser- 

 vation, we should be especially carefully to see that the organism 

 which was made the object of investigation did not present any 

 increase of weight in its organs, or that it was no longer at that 

 period of growth, during which it required to consume more 

 materials than could be again traced in the excretions ; we must 

 further avoid that kind of feeding to which the term "fattening" is 

 applied in agriculture ; in short, the organism should be maintained 

 in all respects in its normal state, in order that a conclusive proof 

 may be drawn from the excreta in reference to the necessary 

 amount of food. The best method, therefore, of finding at least 

 the minimum of the food necessary for the support of life, is after 

 stopping all supplies from without, to determine the quantities of 

 matters which the organism loses by the excretion of urine, faeces, 

 and products of perspiration. The numerous experiments on 

 inanition, which were formerly made on different animals, appeared 

 to present a good basis for this mode of observation. But, how- 

 ever important it may be to know the minimum quantities needed 

 for the continuance of the life of the organism, these kinds of expe- 



