PREFACE Til 



entitled to consider the presence of ammonia in the body as the cause, but only as 

 the effect of a cause. 



Thus medicine, after the fashion of the Aristotelian philosophy, has formed 

 certain conceptions in regard to nutrition and sanguification ; articles of diet have 

 been divided into nutritious and non-nutritious ; but these theories, being founded 

 on observations destitute of the conditions most essential to the drawing of just 

 conclusions, could not be received as expressions of the truth. 



How clear are now to us the relations of the different articles of food to the 

 objects which they serve in the body, since organic chemistry has applied to the 

 investigation her quantitative method of research ! 



When a lean goose, weighing 4 Ibs/, gains, in thirty- six days, during which it 

 has been fed with 24 Ibs. of maize, 5 Ibs. in weight and yields 3 J Ibs. of pure fat, 

 this fat cannot have been contained in the food, ready formed, because maize does 

 not contain the thousandth part of its weight of fat, or of any substance resembling 

 fat. And when a certain number of bees, the weight of which is exactly known, 

 being fed with pure honey, devoid of wax, yield one part of wax for every twenty 

 parts of honey consumed, without any change being perceptible in their health or 

 in their weight, it is impossible any longer to entertain doubt as to the formation 

 of fat from sugar in the animal body. 



We must adopt the method which has thus led to the discovery of the origin of 

 fat, in the investigation of the origin and alteration of the secretions, as well as in 

 the study of all the other phenomena of the animal body. From the moment that 

 we begin to look earnestly and conscientiously for the true answers to our ques- 

 tions, that we take the trouble, by means of weight and measure, to fix our obser- 

 vations, and express them in the form of equations, these answers are obtained 

 without difficulty. 



However numerous our observations may be, yet, if they only bear on one side 

 of a question, they will never enable us to penetrate the essence of a natural phe- 

 nomenon in its full significance. If we are to derive any advantage from them, 

 they must be directed to a well defined object ; and there must be an organized 

 connection between them. 



Mechanical philosophers and chemists justly ascribe to their methods of research 

 the greater part of the success which has attended their labours. The result of 

 every such investigation, if it bear in any degree the stamp of perfection, may 

 always be given in few words; but these few words are eternal truths, to the 

 discovery of which numberless experiments and questions were essential. The 

 researches themselves, the laborious experiments and complicated apparatus, are 

 forgotten as soon as the truth is ascertained. They were the ladders, the shafts, 

 the tools, which were indispensable to enable us to attain to the rich vein of ore ; 

 they were the pillars and air passages which protected the mine from water and 

 from foul air. 



Every chemical or physical investigation, however insignificant, which lays 

 claim to attention, must in the present day possess this character. From a certain 

 number of observations it must enable us to draw some conclusion, whether it be 

 extended or limited. 



The imperfection of the method or system of research adopted by physiologists 

 can alone explain the fact, that for the last fifty years they have established so few 

 new and solid truths in regard to a more profound knowledge of the functions of 

 the most important organs, of the spleen, cf the liver, and of the numerous glands 



