CONSUMPTION OF AZOTISED MATTER. 527 



The two essential facts to be borne in mind in this kind 

 of inquiry are, first, that in an animal which is stationary in 

 point of weight, the quantity of carbon in the whole daily food 

 is an index of the amount of carbonic acid produced in the 

 body during that time ; and that the quantity of carbonic acid 

 thrown off' by the lungs in that time is, with a merely trifling 

 exception, a measure of the entire work performed in the mean- 

 while by such an animal body, including vital work, heat work, 

 mechanical work, and mental work the rate being equal to the 

 elevation of 685,1 50 lb. of matter to the height of one foot 

 for every ounce of carbon contained in the carbonic acid so 

 produced : and again, that in a stationary animal, the quantity 

 of urea excreted in 24 hours will to a certain extent indicate, 

 by the amount of contained nitrogen, how much azotised mat- 

 ter lias been consumed in that period. 



Such is the view which Professor Playfair adopts, in the belief 

 that the daily excretion of urea in a hard-worked man may 

 reach nearly 2 oz., or enough to represent, on the data stated 

 on the preceding page, a daily effort of a million and a half 

 foot-pounds (1,557,896). Several high authorities, however, 

 have more recently raised considerable doubts on this point. 

 These do not, indeed, deny that urea, and the other nitrogenised 

 constituents of the urine and faeces, represent the disintegration 

 of the azotised constituents of the solids and the blood, but 

 they contend that such nitrogenised constituents of the excre- 

 tions do not represent more than a part of the mechanical work 

 done by the living frame within a corresponding time. They 

 say that the amount of urea excreted in a given time is not an 

 index of the mechanical work done in that period not only be- 

 cause it falls short of the required amount, but also because a 

 part of it may be derived from an excess of nitrogenised ali- 

 ment that has never become incorporated with the living solids. 

 This controversy is one of much moment ; but as it is going 



