THE KINDS OF WORK CARRIED ON BY LIVING BODIES. 485 



rounding medium, depends on a slow combustion, now often 

 termed eremacausis, chiefly between the oxygen admitted in 

 respiration, and the carbon in the blood, as well as the 

 carbon of such solids as are continually decomposed in the 

 actions, whether assimilative or relative, of the living frame. 

 The quantity of carbonic acid thrown off in a given time is 

 the index of the amount of this slow combustion in the same 

 period. And since it has been ascertained that the amount of 

 heat produced by the combustion of fixed quantities of carbon 

 and oxygen is the same whether the combustion be rapid or 

 slow, the sum of the heat generated in a given time in a 

 living body, whether sensible or metamorphosed, is determined 

 when the quantity of carbonic acid thrown off in that period 

 is ascertained. The effect of the combustion of hydrogen, 

 sulphur, phosphorus, is often omitted in such calculations 

 in the mean time, partly for the sake of simplicity, partly 

 because the oxygen of the solids decomposed supplies nearly 

 what is required for their combustion. It is a part, however, 

 of the " doctrine of the conservation of energy " that all the 

 heat that is produced in a living body by the slow combustion 

 in question does not become sensible, but that a part of it 

 passes into, or at least represents, the motive forces continually 

 exercised in a greater or less degree in an animal body. And 

 there are even other actions besides mere motive operations, 

 particularly in the human body, that cause the disappearance 

 of a portion of the heat generated. Thus Dr Lyon Playfair, 

 in one of his excellent memoirs on this subject, says : " In the 

 functions of bodily activity there are four kinds of work which 

 a man has to perform : (1) Vital work, (2) Heat work, (3) 

 Mechanical work, (4) Mental work. The ' vital work ' refers 

 to that proceeding in the body, in the direction of which man 

 is more or less an unconscious agent. The heart beats, the 

 blood circulates, the lungs play digestion, assimilation, and 



