324 THE MECHANICAL EQUIVALENT OF HEAT. 



enumeration of its useful applications ; and, for the same 

 reason, a few historical remarks touching the circumstances 

 attending the discovery of the foregoing fundamental law, 

 will not be out of place here. 



In the summer of 1840, on the occasion of bleeding Eu- 

 ropeans newly arrived in Java, I made the observation that 

 the blood drawn from the vein of the arm possessed, almost 

 without exception, a surprisingly bright red colour. 



This phenomenon riveted my earnest attention. Starting 

 from Lavoisier's theory, according to which animal heat is 

 the result of a process of combustion, I regarded the twofold 

 change of colour which the blood undergoes in the capillaries 

 as a sensible sign as the visible indication of an oxidation 

 going on in the blood. In order that the human body may 

 be kept at a uniform temperature, the development of heat 

 within it must bear a quantitative relation to the heat which 

 it loses a relation, that is, to the temperature of the sur- 

 rounding medium ; and hence both the production of heat 

 and the process of oxidation, as well as the difference in col- 

 our of the two kinds of blood, must be on the whole less in 

 the torrid zones than in colder regions. 



In accordance with this theory, and having regard to the 

 known physiological facts which bear upon the question, the 

 blood must be regarded as a fermenting liquid undergoing 

 slow combustion, whose most important function that is, 

 sustaining the process of combustion is fulfilled without the 

 constituents of the blood (with the exception, that is, of the 

 products of decomposition) leaving the cavities of the blood- 

 vessels or coming into such relation with the organs that an 

 interchange of matter can take place. This may be thus 

 stated in other words : by far the greater part of the assimi- 

 lated food is burned in the cavities of the blood-vessels them- 

 selves, for the purpose of producing a physical effect, and a 

 comparatively small quantity only serves the less important 

 end of ultimately entering the substance of the organs them- 



