GENERATION OF WARMTH DURING CONTRACTION. 77 



antimony and bismuth, and having covered the surface 

 of each of the ends with a muscle from the lower leg 

 of a frog, he waited until both had assumed an equal 

 temperature. He then by irritation induced activity 

 in one muscle, and owing to the sensitiveness of the 

 apparatus he was not only able to determine the warmth 

 arising during a single pulsation, but even to indicate 

 differences in this according to the circumstances 

 (burden, &c) under which the pulsation occurred. 



The law of the conservation of energy would lead 

 us to expect that in cases in which the muscle ac- 

 complished a greater amount of mechanical work, the 

 production of warmth would be less, and vice versa. 

 When weights are applied, as burden, to the muscle, 

 the labour performed increases, as we found, up to a 

 certain point with every increase in weight. The 

 generation of warmth should accordingly decrease in 

 this case. This was not, however, the case in the 

 experiments made by Heidenhain. As we cannot sup- 

 pose that the law of the conservation of energy,' which 

 is elsewhere throughout nature universally valid, is 

 invalid as regards muscle, we can only suppose that 

 the number of chemical modifications occurring at each 

 muscular pulsation is not always the same, but that 

 wiien greater weight is applied a larger amount of 

 substances are consumed in the muscle, so that both 

 the production of warmth and the work accomplished 

 may, though the irritant remains the same, differ 

 according to the degree of tension of the muscle. On 

 the other hand, it is quite in accordance with the law 

 of the conservation of energy that the muscle generates 



' On this law see the admirable work of Balfour Stewart (Inter- 

 national Scientific Series, vol. vi.). 



