35« LIFE : OUTLINES OF GENERAL BIOLOGY 



yielded heat; thirdly, it was possible that the animal produced 

 carbon dioxide more rapidly when it was in the ice-chamber than 

 when it was at a comfortable temperature, during the determination. 

 When all these points were considered and allowed for, the figures 

 as corrected were in sufficiently good agreement for Lavoisier to be 

 sure that he had satisfactorily accounted for the production of 

 animal heat ; and more accurate repetitions of this experiment since 

 his day have amply confirmed him. 



The last of the three points taken into consideration in correcting 

 the gross results led Lavoisier to a further series of experiments 

 which were arrested by his execution, but which were of the greatest 

 importance. He showed that a man, just as much as a guinea-pig, 

 did use up more oxygen and set free more carbon dioxide at a low 

 temperature than at a temperature corresponding to that of the 

 body — naturally, since the body temperature has to be kept up in 

 spite of increased loss of heat; further, that a man during the 

 digestion of a meal used up more oxygen than he did when fasting 

 — thus proving that work was done during digestion; and finally, 

 that a man doing hard work, in lifting a weight, used up oxygen 

 some three times as fast as a man resting. 



The Cost of Living. — Experiments on the same principle as 

 Lavoisier's are constantly being made in laboratories all over the 

 world, with costly but accurate apparatus, on a larger scale. There 

 are automatic machines which record the amount of carbon dioxide 

 in the air; calorimeters are built as large as small rooms, in which 

 a man can rest or perform muscular work, as the experiment 

 demands. The object of the experiments has changed too: it is no 

 longer necessary to seek proof of Lavoisier's idea, that the animal 

 body is comparable to a machine in its intake and output of energy, 

 and obeys the same physical laws. But the method has proved a 

 most valuable one in the investigation of physiological problems, 

 and also in medical diagnosis. 



In either case, special emphasis is laid on the problem of deter- 

 mining the physiological cost of living, that is to say, the amount 

 of energy which is given out by the body — and, if accounts are to 

 balance, must be returned to the body in the form of food — when 

 no unnecessary work is being performed, and the body is merely 

 keeping itself alive. The beating of the heart, the movements of 

 breathing, the maintenance of the normal temperature in spite of 

 loss from the surface — these proces.ses, and doubtless more subtle 

 ones also, demand energy, but apart from these absolute rest is 

 aimed at. Under such conditions, the energy output of the body at 

 rest, the "basal metabolism" as it is called, has been determined 

 for thousands of individuals. 



It is possible to determine this value in many ways — by oxygen 

 consumption, by carbon dioxide formation, by heat production, by 



