PHYSIOLOGICAL 357 



The answer to both questions was found by Lavoisier in 1776 and 

 the following year. He showed, simply and convincingly, that about 

 one-lifth of the volume of ordinary air consisted of a gas to which 

 he gave the name "oxygen", and which had the power of combining 

 with metals, or with the carbon of "organic" substances, or with 

 hydrogen and many other substances, when they were sufficiently 

 heated in air. With metals oxygen forms earthy oxides, such as rust; 

 with hydrogen it forms water; with carbon it forms a gas, the 

 "fixed air" of earlier chemists, which Lavoisier recognised as carbon 

 dioxide. In the burning of a candle, then, the oxygen of the air 

 combined with the carbon of the wax or tallow to form carbon 

 dioxide, and in this process energy was set free, to appear as light 

 and heat. He then showed that mice also used up the oxygen of the 

 air they breathed, and produced carbon dioxide, and he concluded 

 that in the animal body a slow combustion took place, the carbon 

 of the organic materials of the body — continually renewed by the 

 food — combining with the oxygen of the air, with formation of 

 carbon dioxide and liberation of energy for the needs of the body. 

 Where this combustion took place Lavoisier did not know; for some 

 time it was commonly thought to take place in the lungs, then in 

 the blood, and at last it was established that it takes place in every 

 cell in the body, but most vigorously in the muscles. 



To Lavoisier and his contemporaries these theories seemed novel 

 and hazardous; and it was necessary to prove their validity by 

 actual measurement, as accurate as the technique of the time 

 permitted. With the help of the great physicist Laplace, he con- 

 structed a "calorimeter" — that is, an apparatus for measuring the 

 amount of heat given off by an animal or in a given chemical 

 reaction. This consisted of a double- walled can, with ice packed in 

 between the walls to prevent heat entering from outside — since this 

 was long before the invention of the vacuum flask. The interior of 

 the apparatus contained a cage in which a guinea-pig was placed, 

 and round the cage more ice was packed; the water which was 

 formed by the melting of this ice was collected and weighed, and 

 served as a measure of the amount of heat given off by the guinea- 

 pig during the experiment: for instance, it melted 40 grams of ice 

 in an hour. Before or after the experiment the amount of carbon 

 dioxide produced by the animal was measured, and found to be a 

 third of a gram per hour; and Lavoisier and Laplace determined 

 that the burning of carbon at this rate should set free enough heat 

 to melt 32 1 grams of ice in an hour. The agreement between the two 

 figures (40 and 325) was not very good, but it was suggestive, and 

 might perhaps become closer if possible sources of error were tracked 

 down. First of all, it had to be remembered that the animal was 

 warm when put into the ice-chamber; secondly, it was certain that 

 combinations of hydrogen with oxygen in the animal's body also 



