24: president's address. 



conception the usual practiceof seeking an explanation in accord 

 with what is known of any phenomenon. For over a thousand 

 years no one noted any more about the breath. The discoveries of 

 carbon dioxide byHelmontand Black and of oxygen by Priestley 

 opened a new era. By the end of the eighteenth century the 

 methods devised for analysing gases were sufficiently accurate to 

 permit the estimation of the constituents of expired air. Lavoisier, 

 MenzieSj and Sir Humphrey Davy collected, in a jar, the air 

 passing from the nose and mouth and determined the compounds. 

 Since this was done by these worthy scientists, thousands of 

 analyses have been made. Great improvements have been intro- 

 duced in the way in which the air has been collected, and in the 

 process of estimating the quantities of the different gases, but 

 the nature of the estimation has not altered. The figures 

 express the amounts of the component gases in the jar holding 

 the expired air. They inform us of expired air on the assump- 

 tion that it is of uniform composition. Some of the earlier in- 

 vestigators thought that the composition of mixed expired air 

 was similar in different breaths, and even from different persons. 

 They therefore paid great attention to the process of analysis, 

 because they supposed that such differences as were noticed, were 

 due to defects in the method of analysis. They devoted much 

 care to modifying the processes for measuring the amounts of 

 gases in mixtures. One investigator took such care that he was 

 able to measure the amount of oxygen in one sample of expired 

 air with an accuracy of one part in 10,000. He obtained the 

 figure 17 - 033 for the percentage of oxygen. The use of accurate 

 methods soon made it evident that the results varied with the 

 depth of respiration, with the rate of breathing, with movements 

 of the body, with the character of the food, and with many other 

 circumstances. A few still held that there was a definite com- 

 position for the expired air which might be discovered by a study 

 of the effects of the factors influencing breathing. The majority 

 ceased to pay much attention to the figures from any single 

 analysis, and recorded the results of the examination of the air 

 expired over long intervals of time. 



