Respiration. 27 



of heat, and the solid material was being oxidised and dissipated 

 into the air as aqueous vapour and carbonic acid gas ; but could 

 they, he asked, reverse the process, and, by supplying the 

 necessary heat, make wood out of carbonic gas and water ? 

 That would be like expecting water to flow up a hill, for the 

 laboratory rule is that chemical changes are such as to produce 

 heat and do not take place in the opposite direction. Yet this 

 is exactly how the wood has been made ; the tree has silently 

 absorbed these very substances and built them up into wood. 



Of the ordinary functions which living bodies perform, the 

 one which is most nearly understood is respiration, and therefore 

 he had chosen it for the subject of that night's lecture. 

 Respiration in its most superficial sense means the breathing in 

 of pure air, and the exhalation of impure air. But they might 

 give a larger meaning to the word. He had alluded to a 

 burning match, and said that the wood of this match was being 

 oxidised by oxygen drawn from the air, that it emitted heat, 

 and that the substance got dissipated. The same process, he 

 pointed out, is taking place continually in every part of the 

 human body. When he moved his finger some oxygen was 

 used up and some carbonic acid gas and water were parted with ; 

 the oxygen was breathed in by the lungs, the carbonic acid and 

 water would in due time be breathed out by them. The 

 problem was to investigate the processes by which the oxygen 

 of the inspired air is carried to the hidden recesses of the body, 

 and those by which the carbonic acid is carried from the tissues 

 to be cast out into the air of the lung. 



They would observe on the screen a slide representing 

 human blood ; it was made up of numerous corpuscles which 

 float in a clear fluid. Each of these corpuscles is a sort of 

 submarine boat plying between the lungs and the tissues, and 

 at every journey it takes in a cargo of oxygen at the lungs, 

 which it unloads on reaching the small blood vessels of a 

 muscle or other tissue. The corpuscle is composed largely of a 

 red material — haemoglobin — to which the colour of the blood 

 is due. This red material has the power of absorbing oxygen 



