266 LECTURES TO SCIENCE TEACHERS. 



another vessel which can be attached to each tube in 

 front of the metre in order simply to saturate the air with 

 watery vapour before it is measured. Thus we ensure that 

 the amount of the watery vapour in the air and in the 

 sample shall be the same. The volume of air differs 

 slightly according to the amount of watery vapour it 

 contains, and the water having been withdrawn from the 

 sample it would be necessary to make a correction for the 

 watery vapour in comparing its volume with that of the 

 total air. A much easier plan is simply to saturate both 

 the whole air and the sample of air with water, and then 

 measure them. 



But besides the means of measuring the total quantity of 

 air passing through the apparatus, and of measuring and 

 analysing the sample, you want a standard of comparison 

 for your sample, and this is got by taking a little of the 

 external air just as it enters the apparatus, and measuring 

 and analysing it in the same way as the other sample. By 

 comparing the amount of carbonic acid and watery vapour 

 and oxygen contained in the air which enters the chamber 

 with the amount of these substances contained in the air, 

 after it has been breathed by the animal, you can make out 

 what changes have been produced in it by the animal's 

 respiration. 



By means of this apparatus some very interesting facts 

 have been made out ; and I think one of the most interest- 

 ing of all of these is that the oxygen we take in is not 

 immediately used up, but may be used up a good while after- 

 wards. For example, the oxygen that we take in during the 

 course of the night is not used up at the time we are sleeping, 

 but it is stored up for work the next day. During the night 

 we take in more oxygen and give off less carbonic acid ; during 

 the day we take in less oxygen and give off more carbonic acid, 

 so that the body as a whole can store up oxygen just as the 

 frog's muscle of which I have several times spoken, and use 

 it up as it is wanted. The day is the period of combus- 

 tion, the night of repair, and the necessity for having well- 

 ventilated bed-rooms becomes self-evident, for otherwise 

 how can the body get the oxygen it requires ? 



I must now pass on to speak of the mode in which the 

 muscles are supplied with fuel. "We have hitherto been 

 dealing only with the respiration of the muscles of a single 



