264 LECTURES TO SCIENCE TEACHERS. 



it would be impossible to analyse the whole quantity 

 breathed by a man during such a long time as an entire 

 day. It is also quite unnecessary, for its composition can 

 be ascertained as well from the analysis of a small sample 

 as from an analysis of the whole, and if both the entire 

 quantity of air respired and that contained in the sample be 

 carefully measured, a simple sum in proportion will give 

 the composition of the whole. The apparatus, then, consists 

 of five parts. 



1. A box. or room, A, in which the animal or man 

 respires. 



2. A fan, or pump, B, to draw a current of air constantly 

 through the room. 



3. Pumps, c, c', c", to suck out a sample of the air after 

 it has passed through the room. 



4. An apparatus,/, D, D', to analyse this sample. 



5. A large metre, E, to measure the total quantity of air 

 which has passed through the room, and small metres, E, E', 

 to measure the samples. 



There is an inlet in A at this side, a, through which the air 

 enters, and an exit at the other side (b), through which it 

 passes out. It is drawn through the apparatus by means of 

 an aspirating machine, B, and the amount which passes is 

 marked by means of a metre, E. We can thus ascertain the 

 amount of air passing through the apparatus in a given 

 time. The next thing is to analyse a sample of it. Here 

 is the tube (c) through which the air is passing, and 

 there is a little tube (d) attached to it, and if we can suck 

 a little of the air through this tube we shall get a sample. 

 This is accomplished by means of a little pump, c', which 

 pumps the air from the tube, and sends it through this 

 apparatus, D, D'. This pump is neither more nor less than a 

 small glass bell-jar, which passes into a vessel containing 

 mercury. Each time it is raised it creates a vacuum into 

 which the air rushes from the large tube leading from the 

 box, and each time it is depressed it drives out the air 

 again along another tube. It is prevented from going 

 backwards by means of a little valve, v, consisting of a small 

 bulb half filled with mercury, so arranged that the air can 

 only pass one way. By having one of these valves here 

 between the tube and the pumping apparatus, and another 

 between the pump and analysing apparatus, we secure 



