APPARATUS FOE PHYSIOLOGICAL USE. 265 



that the air shall not go backwards, and at each movement 

 of the bell- jar we withdraw a certain quantity of air from 

 the tube (c), and drive it into the analysing apparatus, D', 

 and on through the small metre, E. 



The first part of the analysing apparatus consists of a 

 small jar filled with pumice stone and strong sulphuric acid, 

 /, which absorbs the watery vapour. This water, of course, 

 increases the weight of the vessel, and as it is weighed before 

 and after the air has passed through it, you can ascertain 

 from the increase in weight how much water was contained 

 in the air, and . therefore, by a simple calculation, how much 

 water the animal was giving off in a certain time. The air 

 is then drawn through a tube, D, containing a quantity of 

 baryta-water, which absorbs the carbonic acid given off by 

 the animal. From this long tube it passes into a shorter 

 one, D', which is filled, like the first, with baryta- water, 

 and is simply supplementary to it, so that if by any chance 

 the whole of the carbonic acid was not absorbed by 

 the baryta-water in the first tube the second one would 

 take up the remainder. The fact is, however, that the 

 baryta- water in the first tube in almost every case absorbs 

 the carbonic acid so completely that the second one is of 

 no use. Then there is a small metre, E', by which is measured 

 the quantity of air which has passed through the small 

 tube. From that metre you can either let the air go out 

 into the atmosphere, or can collect it over mercury, and 

 analyse it for oxygen. Thus you see you have the quantity 

 of air passing through the apparatus measured by the large 

 metre, then the quantity which is withdrawn for examina- 

 tion is measured by the small metre, and thus you know 

 what proportion your sample bears to the original quantity. 

 You may perhaps have a thousand litres passing through 

 the apparatus, and ten litres taken as -a sample. Then you 

 can say that so much watery vapour has been passed off 

 in these ten litres, and multiplying that by 100 you can 

 say there was so much watery vapour exhaled by the animal 

 in the time the apparatus was at work. By estimating the 

 amount of carbonic acid collected by this baryta-water you can 

 ascertain how much carbonic acid has been contained in the 

 quantity of air passing through this tube, and multiplying 

 that by 100 you get the quantity contained in the whole. 

 These are the essentials of the apparatus. There is, besides, 



