CHAP. I. ] THE EQUIPMENT OF THE SHIP. 43 
of the stop-cock. 6, ¢, and d are ordinary stop-cocks, pierced to 
the bore of the capillary. A general idea of the use of the dif- 
ferent parts of the apparatus will be obtained from the descrip- 
tion of the analysis of a sample of air extracted from sea-water. 
Suppose the instrument to be set up, and with the gaseous mixt- 
ure in the eudiometer, where its volume has been ascertained. 
Fig. 5.—The Gas-analysis Apparatus, 
The mixture consists, we shall say, of oxygen, nitrogen, and car- 
bonic acid. The last of these is determined first by absorption 
by caustic potash. For this purpose mercury is run out of n by 
the stop-cock y, p is raised, and the stop-cocks ¢, 6, and @ open- 
ed. The air is thus driven over out of g into m, mercury being 
allowed to fill the capillary, when the stop-cocks are again shut. 
