386 



Dr. W. Marcet. 



[June 21, 



cation with the external air ; the endiometer is then rapidly depressed 

 and closed. In this position the tube from the hydrogen can be rinsed 

 again, independently of the eudiometer, so that the washing may be 

 considered as complete and thorough. 



The eudiometer being brought into connexion with the hydrogen is 

 again raised, and 18 c.c. of hydrogen gas are taken in under atmo- 

 spheric pressure. 



The hydrogen kept over water is saturated, and a thermometer 

 with its bulb in the bell-jar gives the temperature of the gas, which 

 is very nearly that of the laboratory ; so that by the time the gas 

 is ready to be measured in the eudiometer it shows no tendency 

 either to contract or dilate. The eudiometer now contains the 

 volume of hydrogen required for the analysis, and the stop-cock is 

 turned shutting off the gas from the holder, and opening the V" 

 shaped tubes through and through in readiness for washing out with 

 the air to be analysed. 



The air from the large glass jar is introduced into the eudiometer 

 in the following way. Having filled the funnel referred to above 

 with water, the latter is let into the jar by opening slightly the 

 pinch-cock closing the funnel ; at the same time the glass jar having 

 been connected with the \/- sna P e d tube of the eudiometer by india- 

 rubber tubing, is opened towards the instrument, when the air 

 displaced by the water added rinses out the india-rubber and steel 

 tubings. There is plenty of air in the jar, so that no necessity occurs 

 to be saving ; when the tubes are rinsed the eudiometer is raised in 

 the mercury up to about 45 c.c, carrying a column of mercury with 

 it ; then the two-way stop-cock is very carefully turned so as to 

 admit the air to be analysed, which is aspired by the mercury as it 

 subsides. Thus some 27 c.c. of air are introduced. The aspiration 

 must be fairly rapid, and the fall of mercury in the tube should be 

 stopped by turning the stop-cock before the mercury has quite 

 reached its level in the trough, otherwise there is a risk of a recoil of 

 the mercury, and a " pumping " which it is important to avoid. The 

 mixed gases are left undisturbed for two or three minutes, and their 

 volume is read off under atmospheric pressure, the eudiometer being 

 next moved up and down in the mercury by a few centimetres, so as 

 to effect the perfect mixture of the gases. The instrument is now 

 slightly raised, carrying with it a short column of mercury, and the 

 gases are ignited by the electric spark under reduced atmospheric 

 pressure. This mode of proceeding, recommended by McLeod,* 

 weakens considerably the violence of the explosion, and ensures per- 

 fect safety. Immediately after the explosion the gas in the eudio- 

 meter is brought approximately under atmospheric pressure. 



* McLeod, " On i» new Form of Apparatus for Gas Analysis," ' Chem. Soc. 

 Journ.,' 1869. 



