Prof. J. P. Cooke on Lecture Experiments. 



57 



Fig. 9. 



secured in position. It is next 

 necessary to raise the tempera- 

 ture of the eudiometer-tube to a 

 point considerably above the boil- 

 ing-point of water, and maintain 

 it at this temperature during the 

 rest of the experiment. This is 

 most conveniently done by passing- 

 through the outer tube the vapour 

 of amy lie alcohol (fousel oil), which 

 may be generated in a flask placed 

 at one side and connected by a 

 glass tube with the upper end of 

 the apparatus. The tubulature 

 at the lower end of the tube should 

 at the same time be dipped into 

 the mouth of a glass bottle into 

 which the alcohol may flow as fast 

 as the vapour is condensed ; and 

 since the latent heat of this vapour 

 is very small, this simple method 

 of condensation will be found am- 

 ply sufficient. 



As soon as the temperature of 

 the eudiometer-tube is constant, 

 which can easily be told, because 

 the mercury-column will then remain at a constant height, mer- 

 cury must be poured into the open tube or withdrawn from it 

 by the nipper-tap until the level is the same in both limbs of 

 the apparatus. It remains now only to note the volume of the 

 confined gas, and to explode it by passing an electric spark 

 through the platinum wire already noticed — first, however, 

 tightly closing the end of the open tube with its rubber stopper. 

 When the stopper is removed and the mercury-columns again 

 brought to the same level, it will be found that the volume of 

 the confined gas has been reduced one-third. The result, how- 

 ever, is always a little too small, because aqueous vapour, even 

 at the boiling-point of amylic alcohol (132° C), does not exactly 

 obey Mariotte's law, and is somewhat more condensed than the 

 mixed gases by the atmospheric pressure. It would be better 

 to employ the vapour of a liquid having a still higher boiling- 

 point ; but the results with amylic alcohol are sufficiently accu- 

 rate for a lecture-experiment*. 



* Indeed we can obtain with free steam a result which is sufficiently near 

 the theory for all purposes of illustration ; only it is then important that 

 both the mixed gases? and the resulting vapour should be measured under 



