25 



A' B' = vol. of carbon dioxide. 

 B' C' = vol. of oxygen. 

 C' = vol. of nitrogen. 



The centesimal composition of the mixture is then very easily 

 calculated. 



The following formula may be used to bring the observed 

 volumes to standard temperature and pressure : 



yp 



(1 + -003065 *)760. 



V= observed volume of gas in cubic centimetres. 

 P= pressure upon the dry gas. 



= temperature at the time the volume was measured. 

 For further details, see " Chemical Society's Journal," vol. xxi. 

 p. 109. 



109. Apparatus for the determination of vapour 

 densities by (ray-Lussac's process, and by Dumas' process. 



E. 54. 1874. 



See also Nos. 93 and 94. 



Gay-Lussac's Process. The principle of this method is to 

 convert a known weight of a substance, confined over mercury, 

 into vapour, the volume of which being measured at known 

 temperature and pressure, can be readily reduced by calculation 

 to that at standard temperature and pressure. 



Dumas' Process is the reverse of the preceding. A known 

 volume of a vapour is first measured, and its weight then ascer- 

 tained ; from these data the vapour density under standard con- 

 ditions is calculated. 



Further details may be found in Miller's " Chemical Physics." 



110. Apparatus for showing that hydrogen and chlorine 

 do not contract when they unite to form hydrochloric acid. 



E. 55. 1874. 



This consists of a glass tube having a stopcock at each end, and 

 filled with a mixture of hydrogen and chlorine in equal volumes. 

 By opening one end of this tube under mercury it is proved to 

 be completely full of gas. Then, either by the light of burning 

 magnesium wire, or by an electric spark passed between two 

 platinum wires fused into the tube, the gases are caused to 

 combine. On opening the tube a second time under mercury, 

 after cooling, the volume of the contained gas is shown to be 

 the same as before combination, and solution of the hydrochloric 

 acid gas in blue litmus-water proves that no hydrogen or chlorine 

 is left. (See Hofmann's " Modern Chemistry," Lecture iii.) 



111. Apparatus for the decomposition of ammonia by 

 electric sparks, and for the subsequent combustion of 

 the liberated hydrogen by cupric oxide. 



E. 56. 1874. 



A certain volume of dry ammonia gas is confined in the 

 closed limb of the tube, and the height of the mercury therein, 



