APPENDIX. 681 



a vapour may be applied to the knowledge of its atomic weight, 



and consequently of the number of atoms which it contains. Let 



us take benzoic acid as an example. It is composed of, 



Carbon, . . 74-34 



Hydrogen, . .4-42 



Oxygen, . . 21-24 



100-00 



The specific gravity of its vapour, as determined by Dumas and 

 Mitcherlich, is 4-27. Now the specific gravity of a volume of 

 carbon vapour and hydrogen gas, and of half a volume of oxy- 

 gen gas* is as follows : 



Carbon, . . 0-4166 



Hydrogen, . 0-0694 



Oxygen, . . 0-5555 



The atomic weight is 14-125. Now it is easy to see that C 14 H 5 



O 3 give that atomic weight. To see whether the specific gravity 



of the vapour of benzoic acid agrees with this number, we have, 



14 volumes carbon weigh 5*8333 



5 volumes hydrogen, . 0*3472 



1^ volume oxygen, . 1-6666 



7-8472 



If these 22 atoms were condensed into one volume the speci- 

 fic gravity of the vapour would be 7*8472. But this great con- 

 densation seldom takes place. In general we must divide by 

 2, showing that the atoms are condensed into two volumes in 

 the vapour. Dividing 7-8472 by 2, we have 3-9236, which ap- 

 proaches pretty nearly to the density of the vapour found by ex- 

 periment. 



Let us take another example in which the atomic weight can- 

 not be determined directly ; that we may see the use that may 

 be made of the specific gravity of the vapour. Let the substance 

 be camphor. It is composed of, 



Carbon, . 78-94 



Hydrogen, . 10*53 



Oxygen, . 10-53 



100-00 



* The reason of taking a volume of the first two and half-a-volume of oxy- 

 gen, is, that a volume of carbon and a volume of hydrogen are each reckoned 

 equivalent to an atom, while a volume of oxygen is equivalent to two atoms. 



