26 
POPULAR SCIENCE REVIEW. 
In the year 1808, G-ay Lussac announced the fact that there 
exists a simple relation between the volumes in which elemen- 
tary gases combine, and also between these volumes and the 
volume of the product, provided that product be gaseous. 
Hydrogen and oxygen combine — Gay Lussac showed — in 
the proportion of two volumes to one volume ; the product is 
two volumes of aqueous vapour. Hydrogen and nitrogen com- 
bine in the proportion of three volumes to one volume ; the 
product is two volumes of ammonia gas. Nitrogen and 
oxygen combine in the proportion of two volumes to one 
volume ; the product is two volumes of nitrous oxide gas. 
Extended experiments have shown that, as a very general rule, 
two volumes of any compound gas contain 1, 2, 3, &c. 
volumes of the elementary gases composing the compound 
body ; or, in other words, if we adopt two volumes of a com- 
pound as the standard, we can express the volumes of the con- 
stituent elements in whole numbers. Further, experience has 
shown that the number representing the smallest (relative) 
amount of an element in two volumes of any of its gaseous 
compounds, also represents the proportion, by weight , in which 
that element combines with other elements : we may call this 
number the combining number of the element. Thus, the 
smallest amount of oxygen in two volumes of any gaseous com- 
pound of this element— hydrogen being the unit of the scale — 
is 16. All known gaseous compounds of oxygen may be repre- 
sented as containing 16, or a multiple of 16, parts by weight of 
that element- The smallest amount of nitrogen in two volumes 
of a gaseous compound of nitrogen is 14. All gaseous nitrogen- 
compounds may be represented as containing 14, or a multiple 
of 14, parts by weight of that element. 
Hence, if we adopt two-volume formulae, we are enabled to 
attach to each element a number expressing the proportion by 
weight in which that element combines with other elements to 
form compounds. 
Now, in the equivalent system of notation, as strictly carried 
out, no such generalization was possible. The equivalents 
3, 4, 6, &c. might be assigned to carbon. In marsh gas 
3 parts by weight of carbon are combined with 1 part by 
weight of hydrogen ; in ethane 4 parts of carbon are combined , 
with 1 of hydrogen ; in ethene 6 parts of carbon are combined 
with 1 of hydrogen. Are we to represent carbon compounds as 
contaiuing multiples of 3, multiples of 4, or multiples of 6 parts 
by weight of carbon ? We are perfectly entitled, on the system 
of notation by equivalents, to adopt the formula CH as express- 
ing the composition of each of the gases already mentioned, 
giving to C a different value in each formula. Or, if we 
adopt — as was actually done — 6 as the equivalent of carbon, 
