530 
ON 11ECENT CHANGES IN 
gases combine they do so in volumes which stand in a simple relation to each 
other, and that the same simplicity is observed in the proportion between the 
gas volume of the resulting compound and those of its constituents. Thus :— 
2 measures (say litres) of H combine with 1 litre of O to form 2 litres of 
water vapour ; 1 litre of H unites with 1 litre of Cl to form 2 litres of hydro¬ 
chloric acid gas ; 1 litre of N unites with 3 litres of H, producing 2 litres of 
ammoniacal gas ; 2 litres of N unite with 1 litre of O to form 2 litres of nitrous 
oxide gas, etc. 
It was soon found also that a very striking relation exists between the 
density of a gas and its combining proportion, when hydrogen is taken as the 
standard of specific gravity ; in fact, the number representing the density is 
identical, in the very great majority of the elements, with that express¬ 
ing the combining proportion. Here is a most important fact ; one, indeed, 
which has been in the highest degree productive of results. 
Let us examine it a little further. Elements combine in proportions by 
weight, which are definite and invariable; they also exhibit a definite and 
constant ratio in the volumes corresponding to those weights. For ex¬ 
ample : — 
1 litre of N weighs L251 gramme, and combines with 3 litres of H, 
weighing ‘2679. 
Here we have equal volumes of nitrogen and hydrogen, weighing respec¬ 
tively 1-251 and = *0893. 
How, -0893 : i-25i:: i: u. 
These numbers, therefore, express their densities. This coincidence between 
combining proportion and density might, therefore, be foreseen. If, accord¬ 
ing to the atomic theory, we believe combining proportions to represent the 
weights of the atoms of the elements, then, also, these latter should be 
ascertainable by examination of the specific gravities of those elements in 
the gaseous state. 
Again, in the gaseous state all bodies, simple or compound, expand or con¬ 
tract equall} 7- upon a like increase or diminution of temperature, whilst under 
equal pressures they are reduced to the same fraction of their original 
volume. The proposition, “ equal volumes of gases contain the same number 
of atoms,” which is known as the law of Ampere, follows naturally from a 
review of all these circumstances. Of late years it has received a slight 
modification, whereby it is rendered applicable to compound as well as to 
simple gases. It now stands, “ equal volumes of simple and compound gases 
contain the same number of molecules.” The molecular weights of com¬ 
pound gases are thus proportional to their densities, and the quantity of each 
substance representing its molecular weight occupies, under the same cir¬ 
cumstances of pressure and temperature, double the space of 1 atom of 
hydrogen—briefly, 2 volumes. But, it must be observed, all this simplicity is 
lost if we employ equivalent instead of u atomic” weights ; in other words, 
if we attach to C, S, Se, Te, O, and those metals which, like mercury, form 
volatile combinations, the numbers which have hitherto served as their atomic 
weights, the relations just described are no longer found to have any exist¬ 
ence for those bodies. A few examples will illustrate this 
Elements. 
Density compared with H. 
Former atomic weight. 
New atomic weight. 
Hydrogen 
1 
I 
1 
Oxygen 
15-9 
8 
16 
Nitrogen 
14 
14 
14 
Sulphur 
Chlorine 
32 
16 
32 
352 
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