comprehension of the phenomena of nature. The law may now be formulated from two points of view. In the first place, from a physical aspect: equal volumes of gases (or vapours) at equal temperatures and pressures contain the same number of molecules—or of particles of matter which are neither mechanically nor physically divisible—previous to chemical change. In the second place, from a chemical aspect, the same law may be expressed thus: the quantities of substances entering into chemical reactions occupy, in a state of vapour, equal volumes. For our purpose the chemical aspect is the most important, and therefore, before developing the law and its consequences, we will consider the chemical phenomena from which the law is deduced or which it serves to explain.
When two isolated substances interact with each other directly and easily—as, for instance, an alkali and an acid—then it is found that the reaction is accomplished between quantities which in a gaseous state occupy equal volumes. Thus ammonia, NH 3 , reacts directly with hydrochloric acid, HCl, forming sal-ammoniac, NH 4 Cl, and in this case the 17 parts by weight of ammonia occupy the same volume as the 36·5 parts by weight of hydrochloric acid. Ethylene, C 2 H 4 , combines with chlorine, Cl 2 , in only one proportion, forming ethylene dichloride, C 2 H 4 Cl 2 , and this combination proceeds directly and with great facility, the reacting quantities occupying equal volumes. Chlorine reacts with hydrogen in only one proportion, forming hydrochloric acid, HCl, and in this case equal volumes interact with each other. If an equality of volumes is observed in cases of combination, it should be even more frequently encountered in cases of decomposition, taking place in substances which split up into two others. Indeed, acetic acid breaks up into marsh gas, CH 4 , and carbonic anhydride, CO 2 , and in the proportions in which they are formed from acetic acid they occupy equal volumes. Also from phthalic acid, C 8 H 6 O 4 , there may be obtained benzoic acid, C 7 H 6 O 2 , and carbonic anhydride, CO 2 , and as all the elements of phthalic acid enter into the composition of these substances, it follows that, although they cannot re-form it by their direct action on each other (the reaction is not reversible), still they form the direct products of its decomposition, and they occupy equal volumes.