No. 1.No. 2.No. 3.No. 4.No. 5.
Nitrogen63·6446·6736·8430·4425·93
Oxygen36·3653·3363·1669·5674·07

In casting the eye over this table, no relation will probably be detected between the five cases. But if we write down, not the quantities of nitrogen and oxygen contained in 100 parts of each compound, but the quantity of oxygen which in each compound is united to some fixed quantity of nitrogen, we shall at once detect a remarkable law: thus, taking 28 as the fixed weight of nitrogen, for reasons which need not be here explained:

No. 1.No. 2.No. 3.No. 4.No. 5.
Nitrogen2828282828
{Oxygen1632486480
{    or16 × 116 × 216 × 316 × 416 × 5

Chemists have a sort of shorthand method of expressing the composition of substances, which may be conveniently illustrated by the case before us. Let it be agreed that the letter N shall not only represent nitrogen, but always fourteen parts by weight—grains, ounces, &c., &c.,—of nitrogen; and that, similarly, O shall stand for sixteen parts by weight of oxygen. It is plain that the composition of the compound No. 2 may be represented by simply writing down “NO;” and that of No. 4, in which there is just double the proportion of oxygen, by “NOO.” But to avoid an unnecessary repetition of the same symbol, when it has to be taken more than once, a small figure is written after and a little below it. Thus, for OO, “O2” is written. The proportional composition of each of the five compounds will now be obvious from the following symbols:

No. 1.No. 2.No. 3.No. 4.No. 5.
N2ONON2O3NO2N2O5

These symbols may be regarded as merely a compendious expression of the composition of each substance—as a shorthand statement of the facts of analysis. But to the majority of chemists the symbols have a deeper significance; for they are taken as representing the atoms of each element which enter into each smallest possible particle of a compound; they express a certain theory of the ultimate constitution of matter. Thus, if we suppose that there exist indivisible particles of nitrogen and of oxygen, and that each smallest particle, or molecule, of the compounds under consideration is constituted of a certain definite and invariable number of each kind of atoms; and, further, if we suppose that an atom of oxygen is heavier than one of nitrogen in the proportion of 16 to 14, or 8 to 7, we shall have a simple theoretical explanation of the relations in the proportions already pointed out. In fact, these would result from the simplest combinations of the two kinds of atoms; and we can picture each one of the smallest particles of the several bodies as thus constituted:

No. 1.No. 2.No. 3.No. 4.No. 5.
● ●
○		● ○● ●
○ ○ ○		●
○ ○		● ●
○ ○ ○
○ ○
N2ONON2O3NO2N2O5

The black circles represent nitrogen atoms, and the open ones oxygen atoms; the symbols are placed below in order that their relation to the supposed atomic constitution may be obvious at a glance. While the symbol of a compound must always accord with its percentage composition, the latter does of itself determine the symbol or formula. A number of other circumstances, which cannot here be discussed, are taken into account as evidence of the constitution of the molecule.

This digression on chemical formulæ will, it is hoped, enable the general reader, who may not previously have been acquainted with them, to perceive their significance, instead of passing them over as unintelligible cabalistic letters when they occur in the following pages. With this object, it may be added that the elements, hydrogen, carbon, and chlorine, are respectively represented by H, C, and Cl; and that the proportional quantities, which are also implied in the symbols, and are those by which H, C, and Cl combine with other bodies, are 1, 12, and 35·5 respectively. Another point which should be understood is that the properties and behaviour of a chemical compound are different, and usually extremely different, from those of any of its constituents. This is well illustrated in the subject we are considering. Atmospheric air is a mixture (not a compound) of nitrogen and oxygen gases, and all its properties are intermediate between those of its ingredients taken separately. Nitrous oxide, N2O, has properties not possessed by either constituent separately. For example, it is very soluble in water, whereas oxygen is very slightly so, and nitrogen still less. The other compounds we have referred to differ widely from nitrous oxide and from each other in their properties.

Nitrous oxide is an invisible gas, having a slightly sweetish taste and smell. It is dissolved by water, which, at ordinary temperatures, takes up about three-fourths of its volume of the gas. By cold and great pressure the gas may be condensed into a colourless liquid. The gas is obtained in a pure state by gently heating the salt called ammonium nitrate, which is formed by neutralizing pure nitric acid with carbonate of ammonia. The action which occurs may be explained thus: the hydrogen of the ammonium unites with a portion of the oxygen of the nitric acid, forming water, whilst the remainder of the oxygen combines with the nitrogen. As chemical actions are regarded as either separations or unions of atoms, they can be expressed by what is called a chemical equation, the left-hand side of which shows the arrangement of the atoms before the action, and the right-hand side the arrangement after it, the sign of equality being read as “produce” or “produces.” But the validity of the equations, like that of the symbolic formulæ, is quite independent of the existence of atoms; for the equation always rests on certain facts, namely, the relations between the quantities of the substances which enter into, and those which are produced by, a chemical action. Thus, in the present case the action may be symbolically expressed as follows: