Analyses of this compound show that it is composed of one part by weight of hydrogen and 4.66 parts by weight of nitrogen.
Dalton said one atom of hydrogen combines with one atom of nitrogen to form an atom of ammonia; hence an atom of nitrogen is 4.66 times heavier than an atom of hydrogen; in other words, if the _atomic weight_ of hydrogen is taken as unity, the _atomic weight_ of nitrogen is expressed by the number 4.66.Dalton referred the atomic weights of the elements to the atomic weight of hydrogen as unity, because hydrogen is lighter than any other substance; hence the numbers which tell how much heavier the atoms of the elements are than an atom of hydrogen are always greater than one, are always positive numbers.
When two elements unite in different proportions, by weight, to form more than one compound, Dalton supposed that (in most cases at any rate) one of the compounds is formed by the union of a single atom of each element; the next compound is formed by the union of one atom of the element which is present in smaller quantity with two, three, or more, atoms of the other element, and the next compound is formed by the union of one atom of the first element with a larger number (always, necessarily, a whole number) of atoms of the other element than is contained in the second compound; and so on.

From this assumption, and the Daltonian conception of the atom, it follows that the quantities by weight of one element which are found to unite with one and the same weight of another element must always be expressible as whole multiples of one number.

For if two elements, A and B, form a compound, that compound is formed, by supposition, of one atom of A and one atom of B; if more of B is added, at least one atom of B must be added; however much of B is added the quantity must be a whole number of atoms; and as every atom of B is the same in all respects as every other atom of B, the weights of B added to a constant weight of A must be whole multiples of the atomic weight of B.
The facts which were available in Dalton's time confirmed this deduction from the atomic theory within the limits of experimental errors; and the facts which have been established since Dalton's time are completely in keeping with the deduction.

Take, for instance, three compounds of the elements nitrogen and oxygen.

That one of the three which contains least oxygen is composed of 63.64 _per cent._ of nitrogen, and 36.36 _per cent._ of oxygen; if the atomic weight of nitrogen is taken to be 4.66, which is the weight of nitrogen that combines with one part by weight of hydrogen, then the weight of oxygen combined with 4.66 of nitrogen is 2.66 (63.64:36.36 = 4.66:2.66).