John Dalton's investigations concerned with the atomic theory in chemistry, with which his name is inseparably associated, are the most important of all.

It has been proposed that this theory was suggested to him either by researches on ethylene (olefiant gas) and methane (carburetted hydrogen) or by analysis of nitrous oxide (protoxide of azote) and nitrogen dioxide (deutoxide of azote), both views resting on the authority of Thomas Thomson.

However, a study of Dalton's own laboratory notebooks, discovered in the rooms of the Lit & Phil, concluded that so far from Dalton being led by his search for an explanation of the law of multiple proportions to the idea that chemical combination consists in the interaction of atoms of definite and characteristic weight, the idea of atoms arose in his mind as a purely physical concept, forced upon him by study of the physical properties of the atmosphere and other gases.

The first published indications of this idea are to be found at the end of his paper on the absorption of gases already mentioned, which is read on October 21, 1803, though it will not be published until 1805.

Here he says: Why does not water admit its bulk of every kind of gas alike?

This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases.

Dalton proceeds to print his first published table of relative atomic weights.

Six elements appear in this table, namely hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen conventionally assumed to weigh 1.

Dalton also begins using symbols to represent the atoms of different elements.

In his laboratory notebook under the date 6 September 1803, there appears a list in which he sets out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc., by chemists of the time.

It appears, then, that confronted with the problem of calculating the relative diameter of the atoms of which, he was convinced, all gases were made, Dalton has used the results of chemical analysis to propound his atomic theory of matter.