The atomic theory of Dalton, which had just been announced to the scientific world, moreover, required that the elements contained in a compound should be in the ratio of either their atomic weights or of simple multiples of these. If this were true of air, then there should have been found 22"2 per cent. of oxygen by weight instead of the 23 per cent. which it contains.
Again it had been observed that when two gases combine with each other, there is either a rise or fall in temperature, and generally a contraction of volume, but no such changes were noticeable when air was artificially made by mixing its constituents in the proper proportions.
It was evident, then, that air is a mixture and not a compound.
Nor is the composition of air absolutely constant. By the aid of more refined analytical methods than had been employed before that time, Bunsen, in 1846, succeeded in detecting slight variations in the percentage of oxygen, in samples of air collected in different places and at different times. His figures vary between 20"84 per cent. and 2o'96 per cent., and they have been abundantly confirmed by later investigations.
It requires but little reflection to convince us that the composition of air cannot remain constant, and that other gases besides those I have mentioned must be present in it. Consider the many agencies constantly at work to deprive
In the preparation of this lecture free use has been made of the standard works on the history of chemistry, and especially the writings of Professor Ramsay on this subject.