It has been held that Dalton anticipated Avogadro in assuming that all gases contain equal numbers of “atoms” (molecules). Dr. Debus adopts this view, and assumes that Dalton in 1801 made use of it to explain the phenomena of the diffusion of gases, and that this idea, along with his early experiments on nitric oxide and oxygen, led to his atomic theory. Roscoe and Harden, on the other hand, held that Dalton never definitely held the view that equal volumes of gases contain an equal number of atoms (molecules), nor had such a conception any bearing upon his explanation of the facts of diffusion. Dissatisfied with the theory which he did hold (viz. the repulsion exerted by an atom on all others of its kind, but not on atoms of a different kind), he was led to consider the behaviour of atoms of unequal size, and finding an agreement with the observed facts, he then sought for means of determining whether or not the atoms were actually of unequal size, and so was led to the further developments of the theory. The weight of the evidence goes to show that Dalton arrived at his theory in the latter half of 1803, and that assumption on which it turned was “that no two species [of pure elastic fluids] agree in the size of their particles.” There is no clear indication that Dalton ever imagined that the simple gases were diatomic in structure, which, of course, is the main point in Avogadro’s hypothesis. It seems necessary to set out this matter in some detail as misapprehension appears to exist, especially in Germany, as to the merits of the controversy which arose after the appearance of Roscoe and Harden’s work. * * * * *