Liebig left Paris in 1824. By the year 1830 he had perfected and applied that method for the analysis of organic compounds which is now in constant use wherever organic chemistry is studied; by the same year Dumas had given the first warning of the attack which he was about to make on the great structure of dualism raised by Berzelius. In a paper, "On Some Points of the Atomic Theory," published in 1826, Dumas adopted the distinction made by Avogadro between molecules and atoms, or between the small particles of substances which remain undivided during physical actions, and the particles, smaller than these, which are undivided during chemical actions. But, unfortunately, Dumas did not mark these two conceptions by names sufficiently definite to enable the readers of his memoir to bear the distinction clearly in mind. The terms "atom" and "molecule" were not introduced into chemistry with the precise meanings now attached to them until some time after 1826. Although the idea of two orders of small particles underlies all the experimental work described by Dumas in this paper, yet the numbers which he obtained as representing the actual atomic weights of several elements--_e.g._ phosphorus, arsenic, tin, silicon--show that he had not himself carried out Avogadro's hypothesis to its legitimate conclusions. Two years after this Dumas employed the reaction wherein two volumes of gaseous hydrochloric acid are produced by the union of one volume of hydrogen with one volume of chlorine, as an argument which obliged him to conclude that, if Avogadro's physical hypothesis be accepted, the molecules of hydrogen and chlorine split, each into two parts, when these gases combine chemically. But Dumas did not at this time conclude that the molecular weight of hydrogen must be taken as twice its atomic weight, and that--hydrogen being the standard substance--the molecular weights of all gases must be represented by the specific gravities of these gases, referred to hydrogen as 2. I have already shortly discussed the method for finding the relative weights of elementary atoms which is founded on Avogadro's hypothesis, and, I think, have shown that this hypothesis leads to the definition of "atom" as the smallest amount of an element in one molecule of any compound of that element (see p. 142). This deduction from Avogadro's law is now a part and parcel of our general chemical knowledge. We wonder why it was not made by Dumas; but we must remember that a great mass of facts has been accumulated since 1826, and that this definition of "atom" has been gradually forced on chemists by the cumulative evidence of those facts. One thing Dumas did do, for which the thanks of every chemist ought to be given him; he saw the need of a convenient method for determining the densities of compounds in the gaseous state, and he supplied this need by that simple, elegant and trustworthy method, still in constant use, known as _Dumas's vapour density process_.