Heroes of Science: Chemists

by M. M. Pattison Muir

Only one compound of hydrogen and oxygen was then known; hence it was presumed to be a binary compound, i.e. a compound the smallest particle of which consisted of one atom of hydrogen and one atom of oxygen; and hence, from the data already given on page 130, it followed that the atomic weight of oxygen was 8. Two compounds of carbon and oxygen were known, each containing six parts by weight of carbon, in one case united with eight, and in the other case with sixteen parts by weight of oxygen. From Dalton's rules one of these was a binary, and the other a ternary compound; but as the atomic weight of oxygen had already been determined to be 8, that compound of carbon and oxygen containing eight of oxygen combined with six of carbon was decided to be binary, and that containing sixteen of oxygen (i.e. two atoms) to be ternary; and hence the atomic weight of carbon was determined to be 6.

In the second part of the "New System" Dalton, guided by these rules, determined experimentally the atomic weights of a great many substances; but this was not the kind of work suited to Dalton's genius. His analytical determinations were generally inaccurate; nevertheless, he clearly showed how the values of the atomic weights of elements ought to be established, and he obtained results sufficiently accurate to confirm his general theory. To make accurate determinations of the relative weights of elementary atoms was one of the tasks reserved for the great Swedish chemist Berzelius (see pp. 162-170). When we examine Dalton's rules we must confess that they appear somewhat arbitrary. He does not give reasons for his assertion that "when only one combination of two bodies can be obtained, it must be presumed to be a binary one." Why may it not be ternary or quaternary? Why must the atom of water be built up of one atom of hydrogen combined with one atom of oxygen? Or, when two compounds are known containing the same pair of elements, why must one be binary and the other ternary?

Or, even assuming that this must be justified by facts, does it follow that Dalton's interpretation of the atomic structure of the two oxides of carbon is necessarily correct? These oxides contain 6 of carbon + 8 of oxygen, and 6 of carbon + 16 of oxygen, respectively.

Take the second, 6: 16 = 3: 8; assume this to be a binary compound of one atom of oxygen (weighing 8) with one atom of carbon (weighing 3), then the other will be a ternary compound containing one atom of oxygen (8) and two atoms of carbon (6).

Hence it appears that Dalton's rules were too arbitrary, and that they were insufficient to determine with certainty the atomic weights of some of the elements. Nevertheless, without some such rules as those of Dalton, no great advances could have been made in applying the atomic theory to the facts of chemical combination; and Dalton's rules were undoubtedly founded on wide considerations. In the appendix to Volume II. of his "New System" he expressly states that before the number of atoms of two elements present in the atom of a compound can be determined, it is necessary that many combinations should be examined, not only of these elements with each other, but also of each of these with other elements; and he tells us that to gather together facts bearing on this general question of chemical synthesis was the object of his work from the time of the promulgation of the atomic theory.

When we find that Dalton applied the term "atom" to the small particles of compound bodies, we at once see that by atom he could not always mean "that which cannot be cut;" he simply meant the smallest particle of a substance which exhibits the properties of that substance.

A mass of water vapour was conceived by Dalton as "like a mass of small shot." Each shot exhibited the characteristic chemical properties of water vapour; it differed from the large quantity of vapour only in mass; but if one of these little pieces of shot were divided—as Dalton, of course, knew it could be divided—smaller pieces of matter would be produced. But these would no longer be water; they would be new kinds of matter. They are called oxygen and hydrogen.

As aids towards gaining a clear conception of the "atom" of a compound as a definite building, Dalton made diagrammatic representations of the hypothetical structures of some of these atoms: the following plate is copied from the "New System:"—A represents an atom of alum; B, an atom of nitrate of alumina; C, of barium chloride; D, of barium nitrate; E, of calcium chloride; F calcium nitrate; G, of calcium sulphate; H, potassium carbonate; I, of potash; and K, an atom of soda.

Fig. 3.