Part 9 (1/2)
In the same year a paper by Thomson appeared in the _Philosophical Transactions_, wherein it was experimentally proved that oxalic acid combines with strontia to form two distinct compounds, one of which contains twice as much oxalic acid as the other, the amount of strontia being the same in both. a.n.a.lyses of the oxalates of potash, published about the same time by Wollaston, afforded another ill.u.s.tration of the _law of multiple proportions_, and drew the attention of chemists to Dalton's theory. But the new theory was opposed by several very eminent chemists, notably by Sir Humphry Davy. In the autumn of 1807 Wollaston, Thomson and Davy were present at the dinner of the Royal Society Club, at the Crown and Anchor, in the Strand. After dinner, these three chemists discussed the new theory for an hour and a half, Wollaston and Thomson trying to convince Davy of the truth of Dalton's theory; but ”so far from being convinced, he went away, if possible, more prejudiced against it than ever.”
Soon after this Wollaston succeeded in convincing Mr. Davis Gilbert (afterwards President of the Royal Society) of the justness of the atomic theory, and he in turn so placed the facts and the reasoning before Davy, that from this time he became a supporter of the new theory.
In order that the atomic theory should be fruitful of results, it was now necessary that the values of the atomic weights of many elements should be carefully determined.
Let us consider what knowledge must be acquired before the value to be a.s.signed to the atomic weight of an element can be found.
Hydrogen was the element chosen as a standard by Dalton. He a.s.sumed that the atom of hydrogen weighs 1; the atomic weight of any other element is therefore a number which tells how many times the atom of that element is heavier than the atom of hydrogen. Thus, when Dalton said the atomic weight of oxygen is 8, he meant that the atom of oxygen is eight times heavier than that of hydrogen. How was this number obtained?
Accurate a.n.a.lyses of water show that in this liquid one part by weight of hydrogen is combined with eight parts by weight of oxygen; but (it is said) as the atom of hydrogen weighs 1, the atom of oxygen must weigh 8. In drawing this conclusion it is a.s.sumed that the atom, or smallest particle, of water is built up of one atom of hydrogen and one atom of oxygen. Let it be a.s.sumed that the atom of water contains two atoms of hydrogen and one of oxygen, then the latter atom must weigh sixteen times as much as each atom of hydrogen; let it be a.s.sumed that three atoms of hydrogen combine with one atom of oxygen to form an atom of water, then the weight of the oxygen atom must be twenty-four times that of the hydrogen atom. Any one of these a.s.sumptions will equally satisfy the figures obtained by a.n.a.lyzing water (1: 8 = 2: 16 = 3: 24). Now, had we any method whereby we could determine how many times an atom of water is heavier than an atom of hydrogen we should be able to determine which of the foregoing a.s.sumptions is correct, and therefore to determine the atomic weight of oxygen. Hence, before the atomic weight of an element can be determined, there must be found some method for determining the atomic weights of compounds of that element.
Unless this can be done the atomic theory is of little avail in chemistry.
I conceive it to be one of the signal merits of Dalton that he so clearly lays down rules, the best which could be devised at his time, for determining the atomic weights of compounds, or, what is the same thing, for determining the number of elementary atoms in one atom of any compound. In his ”New System” he says that he wishes to show the importance of ascertaining ”the relative weights of the ultimate particles both of simple and compound bodies, the number of simple elementary particles which const.i.tute one compound particle, and the number of less compound particles which enter into the formation of one more compound particle.”
Considering compounds of two elements, he divides these into binary, ternary, quaternary, etc., according as the compound atom contains two, three, four, etc., atoms of the elements. He then proceeds thus--
”The following general rules may be adopted as guides in all our investigations respecting chemical synthesis:--
”1st. When only one combination of two bodies can be obtained, it must be presumed to be a _binary_ one, unless some cause appear to the contrary.
”2nd. When two combinations are observed, they must be presumed to be a _binary_ and a _ternary_.
”3rd. When three combinations are obtained, we may expect one to be _binary_ and the other two _ternary_.
”4th. When four combinations are observed, we should expect one _binary_, two _ternary_, and one _quaternary_,” etc.
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 a.n.a.lytical 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 a.s.sertion 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 a.s.suming 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; a.s.sume 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 ma.s.s of water vapour was conceived by Dalton as ”like a ma.s.s of small shot.” Each shot exhibited the characteristic chemical properties of water vapour; it differed from the large quant.i.ty of vapour only in ma.s.s; 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, pota.s.sium carbonate; I, of potash; and K, an atom of soda.
[Ill.u.s.tration: Fig. 3.]
But I think if we consider this application of the term ”atom” to elements and compounds alike, we shall see objections to it. When an atom of a compound is divided the smaller particles so produced are each very different in chemical properties from the atom which has just been divided.
We may, if we choose, a.s.sume that the atom of an element could in like manner be divided, and that the products of this division would be different from the elementary atoms; but such a division of an elementary atom has not as a matter of fact been yet accomplished, unless we cla.s.s among elements substances such as potash and soda, which for many years were universally regarded as elements, and rightly so regarded because they had not been decomposed. In Dalton's nomenclature then, the term ”atom” is applied alike to a small particle with definite properties known to be divisible into smaller particles, each with properties different from those of the undivided particle, and to a small particle which, so far as our knowledge goes, cannot be divided into any particle smaller than or different from itself.
Nevertheless, if the atomic theory was to be victorious, it was necessary that it should be applied to elements and compounds alike. Until a clear conception should be obtained, and expressed in accurate language, of the differences in structure of the ultimate particles of compounds and of elements, it was perhaps better to apply the term ”atom” to both alike.
These two difficulties--(1) the difficulty of attaching to the term ”atom”