All the analytical investigations undertaken for nearly a century lead us to assume that compound substances possess a definite, constant, invariable, composition. To the ratios deduced from these researches the names of proportional numbers, of chemical proportions, of chemical equivalents, of atoms have been given in turn. The illustrious Berzelius devoted a great part of his life to fixing the weights of the chemical proportions. His investigations on this subject will remain an imperishable monument to his sagacity and genius. The minute and repeated checks to which I have had the assurance or temerity to submit them, have convinced me that his analytical skill has never been surpassed, if indeed it has ever been equalled. Berzelius, we know, concluded from his researches that there is no simple relation between the weights of the atoms of substances; he remained all his life convinced of this truth.
As early as 1815, Dr. William Prout, in a memoir entitled "On the relation between the specific gravities of bodies in their gaseous state and the weights of their atoms" propounded the idea that the atomic weights of the substances which at that time had been well determined could be represented by multiples of the atomic weight of hydrogen. Prout had so little faith in the exactness of his hypothesis that he published his paper under the veil of anonymity. Whatever may be the opinion one holds about this hypothesis, whatever fate may be reserved for it in the future, it is impossible not to pay homage to the rare penetration of its author. Right or wrong, it permitted him to divine the specific weight of hydrogen forty years before it was determined experimentally with any degree of accuracy.
From the point of view of the philosophy of nature the import of Prout's idea is immense. Those components of compound substances which we regard as elementary on account of their absolute immutability for us, would themselves be only compounds. Such elements, whose discovery is the glory of Lavoisier and has immortalised his name, can thus be considered as being derived from the condensation of a unique substance: we are thus naturally conducted to the unity of matter, although we in fact observe its plurality, its multiplicity.
In England the hypothesis of Dr Prout was almost universally accepted as absolute truth. The work executed by Professor Thomas Thomson of Glasgow in order to base it on analytical experiments, greatly contributed to this result. Nevertheless, the same thing did not hold for Germany or France. The immense prestige which surrounded the name of Berzelius and legitimate confidence inspired by his work on the weights of the atoms were incontestably the cause of this. Even in England doubts arose: as early as 1833 Professor Turner, on the invitation of the British Association for the Advancement of Science, undertook a series of analyses, from which he drew the conclusion that the hypothesis of Dr Prout was not exact. In 1839, Professor Penny of Glasgow arrived at the same conclusion, although his results differed in some points from those obtained by Turner. The determination of the atomic weight of carbon made by M. Dumas and myself in 1839-40 called the attention of chemists anew to this subject. The atom of carbon deduced from our syntheses of carbonic acid agrees in fact entirely with Prout's hypothesis. The new syntheses of water which M. Dumas published in 1843, and which were confirmed by the work of MM. Erdmann and Marchand, led to the same result.
In 1842 and in 1843, M. de Marignac of Geneva undertook a series of researches on chlorine, bromine, iodine, nitrogen, silver, and potassium for the purpose of submitting Prout's law to a new and careful examination. It has been shown in the most certain fashion by the Genevan chemist, whose accuracy is generally recognised, and to whose skill and conscientiousness Berzelius has paid a justly merited tribute, that the unit adopted by Prout is twice too great as far as chlorine is concerned. But he declared himself to be of the firm opinion that taking into account the extreme difficulty of experimentally attaining results of absolute exactness, we cannot consider as contrary to Prout's law the atoms he has found for silver, potassium, bromine, iodine, and nitrogen. Thus, according to M. de Marignac, the principle may be exact, only the unit should be diminished by half.
At the end of 1857 M. Dumas published a Memoir on the Equivalents of the Elements. As might be expected this memoir produced a great sensation. The illustrious French chemist, taking as basis M. de Marignac's researches on silver, found that all well known substances which he had occasion to examine, perfectly conform to the principle of Prout, when certain modifications are applied to this principle. According to him, the elements are multiples of hydrogen by 1, or by 0.5, or finally by 0.25. My illustrious master accompanied his work by philosophical considerations which though very important have no connection with the question I propose to treat.
Such, at the present time, is the state of knowledge regarding the relations between the atomic weights of the elements.
For a great number of years I have dedicated all my leisure to elucidate this problem. I confess frankly that when I undertook these researches, I had an almost absolute confidence in the exactness of Prout's principle. The doubt at which I had hinted in my work on the atom of carbon, in 1845, had its origin in the result of two determinations which were in poor agreement with my other syntheses.
In my investigations I have dealt with chlorine, bromine, fluorine, sulphur, nitrogen, potassium, sodium, lithium, calcium, barium, lead, and silver. In the course of the long and laborious researches to which I have devoted myself, it may easily be conceived that I obtained successively the most discordant results, according to the substances and methods I employed. Sometimes these results were in absolute agreement with the principle of Prout: this was the case for example with lead, silver, sodium, and calcium determined as a function of the atom of carbon; sometimes they were completely irreconcilable with the law: such was the case of lead determined as a function of the atom of nitrogen and of sulphur; of silver determined as a function of nitrogen and of chlorine; of potassium determined with regard to oxygen and chlorine.
I dare affirm without fear of being contradicted that any chemist who has pursued research on the weights of the atoms, if he has varied his methods, if he has determined the weight of the atom of one substance as a function of the atom of two or three different substances, has encountered the same difficulties, the same contradictions. I accordingly experienced for long many a painful perplexity.
However, as I lately intimated to the Academy in requesting it to take note of my declaration, for some time all doubt has vanished from my mind. I have reached the complete conviction, the entire certainty, as far as certainty can be attained on such a subject, that Prout's law, with all the modifications due to M. Dumas, is nothing but an illusion, a pure hypothesis expressly contradicted by experiment. Chemists after examining the work which I have the honour of now presenting in detailed analysis to the Academy, if they can cast away their prejudices and mental prepossessions, and place their reliance on experiment, will soon share my conviction, namely, that there does not exist a common divisor for the weights of the elements which unite to form all definite compounds.
The paper which I now publish only embodies my researches on nitrogen, chlorine, sulphur, potassium, sodium, lead and silver. I chose these substances because generally they are the best known, form the stablest compounds, and because generally they have been regarded as obeying Prout's law. I have sufficiently emphasised that my aim was not to determine the weight of the atom of these substances, but rather the ratios of the weights of the atoms: in order to see if a common divisor exists for these ratios I have employed methods which in my opinion would most surely lead to a definite result. Whilst varying the methods I have preferred as far as possible to adopt those already employed by chemists recognised for their proved accuracy. This I did in order to obtain a more efficacious control. It is clearly evident that if results repeated sufficiently often agree with those previously obtained, the probability in favour of their exactness becomes very great; if on the contrary they differ, it is possible from the thorough study of all the conditions of experiment to find the means of assuring oneself on what side the truth lies. In this way I have succeeded in discovering errors which otherwise would have undoubtedly escaped me.
Although I only aimed at determining the ratios of numbers, I have nevertheless made two determinations, either of which would at once permit the deduction from all my experiments of the atom of these seven elements relatively to oxygen.
We shall see later that the weights of these atoms are in close accordance with those generally accepted, save in the case of nitrogen.
The determinations of the ratios have been made by way of synthesis, or by double decomposition. I have only had recourse to analysis to procure the data necessary for the calculation of the atomic weights as a function of oxygen.
If we cast our eyes on the atomic weights of ammonium and nitrogen we see that they differ by 4.02 instead of 4. From this it undubitably follows either that my syntheses of nitrate of silver are inaccurate and that the quantity of this salt produced from metallic silver is a little higher than what I found, which removes me still further from Prout's law, or else that the atomic weight of hydrogen itself is in error by 1/200 of its value.
My researches as a whole lead me to believe that the error exists rather in the atomic weight of hydrogen than in that of nitrogen.
If this opinion, to which with all reserve I would direct the attention of chemists, is correct--and I mean soon to investigate this point by carrying out the synthesis of water by a new method--it follows that the basis on which Dr Prout had erected in his law, is itself without foundation.
Whatever doubt I raise with regard to the accuracy of the atomic weight of hydrogen being represented by 1, oxygen being 8, there can remain no doubt concerning the principle itself. I conclude then by saying: as long as we hold to experiment for determining the laws which regulate matter, we must consider Prout's law as a pure illusion, and regard the undecomposable bodies of our globe as distinct entities having no simple relation by weight to one another. The incontestable analogy of properties observed amongst certain elements must be sought in other causes than those originating in the ratio of weight of their reacting masses.
Phil. Trans. for 1839, 13-33. [Alembic Club note.--CJG]
Ann. chim. phys.  1 (1841), 5-38. [Alembic Club note.--CJG]
Ibid.  8 (1843), 189-207. [Alembic Club note.--CJG]
J. pr. Ch. 26 (1842), 461-478. [Alembic Club note.- -CJG]
C. R. 14 (1842), 570-573; Bibl. Univ. 46 (1843), 350-377. [Alembic Club Note.--CJG]
Ann. chim. phys.  55 (1859), 129-210. [Alembic Club note.--CJG]
Brux. Acad. Sci. Bull. 16 (1849), 33. [Alembic Club note.--CJG]
The figures are:-- Ammonium, 18.06; nitrogen, 14.04. [This Alembic Club note quotes these figures from the lengthy results section omitted from its excerpt.--CJG]