Part 10 (1/2)
But in Avogadro's law we have a far more accurate and trustworthy method for determining the molecular weights of compounds than any which Dalton was able to devise by his study of chemical combinations.
We have thus got a clearer conception of ”atom” than was generally possessed by chemists in the days of Dalton, and this we have gained by introducing the further conception of ”molecule” as that of a quant.i.ty of matter different from, and yet similar to, the atom.
The task now before us will for the most part consist in tracing the further development of the fundamental conception of Dalton, the conception, viz., of each chemical substance as built up of small parts possessing all the properties, other than the ma.s.s, of the whole; and--what we also owe to Dalton--the application of this conception to explain the facts of chemical combination.
The circ.u.mstances of Dalton's early life obliged him to trust largely to his own efforts for acquiring knowledge; and his determination not to accept facts at second hand but to acquire them for himself, is very marked throughout the whole of his life. In the preface to the second part of the ”New System” he says, ”Having been in my progress so often misled by taking for granted the results of others, I have determined to write as little as possible but what I can attest by my own experience.”
We should not expect such a man as this to make any great use of books; one of his friends tells us that he heard him declare on a public occasion that he could carry his library on his back, and yet had not read half of the books which comprised it.
The love of investigation which characterized Dalton when young would naturally be increased by this course of intellectual life. How strong this desire to examine everything for himself became, is amusingly ill.u.s.trated by a story told by his medical adviser, Dr. Ransome. Once when Dalton was suffering from catarrh Dr. Ransome had prescribed a James's powder, and finding his patient much better next day, he congratulated himself and Dalton on the good effects of the medicine. ”I do not well see how that can be,” said Dalton, ”as I kept the powder until I could have an opportunity of a.n.a.lyzing it.”
As Dalton grew older he became more than ever disinclined to place much trust in the results obtained by other naturalists, even when these men were acknowledged to be superior to himself in manipulative and experimental skill. Thus, as we have already learned, he could not be brought to allow the truth of Gay-Lussac's experimentally established law regarding gaseous combinations; he preferred to attribute Gay-Lussac's results to errors of experiment. ”The truth is, I believe, that gases do not unite in equal or exact measures in any one instance; when they appear to do so it is owing to the inaccuracy of our experiments.”
That Dalton did not rank high as an experimenter is evident from the many mistakes in matters of fact which are to be found in the second part of his ”New System.” A marked example of his inaccuracy in purely experimental work is to be found in the supposed proof given by him that charcoal, after being heated to redness, does not absorb gases. He strongly heated a quant.i.ty of charcoal, pulverized it, and placed it in a Florence flask, which was connected by means of a stopc.o.c.k with a bladder filled with carbonic acid: after a week he found that the flask and its contents had not sensibly increased in weight, and he concluded that no carbonic acid had been absorbed by the charcoal. But no trustworthy result could be obtained from an experiment in which the charcoal, having been deprived of air by heating, was again allowed to absorb air by being pulverized in an open vessel, and was then placed in a flask filled with air, communication between the carbonic acid and the external air being prevented merely by a piece of bladder, a material which is easily permeated by gases.
Dalton used a method which can only lead to notable results in natural science when employed by a really great thinker; he acquired a few facts, and then thought out the meaning of these. Almost at the beginning of each investigation he tried to get hold of some definite generalization, and _then_ he proceeded to ama.s.s special facts. The object which he kept before himself in his experimental work was to establish or to disprove this or that hypothesis. Every experiment was conducted with a clearly conceived aim. He was even willing to allow a large margin for errors of experiment if he could thereby bring the results within the scope of his hypothesis.
That the _law of multiple proportions_ is simply a generalization of facts, and may be stated apart from the atomic theory, is now generally admitted.
But in Dalton's mind this law seems to have arisen rather as a deduction from the theory of atoms than to have been gained as a generalization from experiments. He certainly always stated this law in the language of the atomic theory. In one of his walking excursions he explained his theory to a friend, and after expounding his views regarding atomic combinations, he said that the examples which he had given showed the necessary existence of the principle of multiple proportions: ”Thou knowest it must be so, for no man can split an atom.” We have seen that carburetted hydrogen was one of the compounds on the results of the a.n.a.lysis of which he built his atomic theory; yet we find him saying of the const.i.tution of this compound that ”no correct notion seems to have been formed till the atomic theory was introduced and applied in the investigation.”
When Dalton was meditating on the laws of chemical combination, a French chemist, M. Proust, published a.n.a.lyses of metallic oxides, which proved that when a metal forms two oxides the amount of metal in each is a fixed quant.i.ty--that there is a sudden jump, as it were, from one oxide to another. We are sometimes told that from these experiments Proust would have recognized the law of multiple proportions had his a.n.a.lyses only been more accurate; but we know that Dalton's a.n.a.lyses were very inaccurate, and yet he not only recognized the law of multiple proportions, but propounded and established the atomic theory. Something more than a correct system of keeping books and balancing accounts is wanted in natural science. Dalton's experimental results would be the despair of a systematic a.n.a.lyst, but from these Dalton's genius evolved that splendid theory which has done so much to advance the exact investigation of natural phenomena.
Probably no greater contrast could be found between methods of work, both leading to the establishment of scientific (that is, accurate and precise) results, than that which exists between the method of Dalton and the method pursued by Priestley.
Priestley commenced his experiments with no particular aim in view; sometimes he wanted to amuse himself, sometimes he thought he might light upon a discovery of importance, sometimes his curiosity incited him to experiment. When he got facts he made no profound generalizations; he was content to interpret his results by the help of the prevailing theory of his time. But each new fact only spurred him on to make fresh incursions into the fields of Nature. Dalton thought much and deeply; his experimentally established facts were to him symbols of unseen powers. He used facts as Hobbes says the wise man uses words: they were his counters only, not his money.
When we ask how it was that Dalton acquired his great power of penetrating beneath the surface of things and finding general laws, we must attribute this power in part to the training which he gave himself in physical science. It was from a consideration of physical facts that he gained the conception of ultimate particles of definite weight. His method was essentially dynamical; that is, he pictured a gas as a ma.s.s of little particles, each of which acted on and was acted on by, other particles. The particles were not thrown together anyhow; definite forces existed between them. Each elementary or compound gas was pictured as a system of little particles, and the properties of that gas were regarded as dependent on the nature and arrangement of these particles. Such a conception as this could only be gained by a careful and profound thinker versed in the methods of physical and mathematical science. Thus we see that although Dalton appeared to gain his great chemical results by a method which we are not generally inclined to regard as the method of natural science, yet it was by virtue of his careful training in a branch of knowledge which deals with facts, as well as in that science which deduces particular conclusions from general principles, that he was able to introduce his fruitful conceptions into the science of chemistry.
To me it appears that Dalton was pre-eminently distinguished by the possession of imagination. He formed clear mental images of the phenomena which he studied, and these images he was able to combine and modify so that there resulted a new image containing in itself all the essential parts of each separate picture which he had previously formed.
From his intense devotion to the pursuit of science the development of Dalton's general character appears to have been somewhat dwarfed. Although he possessed imagination, it was the imagination of a naturalist rather than that of a man of broad culture. Perhaps it was a want of broad sympathies which made him trust so implicitly in his own work and so readily distrust the work of others, and which moreover led him astray in so many of his purely experimental investigations.
Dalton began his chemical work about six years after the death of Lavoisier. Unlike that great philosopher he cared nothing for political life. The friends in whose family he spent the greater part of his life in Manchester were never able to tell whether he was Whig or Tory. Unlike Priestley he was content to let metaphysical and theological speculation alone. In his quiet devotion to study he more resembled Black, and in his method, which was more deductive than that usually employed in chemistry, he also resembled the Edinburgh professor. Trained from his earliest days to depend on himself, nurtured in the creedless creed of the Friends, he entered on his life's work with few prejudices, if without much profound knowledge of what had been done before him. By the power of his insight into Nature and the concentration of his thought, he drew aside the curtain which hung between the seen and the unseen; and while Herschel, sweeping the heavens with his telescope and night by night bringing new worlds within the sphere of knowledge, was overpowering men's minds by new conceptions of the infinitely great, John Dalton, with like imaginative power, was examining the architecture of the ultimate particles of matter, and revealing the existence of law and order in the domain of the infinitely small.
FOOTNOTES:
[7] See Fig. 2, which is copied from the original in the ”New System of Chemical Philosophy,” and ill.u.s.trates Dalton's conception of a quant.i.ty of carbonic acid gas, each atom built up of one atom of carbon and two of oxygen; of nitrous oxide gas, each atom composed of one atom of nitrogen and one of oxygen; and of hydrogen gas, const.i.tuted of single atoms.
[8] More accurate a.n.a.lysis has shown that there are six parts of carbon united respectively with one and with two parts by weight of hydrogen in these compounds.
CHAPTER IV.
ESTABLISHMENT OF GENERAL PRINCIPLES OF CHEMICAL SCIENCE (_continued_)--PERIOD OF DAVY AND BERZELIUS.
_Humphry Davy_, 1778-1829. _Johann Jacob Berzelius_, 1779-1848.
We may roughly date the period of chemical advance during which the connections between chemistry and other branches of natural knowledge were recognized and studied, as beginning with the first year of this century, and as continuing to our own day.
The elaboration of the atomic theory was busily carried on during the second and third decades of this century; to this the labour of the Swedish chemist Berzelius largely contributed.