Sept. I, 1887] 



NATURE 



417 



forces, even though we cannot in every case explain its pheno- 

 mena in terms of these forces ; that whether these tend to con- 

 tinue or to end existence depends upon their nature and amount, 

 and that disease and death are as much a consequence of the 

 operation of chemical and physical laws as are health and 

 life. 



Loolving back again to our point of departure fifty years ago, 

 let us for a moment glance at Dalton's labours, and compare his 

 views and those of his contemporaries with the ideas which now 

 prevail. In the first place it is well to remember that the key- 

 stone of his atomic theory lies not so much in the idea of the 

 existence and the indivisible nature of the particles of matter — 

 though this idea « as so firmly implanted in his mind that, being 

 questioned on one occasion on the subject, he said 10 his friend 

 the late Mr. Ransome, "Thou knowst it must be so, for no man 

 can split an atom " — as in the assumption that the weights of these 

 particles are different. Thus whilst each of the ultimate jjarticles 

 of oxygen has the same weight as every other particle of oxygen, 

 and each atom of hydrogen, for example, has the same weight 

 as every other particle of hydrogen, the oxygen atom is sixteen 

 times heavier than that of hydrogen, and so on for the atoms of 

 every chemical element, each having its own special weight. It 

 was this discovery of Dalton, together with the further one that 

 the elements combine in the proportions indicated by the relative 

 weights of their atoms or in multiples of these proportions, which 

 at once changed chemistry from a qualitative to a quantitative 

 science, making the old invocation prophetic, "Thou hast ordered 

 all things in measure and number and weight." 



The researches of chemists and physicists during the last fifty 

 years have not only strengthened but broadened the foundations 

 of the great Manchester philosopher's discoveries. It is true that 

 his original numbers, obtained by crude and inaccurate methods, 

 have been replaced by more exact figures, but his laws of com- 

 bination and his atomic explanation of those laws stand as the 

 great bulwarks of our science. 



On the present occasion it is interesting to remember that 

 within a stone's-throw of this place is the small room belonging 

 to our Literary and Philosophical Society which served Dalton 

 as his laboratory. Here, with the simplest of all possible 

 apparatus — a few cups, penny ink-bottles, rough balances, and 

 self-made thermometers and barometers — Dalton accomplished 

 his great results. Here he patiently worked, marshalling facts 

 to support his great theory, for as an explanation of his laborious 

 experimental investigations the wise old man 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 1 can attest by my own experience." Nor ought we 

 when here assembled to forget that the last three of Dalton's 

 experimental es- ay.-. — one of which, on a new method of measuring 

 water of crystallization, contained more than the germ of a great 

 discovery — were communicated to our Chemical Section in 1842, 

 and that this was the last memorable act of his scientific life. In 

 this last of his Contributions to science, as in his first, his method 

 of procedure was that which has been marked out as the most 

 fruitful by almost all the great searchers after Nature's secrets ; 

 namely, the assumption of a certain view as a working hypo- 

 thesis, and the subsequent institution of experiment to bring this 

 hypothesis to a test of reality upon which a legitimate theory is 

 afterwards to be based. "Dalton," as Henry well says, 

 "valued detailed facts mainly, if not solely, as the stepping- 

 stones to comprehensive generalizations." 



Next let us ask what light the research of the last fifty years 

 has thrown on the subject of the Daltonian atoms : first, as 

 regards their size ; secondly, in respect to their indivisibility and 

 mutual relationships ; and thirdly, as regards their motion^-. 



As regards the size and shape of the atoms, Dalton offered 

 no opinion, for he had no experimental grounds on which to 

 form i^, believing that they were inconceivably small and alto- 

 gether beyond the grasp of our senses aided by the most power- 

 ful appliances of art. He was in the habit ofrepresenting his atoms 

 and their combinaiions diagrammatically as round disks or spheres 

 made of wood, by means of which he was fond of illustrating his 

 theory. But such mechanical illustrations are not without their 

 danger, for I well remember the answer given by a pupil to a 

 question on the atomic theory: "Atoms are round balls of 

 wood invented by Dr. Dalton." So determinedly indeed did 

 he adhere to his mechanical method of representing the chemical 

 atoms and their combinations that he could not be prevailed 

 upon to adopt the system of chemical formulae introduced by 

 Berzelius and now universally employed. In a letter addressed 



to Graham in April 1837, he writes : " Berzelius's symbols are 

 horrifying. A young student in chemistry might as soon learn 

 Ilebi-ew as make himself acquainted with them." And again: 

 "They appear to me equally to perplex the adepts in science, 

 to discourage the learner, as well as to cloud the beauty and 

 simplicity of the atomic theory." 



But modern research has accomplished, as regards the size of 

 the atom, at any rate to a certain extent, what Dalton regarded as 

 impossible. Thus in 1865, Loschmidt, of Vienna, by a train of 

 reasoning which I cannot now stop to explain, came to the con- 

 clusion that the diameter of an atom of oxygen or nitrogen was 

 1/10,000,000 of a centimetre. With the highest known magni- 

 fying power we can distinguish the 1/40,000 part of a centi- 

 metre ; if now we imagine a cubic box each of whose sides has 

 the above length, such a box when filled with air will contain 

 from 60 to 100 millions of atoms of oxygen and nitro^ien. A 

 few years later William Thomson extended the methods of 

 atomic measurement, and came to the conclusion that the 

 distance between the centres of contiguous molecules is less than 

 1/5,000,000 and greater than 1/1,000,000,000 of a centimetre ; 

 or, to put it in language more suited to the ordinary/ mind, 

 Thomson asks us to imagine a drop of w ater magnified up to 

 the size of the earth, and then tells us that the coarseness of the 

 graining of such a mass would be something between a heap of 

 small shot and a heap of cricket balls. Or again, to take 

 Clifford's illustration, you know that our best microscopes 

 magnify from 6000 to 8000 times ; a microscope which would 

 magnify that result as much again would show the molecular 

 structure of water. Or again, to put it in another form, if we 

 suppose that the minutest organism we can now see were pro- 

 vided with equally powerful microscopes, these beings would be 

 able to see the atoms. 



Next, as to the indivisibility of the atom, involving also the 

 question as to the relationships between the atoaiic weights and 

 properties of the several elementary bodies. 



Taking Dalton's aphorism, " Tnou knowst no man can split 

 an atom," as expressing the view of the enunciator of the 

 atomic theory, let us see how far this idea is borne out by sub- 

 sequent work. In the first place, Thomas Thomson, the first 

 exponent of Dalton's generalization, was torn by conflicting 

 beliefs until he found peace in the hypothesis of Prout, that the 

 atomic weights of all the so-called elements are multiples of a 

 common unit, which doctrine he sought to establish, as Thorpe 

 remarks, by some of the very worst quantitative determinations 

 to be found in chemical literature, though here I may add that 

 they were not so incorrect as (Walton's original numbers. 



Coming down to a somewhat later date, Graham, whose life 

 was devoted to finding what the motion of an atom was, freed 

 himself from the bondage of the Daltonian aphorism, and 

 defined the atom not as a thing which cannot be divided, but as 

 one which has not been divided. With him, as with Lucretius, 

 as Angus Smith remarks, the original atom may be far down. 



But speculative ideas respecting the constitution of matter 

 have been the scientific relaxation of many minds from olden 

 time to the present. In the mind of the early Greek the action 

 of the atom as one substance taking various forms by unlimited 

 combinations was sufficient to account for all the phenomena of 

 the world. And Dalton himself, though upholding the indi- 

 visibility of his ultimate particles, says : " We do not know that 

 any of the bodies denominated elementary are absolutely inde- 

 composable." Again, Boyle, treating of the origin of form and 

 quality, says : " There is one universal matter common to all 

 bodies — an extended divisible and impenetrable substance." 

 Then Graham in another place expresses a similar thought when 

 he writes : "It is conceivable that the various kinds of matter 

 now recognized as different elementary substances may possos 

 one and the same ultimate or atomic molecules existing in 

 different conditions of movement. The essential unity of matter 

 is an hypothesis in harmony with the equal actijD of gravity 

 upon all bodies." 



What experimental evidence is now before us bearing upon 

 these interesting speculations ? In the first place, then, the space 

 of fifty years has completely changed the face of the inquiry. 

 Not only has the number of distinct well-established elementary 

 bodies increased from fifty-three in 1837 to seventy in 1887 (not 

 including the tiuenty or more new elements recently said to have 

 been discovered by Kriiss and Nilson in certain rare Scandi- 

 navian minerals), but the properties of these elements have been 

 studied, and are now known to us with a degree of precision 

 then undreamt of. So that relationships existing between these 



