ADDRESS. 5^ 



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 assump- 

 tion of a certain view as a working hypothesis, and the subsequent in- 

 stitution 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 generalisations.' 



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 motions. 



As regards the size and shape of the atoms, Dalton ofi'ered no opinion, 

 for he had no experimental grounds on which to form it, believing 

 that they were inconceivably small and altogether beyond the grasp of 

 our senses aided by the most powerful appliances of art. He was in the 

 habit of representing his atoms and their combinations diagrammatically 

 as round discs 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' symbols are 

 horrifying. A young student in chemistry might as soon learn Hebrew 

 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 conclusion that the diameter of an 

 atom of oxygen or nitrogen was io,o(!o,ooo of a centimetre. With the 

 highest known magnifying power we can distinguish the 40^00 P^^t of a 

 centimetre ; 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 nitrogen. 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 5000,000 and greater than f;oo b,ooo,ooo o^ ^ centimetre ; or, to put 

 it in language more suited to the ordinary mind, Thomson asks us to 

 imagine a drop of water magnified up to the size of the earth, and then 

 tells us that the coarseness of the graining of such a mass would be 



