HISTORY OF THE ATOMIC THEORY. 33 



did not do so, on which he says, " If the particles of water 

 during the agitation had not actually communicated their 

 heat, the hot ones ought to have risen to the top, and the 

 cold ones subsided, so as to have made a material difference 

 in the temperature." This shows, that even at that period 

 he was accustomed to think habitually of matter as decidedly 

 atomic in its constitution. 



On the theoretical conclusion to be drawn here, we find his 

 genius taking the lead ; he is accurate in spite of the rudeness 

 of his experiments. He concludes that water conducts heat 

 a little, and that the expansion of water is the same both above 

 and below the point of maximum density. But when he comes 

 to determine the precise place at which that point is found, as 

 it is a matter of experiment, and cannot be got by the mind 

 only, he is at fault; in subsequent experiments learning to 

 become accurate. 



He seems to have lowered the point to 36°, and after- 

 wards considered it 38°, the point now apparently fixed on 

 is 39°, or 39.101. (Playfair and Joule.) Dr. Hope's experi- 

 ments gave it as between 39^ and 40 degrees. In this 

 investigation Dalton's mind again analyses itself, dividing 

 to great clearness of conception on the one side, and 

 carelessness of minute observation on the other. 



In 1830 on reading over some old letters which he was 

 arranging, he found one from Dr. Hope, saying, " notwith- 

 standing the caution you gave me, I venture to publish my 

 pamphlet on the contraction of water by heat,'* Dr. Dalton 

 said, "aye, he had the advantage of me there, but not so 

 much as it appeared at first sight." 



In this paper he makes an observation on the power of 

 capillary tubes to prevent the freezing of water, a circum- 

 stance which has not been thoroughly inquired into, nor the 

 cause assigned its proper place. 



In May, 1800, Dalton was elected secretary of the 

 Literary and Philosophical Society of Manchester, in the 

 F 



