lO Meldrum, Development of the Atomic Theory. 



Experiments connected with the vapour pressure of water. 



A paper which Dalton read April i8th, 1800, marks 

 another stage on the way. The title, which is significant 

 of much, runs as follows : — " Experimental Essays, to 

 determine the Expansion of Gases by Heat, and the 

 maximnm of steam or aqueous vapour, which any gas of 

 a given temperature can admit of ; with observations on 

 the common or improved Steam Engines." 



On this title four remarks may be made, (i) Dalton 

 had arrived by April, 1800, at the idea, which forms 

 the central fundamental conception of the second and 

 third of the "Experimental Essays" of October, 1801, 

 of the vapour pressure of water. He had begun to con- 

 sider other gases besides the air, and knew that the 

 maximum of water vapour in any gas is independent of 

 the nature of the gas. It was in order to show this at 

 different temperatures that he began to measure the 

 " expansion of gases by heat." 



(2) There is a practical connection between the expan- 

 sion of gases by heat, and the original topic of the water 

 vapour in the atmosphere. Dalton's explanation of the 

 ■discrepancies betv/een the results of earlier workers on 

 the subject is that it " arose from the want of due care 

 to keep the apparatus and materials free from moisture." ^'^ 



(3) This paper, although passed for publication by the 

 Society, never appeared. Nothing remains of the " Obser- 

 vations on the common or improved Steam Engines." 



(4) Perhaps Dalton had discovered by April, 1800, 

 what we know as Charles' law, that different gases have 

 the same expansion by heat. But he does not make this 

 claim himself. The law forms the subject of the fourth 

 of the " Experimental Essays," and this fourth essay, 



^' Manchester Memoirs, [i], vol. 5, p. 596, 1802. 



