John D alt on. 205 



Bernoulli, brought up again by Herapath, and so thoroughly 

 studied out by Joule, Clausius, and Maxwell, as to be now 

 almost universally held to be a quality of such molecules as 

 the gases known to us are made of. After all, repulsion is a 

 moving power if not motion, and the result is the same ; but 

 still Dalton's idea does not explain so many conditions as 

 that of Bernoulli would explain ; but Bernoulli did not use 

 his ideas for the larger ends seen by Dalton. The idea of re- 

 pulsion without contact is less intelligible than that of constant 

 motion and impact. To attain repulsion it was necessary 

 to have atoms, the old Greek atoms, the atoms of Lucretius, 

 the atoms defined more exactly by Newton as to external 

 qualities. To these, every exact thinker has been brought 

 when picturing matter; if we give up atoms we come to 

 indefinite bodies, and pass the boundaries of the intelligible. 

 We do not say that there is nothing beyond this boundary. 

 On the contrary, we know that there must be something, 

 but every thinker that has passed into this land of the 

 indefinite has been penetrated with the same quality, and 

 lost himself in seeking even the smallest portion of rest for 

 the soles of his feet ; even non-atomists find a stage in 

 which atoms, or at least undecomposable molecules, phy- 

 sically the same to us at present, exist. 



Dalton was led to seek ' a true theory of evaporation.' 

 The separate action of vapour of water and other gases 

 was an opinion held, as Dalton himself says, by M. Pictet 

 and others ; but Dalton claims chiefly to have removed the 

 last objection. He shows that the weight of the atmo- 

 sphere has no ultimate influence on the evaporation of 

 liquids, although the rise of steam is obstructed by the 

 inertia of the particles of air. 



It is not intended to give Dalton's long and elaborate 



