HISTORY OF THE ATOMIC THEORY. 23 



the vapour so existing is one and the same thing with steam^ 

 a vapour of the temperature of 2X2° or upwards. The idea, 

 therefore, that vapour cannot exist in the open atmosphere 

 under the temperature of 212°, unless chemically combined 

 therewith, I consider as erroneous ; it has taken its rise from a 

 supposition, that air pressing upon vapour condenses the 

 vapour equally with vapour pressing upon vapour, a supposi- 

 tion we have no right to assume, and which I apprehend will 

 plainly appear to be contradictory to reason and unwarranted 

 by facts; for, when a particle of vapour exists between two 

 particles of air, let their equal and opposite pressures upon it 

 be what they may, they cannot bring it nearer to another 

 particle of vapour, without which no condensation of vapour 

 can take place, all other circumstances being the same; and 

 it has never been proved that the vapour in a receiver from 

 which the air has been exhausted, is precipitated upon the 

 admission of perfectly dry air. Hence, then, we ought to 

 conclude, till the contrary can be proved, that the condensa- 

 tion of vapour exposed to the common air does not in any 

 manner depend upon the pressure of the air" 



At p. 1 35, after contending for the theory that the vapour 

 of water is mixed with the air and not combined, he explains 

 how it is precipitated by cold and taken up by heat, and how 

 it is that clouds consisting of light diops do not fall so readily 

 as clouds with heavy drops, as the resistance of the drops is 

 as the square of the diameter, in which his mathematical 

 knowledge helps his meteorology. This was suggested by 

 Mr. Gough. 



There is in these Essays, and everywhere in Dal ton's 

 writings, a great rapidity of reasoning, a direct passage from 

 premise to conclusion without fear, as if more than usually 

 persuaded that true reason could not misguide him, so that he 

 is utterly regardless of consequences. 



At p. IG8 we find a fair example of his mode of reasoning, 

 and one also of his daring theories. 



