22 MEMOIR OF DR. DALTON, AND 



tuation in the density of the air to be effected, and in what 

 manner the preceding general facts relative to the variation 

 of the barometer may be accounted for, is what we shall now 

 attempt to explain." This is referred to the varying amount 

 of vapour. 



In section 5, " observations on the height of clouds," there 

 is given the summary of 5381 observations, made by Mr. Cros- 

 thwaite, an evidence of the intellectual diligence to be found 

 at the lakes even before they became the haunt of poets. 



At p. 127 he gives a table of the temperature of water 

 made to boil at different atmospheric pressures, bearing on the 

 fact, which he is there explaining, that " aqueous vapour 

 always exists as a fluid sui generis diffused amongst the rest of 

 the aerial fluids;" and at p. 129, "that it may be determined a 

 priori what weight of vapour a given bulk of dry air will admit 

 of, for any temperature, provided the spec. grav. of the vapour 

 be given." These conclusions appear more fully in a note* to 

 a paper read in 1797, after having made confirmatory experi- 

 ments. This must be taken as an elucidation of a subject 

 which afforded much discussion at that period. We know 

 that Saussure and others knew well that moisture existed in 

 the air at very low temperatures, and there was a variety of 

 opinions as to the state in which it existed. Many writers of 

 the period believed, that because warm air was sensibly drier, 

 it contained less moisture than cold air; all these points 

 Dalton has elucidated and spoken on with decision. 



In the third and sixth essays, the precise point where the 

 burden of his argument is contained is difficult to find, the 

 reasoning being a constant process to prove what it is supposed 

 we knew beforehand to be the result. In the appendix, he says 

 more pithily, p. 188, " I am confirmed in the opinion that the 

 vapour of water, and probably of most other liquids, exists 

 at all times in the atmosphere, and is capable of bearing any 

 known degree of cold without a total condensation, and that 



* Memoirs of the Manchester Philosophical Society. Vol. V., p. 35 1. 



