132 MEMOIR OF DR. DALTON, AND 



similar parts, less and less, without end, as water into vapour, 

 more or less, subtile and attenuated. Aristotle and his fol- 

 lowers esteemed earth, air, fire, and water, to be elements, 

 simple and uniform in their several kinds, essentially distinct 

 and utterly incapable of being converted into one another, yet 

 easily uniting together, and by their different arrangements, 

 and properties and mixtures, composing every body in the 

 universe. Many modern chemists have adopted this idea; 

 others have increased the number of elements, by adding a 

 saline principle ; others have contended that some of these 

 elements, air and fire for instance, are themselves compound 

 bodies ; and others, lastly, are persuaded, that there is only 

 one elementary homogeneal principle, and that all the varieties 

 of bodies, as well as of what are most commonly esteemed 

 elements, ought to be attributed to the different magnitudes 

 and figures of the particles composing them ; and as the com- 

 ponent parts of water or air, or any other body, are by no 

 means supposed to be elementary particles of matter, but to 

 be made up of different numbers of elementary particles, 

 arranged in different forms, it may be thought probable, that 

 mechanical causes may either diminish or augment the 

 number, or change the disposition of the particles, and thus 

 effect the several varieties observ^able in nature. 



" It would be improper in this place to enlarge on a sub- 

 ject, concerning which both ancient and modern philosophers 

 have been so much divided in opinion. Their great diversity 

 of sentiment may suggest a suspicion that the full compre- 

 hension of it does not fall within the reach of the human 

 understanding. The following observation may, perhaps, 

 tend a little to illustrate this matter. Let us suppose that 

 this terraqueous globe was not surrounded with any air or 

 atmosphere, and that by an approach to the sun, or an in- 

 crease of subterranean fires, by some means or other it should 

 become exposed to a heat four times greater than the medium 

 heat of summer, which we may reckon to be about 60 degrees 



