1819.] P ro f- Leslie on Heat and Climate. 9 



the other focus. The experiment will succeed though the 

 mirrors have not a finished surface.* It is only necessary that 

 they be pretty near each other, that their concavities be suited 

 to the position of their foci, and that the apartment be kept still 

 and undisturbed. If a cold body be held in one of the foci, a 

 thermometer will sink in the other; and this fact has been 

 alleged a« a proof, that cold, as well as heat, is a positive quality. 

 But there is no occasion to have recourse to such an hypothesis, 

 since the body will evidently be environed by a chill atmosphere, 

 which will afford a constant aerial efflux. From these principles 

 we may gather, that the conducting quality of air is not propor- 

 tioned to its density ; for though a greater number of particles 

 come in contact with the heated body at one time, their succes- 

 sion is slower by reason of the diminished volubility of the fluid. 



These general observations prepare us for a closer investigation 

 of the nature, properties, and effects, of heat. I am aware of 

 the difficulties to be encountered in the attempt, and of the 

 temerity of searching into the constitution of matter. The 

 magnificent spectacle of the celestial bodies invites our atten- 

 tion ; and the application of the simple laws, to which their 

 motions conform, affords the finest and most satisfactory exer- 

 cise of the understanding. But in the elemental structure of the 

 universe, nature seems wrapped in impenetrable secrecy. All is 

 dark and forbidding, and to obtain even a glimpse of her 

 abstruser operations would prove highly gratifying to a liberal 

 mind. 



It is an obvious remark that no substance in nature is ever 

 permanent. Matter assumes an infinite variety of forms. Water 

 •changes into ice, and hail, and snow, as well as into invisible 

 steam. The metals liquefy and rise into vapour ; nor are the 

 hardest rocks exempted from similar alterations in their consti- 

 tution. Composition and dissolution form the whole of the 

 science of chemistry. But the powers of nature are incompara- 

 bly superior to the resources of art. Water and air alone are 

 found in many cases to be sufficient for the growth and support 

 of animals and vegetables. Yet how astonishing the variety of 

 substances developed in plants ? They are composed of charcoal, 

 the primitive earths, several kinds of salts, gums, resins, oils ; 

 and, what is still more remarkable, they contain a portion of iron, 

 manganese, and gold. The products of the animal kingdom are 

 even more complicated, and thus the apparently simple sub- 

 stance8, water and air, are, by a change of combinations, ex- 

 hibited in an almost endless variety of forms. Nor can we hesitate 

 to infer that any one substance may be converted into any other, 

 though it may exceed our limited powers to produce the change. 



* It wilt now be perceived that some of these assertion! are unguarded and in- 

 accurate. (Jut m\ attempts afterwards to rpduce them t.> experimental proof led 

 to most of the discoveries explained in the " Inquiry into the Nature and Propa- 

 gation of Heat." — A. 



