1613.] Leslie on Air, Heal, and Moisture. 461 



but it heats the black ball according to its intensity ; and this 

 beat, by depressing the liquid in the tube, marks the intensity 

 of the light. Mr. Leslie has given a curious and valuable col- 

 lection of facts established by means of this instrument ; some 

 of them published long ago in Nicholson's Journal ; others new. 

 I should have been sorry had these curious facts been omitted, 

 which, in strict propriety, they ought to have been, as they have 

 no connexion whatever with the subjects which the book pro- 

 fesses to treat of. 



4. The Atmometer. — This instrument has been already de- 

 scribed in the Annals of Philosophy, vol. i. p. 467. It is an 

 imperfect instrument ; because it gives us no immediate infor- 

 mation by inspecting it, as all the other instruments do. We 

 must observe it twice in succession, and know the time between 

 the two observations, before we can draw any conclusions from it. 



5. Ivory Hygrometer. — This instrument has also been de- 

 scribed in the Annals of Philosophy, vol. i. p. 468. The con- 

 struction of it is highly ingenious, and does credit to Mr. Leslie's 

 inventive powers : but it is from its very nature imperfect j and 

 is always liable to alter by time. These imperfections were not 

 unknown to Mr. Leslie, who points them out with great can- 

 dour. 



III. Dry'rng Power of different Bodies. — Mr. Leslie took 

 different earths and various other bodies, dried them well before 

 the fire, and then put them into well stopped phials. By putting 

 a little of each into a large glass bottle along with a hygrometer, 

 he ascertained the degree of dryness which it produced in the 

 air. These results are curious, and unexpected. They are as 

 follows : — 



Alumina made the hygrometer stand at. . 84° 



Carbonate of magnesia J5 



Carbonate of lime 7 U 



Silica 40 



Carbonate of barytes 32 



Carbonate of strontian 23 



Pipe-clay 85 



Green-stone 80 



Sea sand with shells JO 



The degree of dryness produced by the different soils he 

 found proportional to the fertility of the soil. Hence he con- 

 jectures that the fertility depends upon this property. There can 

 be no doubt that the fertility of soils is very much connected 

 with the power which they possess of retaining the proper degree 

 of moisture. 



Connected with these curious results are the following : — 



