466 DiJ'cripton of a new Hygrometer and Pholometer. ^ 



llandard of the wet ball. The law, however, being known of the diffolving power of air 

 as afFefted by heat, it is eafy from the difpofition of the air with refpefl to humidity at 

 one temperature to derive that at any other. It will fuffice to mention the refult of a 

 number of careful experiments : — fuppofing air at the freezing point to be capable of 

 holding 50 parts of moifture ; at 10 degrees centigrade, it will hold 100; at 20°, 200 ; at 

 30", 400 •, — thus doubling at each increafe of 10 degrees. Hence a table may be con- 

 llru£lcd by which tiiofe converfions will be eafily made. 



To omit nothing that tends to elucidate the theory of the inftrument, I muft obferve, 

 tli:it the air in its contacl with the humid furface is not abfolutely cooled to the fame 

 temperature : the air and water really meet each other at an intermediate point deter- 

 mined by their compounded denfity and capacity. Confequently the indications of the 

 hygrometer ought to be augmented by the ^-^^ part, or -j- x tto- J^"' this quantity is too 

 fmall in any cafe to be regarded. 



Inftead of water, the coated ball may be wetted with jether, alcohol, or other volatile 

 liquids, and hence will be determined the attraftions of air for thofe fubftances under the 

 feveral modifications of heat, preflure, and previous moifture. It is eafy likewife, to re- 

 uerfe the experiments by covering tl>e upper ball with fulphuric acid, potafli, or any dcli- 

 quefcent fait. A vaft field of inquiry thus opens to view,'prefenting fadts not only im- 

 portant in themfelves, but calculated to correct and expand our ideas on the fubje£l of 

 qhemical aflinities. 



By help of this hygrometer I have colle£led a number of meteorological obfervations, 

 *nd inveftigated the nature and produdlion of dews and their curious affedions to the 

 metals, glafs, and vegetable fubftances. But what is more important, I have afcertained 

 the attra£lions of air for humidity under different prefTures, and with different tempera- 

 tures. I have examined likewife the properties of other gafes in thofe refpe£ls, and have 

 fucceeded in accommodating the refult to a few geometric laws of beautiful fimplicity. 

 I forbear to fay more at prefent, but cannot help confidering the fubjedt as almoft ex- 

 haufted, and the modifications of the atmofphere as at length reduced to a fcience. 



Having fo minutely defcribed the hygrometer, I fhall difpatch the photometer, though 

 the more curious inftrument, in a few words. In fa£l it is conftruflied in the fame man- 

 ner, only the upper ball is blown of black glafs, or is blackened, and the lower one is 

 quite diaphanous and free of fpecks. The former detains the incident light, while the 

 latter tranfmits it freely. But light, in proportion to its abforption, caufes heat, whether 

 uniting with bodies it really conftitutes the matter of heat, or only excites heat in the aft 

 of combination. But though the black ball acquires conftant additions of heat, its tem- 

 perature will not uniformly and perpetually increafe ; for the accumulated heat will at laft 

 be conduced off by the furrounding air exaftly as it is received. The depreflion of the 

 liquor, therefore, will meafure the momentary afflux of light. To prevent the irregular 

 cfFefts of winds, which might accelerate that difperfion, tlie inftrument is inclofed within 

 . , a glafs 



