4^4 Defcript'ton of a new Hygromter and Photometer. 



to render it conveniently portable. At laft I attained my wiflies fo completely, that the 

 liygrometer when once adjufted can fcarcely be deranged by the rougheft treatment. It 

 only remained to hit on fome coloured liquor which (hould permanently retain its colour, 

 and at the fame time fliculd not, by any change of temperature, be liable to modify the 

 elafticity of the air included with it, either by abforbing or parting wiih humidity. Alka- 

 line-lye, tinged with carmine, feemed to anfwer the bed ; but Hill I found that, by the in- 

 fluence of ftrong light, the liquor gradually changed its colour, while it fliifted fomewhat 

 its place and advanced nearer the ball where it expofed the mod furface. I no longer 

 doubted that the oxygene of the included air united with the colouring matter, and formed 

 a precipitate. Inftead of common air, therefore, I filled the balls with hydrogene gas, 

 which appears fully to realize my expectation, and from the experience of nearly two 

 months of the brighteft feafon of the year, I believe I may confidently promife on the du- 

 lability of the colour*. I fhall now pro'ceeed to defcribe the hygrometer, and to give 

 fuch direftions as may enable an intelligent perfon, acquainted with fuch manipulations, 

 to conftruft it. 



To the one end of a flender tube, from 4 to 8 inches long, and with an uniform bore from -^ 

 to rs'o of an inch wide, is blown a ball about 4 or 6 loths of an inch in diameter, of coloured 

 glafs, fuch as black, blue, or green, and is bent inwards, fo that its pofterior furface fhall be 

 in a line with the nearcft edge of the tube. A fimilar tube, though fomewhat fhorter, with a 

 bore of equal or greater width, has this enlarged at the one end into a cylinder capable of 

 holding as much liquor as would fill the ftem of the other tube, and terminated with a clear 

 ball of the fame fize as the former. The tubes are a little widened at their other ends, to facili- 

 tate the jun£tion. The balls are next to be filled with hydrogene gas, which may be done 

 in various ways. One of the fimplcft is to infert each tube, and faften it with bees-wax ia 

 a narrow necked flafk containing the gas; then heat the ball with the flame of a candle, 

 and fuffer it to cool, and repeat this operation two or three times. The fhorter piece is 

 dipped in deliquiated potafh tinged with carmine, and a few bubbles of hydrogene gas are 

 forced out by the heat of the hands till, on being allowed to cool, a proper quantity of the 

 coloured liquor lifes. The open ends of the two pieces are then dryed, gently heated, 

 ^nd united by the flame of a blow-pipe in one fl;raight tube. By forcing air with the 

 heat of the hand from one ball into the other, it is eafy to make the liquor hang to- 

 gether with its fummit near the middle of the longer ftem. The whole is then fufpended 

 in a clofe room from the coloured ball, and the other ball plunged in a veflel of water 

 while a temporary fcale is attached to the upper ftem. On adding cold water, the liquor 

 defcends to near the juncture, and on adding warm water it rifes near the upper 

 ball; the difference between thofe two temperatures meafured by a thermometer, and 



• It would ftiH be moft defireable to employ common air only. Some acid or alkaline liquor, difpol'ed 

 neither to abftraft nor communicate moifture, may be found capable of retaining its colour under the joint 

 influence of light and oxygene. Probably certain metallic folutions have that property. I have very lately 

 tried indigo diffolvcd in fulphuric acid. It feems to anfwer well j but at the brumal folftice, and in tlifi 

 lauiky atmofpere of London, I cannot eafily jgdge what efFeft ftrong light would produce. 



referrei 



