On the variable Action of the Electric Column. 251 



being more and more covered with moisture, its surface was at 

 last become such a conductor of the electric fluid from the posi- 

 tive to the negative end, that it became neutral. 



The la~t experiments of Mr, Ronakls would have surely recon- 

 ciled our o])inioas on this subjeci;, had they been made on pur- 

 pose for ics investigation ; being made with a column similar to 

 mine, composed of 800 groujjes, supported between three glass 

 pillars covered svith sealin:;-wax ; but these experiments were 

 made for a different purpose. He placed an hygrometer, a 

 tliermometer, and an electrometer under a receiver inverted over 

 mercury, m order to introduce successively an acid and an alkali, 

 and to obser^■e the degree of dryr.ess they would proihice : with 

 the acid, the hvgronieter descended from 464 to 29 y by the 

 temjterature .59t; having then removed the acid and substi- 

 tuted potash, in the course of a dav, it brought the hygrometer 

 to 24. 



These experiments, and the following, in which Mr. Ronalds 

 made moisture increase under the receiver by introducing in it 

 a moistened card, were made \vith the view of trying the effect 

 of more or less moisture for increasing the action of the column, 

 shown by the numlicr of strikings of the gold-leaf in the electro- 

 meter in a given time, compared with the effects of the changes 

 of heat: they certainly prove that the increase of heat acce- 

 lerates the slrikings, as Mr. Singer had supposed ; but they show 

 at the same time, wliat small quantity of evaporated water in 

 a given space, produces in it extreme moisture, and tliat the in^ 

 crease of fieut with the same quantity of water, tends to di- 

 minish moisture. 



This I know I)v mv own experiments, which are the ohject of 

 my paper in volume xxxiii. of Mr. Nicholson's Philosophical 

 Journal, giving an account of two series of experiments, agreeing 

 with each other, by which I determine the number oi grains vf 

 water \\liich can remain in tlie state of vapour in the space of 

 one cubic foot, by each degree of mv hygrometer and of the 

 thermometer. It mav be seen in the table of the results, that 

 scoen grains of water evaporated iii 07ie cubic foot, by the tem- 

 perature of (iO, brought the hygrometer to 9(vG degrees, and 

 that then the smallest diminution of heat caused a deposit of 

 water on the sides of the vessel. This shows what small (juanti- 

 ties of evaporated iVater act on the hygrometer in all its de- 

 crees ; an important circumstance to be attended to in meteoro- 

 logical systems ; a subject to which I shall return. 



The above experiments made with the greatest care in a glass 

 vessel, which was air-tigiit, show that such experiments made 

 under a receiver are more dilficult than is connnonly imagined, 



and 



