CURRENT ACTION. 123 



arid driving them to the coldest surface. We are therefore reduced to the supposition, 

 that when the light of the suii impinges on a surface of glass, it places that surface in 

 such a condition that it exerts a pressure on the adjacent medium, immediately follow- 

 ed by a condensation of that medium. The state of the force here spoken of applies 

 to the glass surface alone ; it is not an action between the solar ray and the forces that 

 produce crystallization, seeing that it equally takes place in the deposite of aqueous 01 

 mercurial dew, and even of solid gold from a solution of its chloride. In other words, 

 if a ray of the sun be incident on a surface of glass, it develops a force of attraction on 

 that surface. 



468. A gaseous medium, having its temperature disturbed at any point, has a current 

 determined in it. In a chamber, such as the bell of an air-pump, this current circulates 

 round the walls, ascending on the hot and descending on the cool side ; it might be 

 supposed that to this circumstance was due the fact of no crystals being found on the 

 plate of glass (466). The condensation cannot, however, be attributed to this cause ; 

 for if so, a lamp, or any other source of heat, would be equally effectual; it will, how- 

 ever, be hereafter shown that artificial flames tend to remove these depositions from 

 the side nearest to them, and cause them to be accumulated in the colder regions. 



469. Beneath a receiver (a, Jig. 80) a cubical bottle, b, having flat sides, was placed, 

 and in the bottle a few pieces of camphor; the mouth of the bottle was about half an 

 inch in diameter, and was left open, the pressure of the atmosphere being reduced to 

 1| inches of mercury. Temperature of the ray 57 Fah. On examination, after the 

 lapse of one hour and twenty-five minutes, no crystals whatever could be found on the 

 receiver, and but a few sparsely scattered on the sides of the cubical vial. Now there 

 can be no doubt that the whole receiver was full of camphor vapour, and it does 

 not appear that any reason can be assigned for the anomaly of its non-crystallization. 



470. Will artificial light produce analogous results I To ascertain this, I took a 

 glass globe about one inch and a half in diameter, with a neck four inches long, fitted 

 it with a stopcock, and introduced within it a drop of water. The vapour of this 

 water exhibited extreme mobility ; exposure to the clouds caused its immediate deposi- 

 tion. A farther advantage was gained by the use of this apparatus, for by heating the 

 globe uniformly, until all the moisture on its surface was vaporized, and then allowing 

 it to cool, the particles of water readily obey the forces that solicit them. This glass 

 globe, supported vertically on an appropriate stand (a, Jig. 81), was placed at a distance 

 of nine or ten inches from a brightly-burning argand lamp, A ; to protect it from acci- 

 dental currents of air, and from irregularities of radiation from other sources, the whole 

 arrangement was covered by a bell, b c, open at both ends, and about fifteen inches high. 

 It appeared, at first, that a thin dew lined the inside of the whole globe, instead of being 

 confined to one part; but after a certain space of time, the heat which passed from the 

 lamp through the protecting glass disturbed the results, the dew being driven to the cold- 

 est parts. To get rid of the effects of this heat, at a distance of about three feet from 

 the lamp (A., Jig. 82), a double convex glass lens, c, 2i inches in diameter, was placed, 

 which brought the rays to a focus at a distance of five or six feet, where stood the glass 

 globe, a, covered with its protecting jar. The globe had been previously slightly warmed, 



