124 TEMPERATURE OP THE SIDES OF A JAR. 



so as to expel all the dew from its surface, and give it a uniform temperature ; in sev- 

 eral trials it was found that there were no evidences that the bright flame of an argand 

 lamp exerted any force soliciting the vapour of water to move towards one part of the 

 glass rather than another. 



471. I took the arrangement of 462, and shut it up in a dark closet, having pre- 

 viously made the jar perfectly clean and dry ; it remained there for several days, that 

 it might be found whether those little irregularities of temperature which occur in such 

 confined chambers would cause this dew to pass to one side of the glass rather than 

 another; it did not appear that such was the case, for the glass was as free from 

 moisture when taken out as when shut up. And now, this arrangement being placed 

 in the window, where the sun was brightly shining, exhibited on its perihelion surface, 

 in the course of three and a half minutes, a pearly dew; and in six minutes drops of 

 water were trickling down that side. 



472. But it is not essential to the success of this last experiment that the solar ray 

 itself should impinge on the vessel. The temperature in the shade being 94 Fah., I 

 placed the receiver with its cup of water in a window having a northern exposure, and 

 found that the dew readily made its appearance on that side which was towards the 

 light. 



473. It might be suggested that when a vessel is exposed to the sun, that part of 

 the glass which is nearest to him may actually be the coldest ; such an opinion, it is 

 evident, rests on no sufficient grounds. A jar, a g (Jig. 83), was taken, of such dimen- 

 sions that it could receive the differential thermometer, c db, the balls of which, b and c, 

 touched the opposite sides, and in the dark the liquid stood at zero, but on bringing 

 it into the sunshine, if the side a was exposed, then the ball c was warmest, and if the 

 side g, then the ball b was warmest, as was indicated by the motion of the liquid. 

 Hence we know, that in all cases where crystals of camphor, dew of water, &c., are 

 deposited on the side next the sun, they are so deposited in opposition to an energetic 

 force which tends to remove them. 



474. Light which has suffered reflexion at certain angles seems to have undergone 

 a remarkable modification, being no longer able to put the glass into such a condition 

 that it can cause motion towards the sun. It is not to be inferred that any connexion 

 is here traced between this disturbance of the condition of light and the change im- 

 pressed on it by polarization. A beam of the sun falling on a plate of glass, and being 

 reflected at an angle of 45, may be intercepted by any of the arrangements of sections 

 460, 461, as by the barometer tube. It will be found that the crystallization proceeds 

 with considerable rapidity, not, however, on the perihelion side of the vessel, but on 

 the opposite side. It is probable that this result is not dependant on the polarization of 

 light, inasmuch as it takes place equally well at all the angles, less and greater than the 

 maximum angle of polarization of glass. Light, even that of the sun, having once 

 undergone reflexion, has received some determinate impress, which disables it entirely 

 from causing camphor to crystallize on the perihelion side of vessels. 



475. Another remarkable phenomenon is exhibited by the following arrangement : 

 Take a receiver, a (fig. 84), twelve or fifteen inches high, and three or four in diameter; 



