ACTION OF A SCREEN. 125 



place it, as usual, upon the transfer plate, with its proper charge of camphor, c. Then 

 cover it with a tin cylinder, ef, of sufficient dimensions, so that all the light may be 

 shut out except at one place, g, where there is a hole half or three quarters of an inch 

 in diameter. Under favourable circumstances, as a serene sky and bright sun, let the 

 arrangement be exposed, that a column of light may pass through the aperture g into 

 the glass ; it may or it may not finally fall on the camphor at c. It would, of course, 

 be expected that a collection of crystals would form on the inner surface of the glass 

 corresponding to the aperture g. But on trial it is not so ; for however bright the sun 

 may shine, or however favourable other circumstances may be, not a solitary crystal 

 will make its appearance either there or on any other part of the vessel, provided its 

 temperature has been pretty uniform. On an exceedingly calm and serene day in 

 July, 1835, when every circumstance seemed propitious, I made this experiment, and 

 because the jar that I was using was not ground sufficiently true to fit the transfer 

 plate accurately, it had been fixed thereon with common cap cement, and on exposure 

 to the sun, the temperature of the whole arrangement rose so high that the cement 

 was in almost a semifluid condition ; it was one of those days when the eye cannot be- 

 hold the sky or look on the ground without pain, yet not one crystal could be made to 

 appear opposite to the aperture. But on taking off the metallic screen, and exposing 

 the jar, in a little more than a minute small specks were observable on the glass, and in 

 a quarter of an hour its perihelion side was densely coated with crystals. How are we 

 to explain this ? Do the edges of the aperture g impress any change on the passing 

 light 1 Or is the glass surface placed in such a condition that it can no longer produce 

 the deposite of crystals I We shall see hereafter that there are circumstances yet more 

 remarkable, which put us in possession of an explanation. 



476. For the proper understanding of the rationale of these experiments, it is required 

 to know whether it be essential that the solar ray should impinge on the camphor or 

 not, or whether the action is exerted on the vapour only. A tube was therefore taken, 

 of suitable dimensions ; in the lower part of it a fragment of camphor was deposited, 

 and screened as much as possible from the rays of the sun, while its upper part was 

 freely exposed. Crystals formed without difficulty at a distance of three or four inches, 

 or even a foot, from the camphor, but there appeared to be a limit beyond which they 

 did not readily pass. A tube four feet six inches long and two inches in diameter being 

 exhausted, did not show on its exposed end any appearance of crystallization. Near the 

 camphor the deposite was pretty copious, but in advancing from it the crystals were more 

 sparsely scattered, until, towards the upper extremity, none could be seen. Now the 

 maximum quantity of vapour that can exist in a void, or among other gases, provided 

 the mixture be in equilibrio, depends on the lowness of the temperature of any one 

 part of the vessel ; and hence, a long tube, one of the extremities of which is kept cold, 

 does not exhibit these configurations readily, because the quantity of vapour in it is 

 small, owing to the coldness of one part of the void space. It is not necessary, there- 

 fore, that the sun should shine on the camphor, the effect of the rays taking place en- 

 tirely on the vapour filling the void. 



477. There is a singular action which certain bodies exert over this process. Take 



