120 PERIHELION MOTIONS. 



454. It has been stated by some chemists that, while the violet extremity of the solar 

 spectrum blackened chloride of silver, there are other parts of it which would bleach the 

 salt so blackened. But it is not so, for neither does any part of a very dispersed spec- 

 trum, nor a beam which has passed through a variety of absorbing media, exert such an 

 action. These experiments I tried repeatedly, under all the conditions of variation of 

 temperature and brilliancy of the solar rays, but no observation led to the inference 

 that there was any change of colour, or any sign of an approaching change, even after 

 the lapse of a whole month. Indeed, it would seem that the state of the case does not 

 justify any such expectation ; when the chemical rays have disengaged the chlorine, it 

 is gone, and lost forever to the silver, being scattered abroad in the atmosphere; if, there- 

 fore, the substance ever regains a white colour, chlorine must have been purposely fur- 

 nished from other sources, or the white substance said to result must be some compound 

 of unknown ingredients. 



455. The light of the moon is a remarkable example of luminous rays existing with- 

 out either calorific or chemical rays ; the most delicate thermometric arrangements have 

 hitherto failed to show any rise of temperature in the moonshine. A piece of paper, 

 imbued with chloride of silver, may also be exposed to the rays of the full moon, con- 

 verging from a glass, and it will not exhibit any change ; this I proved by placing such a 

 paper in a situation where, for a whole night, the rays of the moon could reach it. And 

 the same observation applies to terrestrial flames. In none of these has the existence 

 of the chemical rays been detected. Chloride of silver, after being exposed for eight 

 hours to the bright flame of an argand lamp converged by a lens, retained its whiteness. 

 The same effect was witnessed when the flame of alcohol tinged red by strontian was 

 employed, or the yellow flame produced by chloride of sodium, and the green of boracic 

 acid ; in these cases the periods of exposure did not exceed half an hour. 



456. OF PERIHELION MOTIONS. Probably the most remarkable effect exhibited by the 

 solar rays is the motion they produce in media endued with much mobility. For many 

 years it has been known that camphor exposed in a bottle to the rays of the sun, formed 

 a crystallization on that side of the vessel nearest the luminary; but the action is so 

 slow, and requires such a length of time for its completion, that no successful investiga- 

 tion has been made as to the nature of the forces in operation. Some philosophers 

 have assumed, upon insufficient ground, however, that the crystallization was effected 

 on the most illuminated side, merely because it was the coldest, as we know that vapours 

 are always deposited on that part of a surface the temperature of which is the lowest. 



457. About three years ago, I published a series of observations on this point. Hav- 

 ing found, from some theoretical considerations, that the crystallization of camphor took 

 place in vacuo with a rapidity convenient for experimental investigation, I was led to 

 make an extended inquiry into the whole matter. 



458. The sun's rays have the power of causing vapours to pass to the perihelion 

 side of vessels in which they are confined, but, as it would appear, not at all seasons of 

 the year. For example, I have a certain glass fitted up for making these observations, 

 and in this vessel, during the months of December, January, and part of February, 1836- 

 37, a deposite was uniformly made towards the sun ; during the months of March, April, 



