122 DEW OF WATER AND MERCURY. IODINE AND GOLD, ETC. 



quired that there should be a vacuum within the jar. In the course of an hour or two, 

 there will be a copious dew at a, and on farther exposure drops of water will trickle 

 down the side of the glass, but on the opposite side not the least cloudiness will be found. 



463. Barometers, hung up in such a position that the sun's rays can have access to 

 them, exhibit an analogous appearance, the side nearest the light being studded with 

 metallic globules. 



464. In any of these experiments iodine may be substituted for camphor, provided 

 mercury is not present, nor any other substance on which this body acts ; the most 

 advantageous method of using iodine is by heating it in a suitable vessel, and when 

 the vessel is quite full of vapour, presenting it to the sun's ray; deposition goes on, on 

 the sunny side, as the condensation takes place. 



465. Nor is it requisite, in obtaining these results, that the material should be either 

 gaseous or vaporous. The rays of light have the property, as was found by Count 

 RUMFORD, of decomposing an aqueous solution of chloride of gold ; on making this ex- 

 periment in a test tube one third of an inch in diameter {Jig. 78), small spangles 

 of metallic gold will be seen, by reflected light, on the side towards the sun, b; by 

 transmitted light it appears of a pale green tint, as is the colour of gold leaf. Here we 

 find that, under certain circumstances, solutions will deposite metallic matter, in obe- 

 dience to the same laws which cause the crystallization of camphor and the deposite of 

 aqueous dew. 



466. A few pieces of camphor were laid on the plate of an air-pump, and a circle 

 of glass two inches in diameter (a, Jig. 79) was supported on a pedestal in the midst 

 of them, the upper part of the glass being four or five inches above the pump plate ; it 

 was then covered with a jar, and exhaustion performed. On exposure to the sun for 

 a suitable length of time, numerous crystals were found on the jar, but none on the 

 circular plate, although it had received the full beams of that luminary. This experi- 

 ment was made with a view of determining what peculiar condition a glass surface was 

 placed in by exposure to the light ; for experimental purposes, the rounded form of the 

 glass receivers being very unsuitable. It was not, therefore, without surprise I observed 

 that, however long the plate was continued in the beams of light, no crystallization 

 would ensue. A flat surface, however, being essential in the course of experiment pur- 

 sued, trials were repeatedly made, by various changes in the arrangement, to cause a 

 deposition of camphor upon such a crown glass plate ; but though in five days I could 

 procure starry crystals upon the bell jar of more than half an inch in diameter, in 

 no instance was a solitary one found on the glass plate. 



467. Two circumstances may determine the precipitation of camphor crystals on a 

 surface : 1st. Reduction of temperature ; 2d. Increase of pressure. To the former we 

 cannot look for an explanation in the case before us, for there is an actual increase of 

 temperature in every part, and more especially on that side of the vessel which is next 

 to the sun. Why, then, does this condensation take place on the hottest surface, the 

 side nearest to the sun ? we cannot admit that the rays of heat have any active part 

 in bringing about the phenomenon. On the other hand, they ought rather to exert 

 a contrary effect, antagonizing the powers that solicit the camphor crystals to form, 



