Camphor towards the Light, 367 



Whatever, then, protects the bottles from radiation, either 

 wholly or in part, prevents the formation of deposits. The 

 wooden scale of a thermometer hanging in a window a few inches 

 from a bottle opposite the light, prevents a deposit in a broad 

 line exactly corresponding with the form of the scale. A bottle 

 placed near a bar of the window-frame will mark out the form 

 of the bar by the camphor crystals coming up to within a certain 

 distance of the bar, leaving that portion of the bottle naked 

 which coincides with the form of the bar. In such a case as this, 

 not only does the bar of wood prevent radiation from a portion 

 of the bottle, but, by absorbing heat from the sun, it acts as a 

 source of heat for hours afterwards. So also if a glass contain- 

 ing camphor stand on wood, the lower part is permanently 

 warmer than the upper, as is shown by the repulsion of the de- 

 posit from the lower part, and that chiefly at the back — the front, 

 where it is exposed to the light, being kept colder from being 

 nearer the window. When bottles are tied over with flannel or 

 india-rubber cloth, there is no deposit under these flaps. When 

 glasses are closed with bungs, these retain the heat and keep the 

 upper parts of vessels warmer than the middle parts, so that no 

 deposit is made within half an inch or an inch of the bung. 

 Coloured glass bottles produce furthest deposits, when exposed 

 to the sun, more readily than white ones, because they sooner 

 become heated on the exposed side. 



The tinfoil rings of Dr. Draper act as screens in preventing 

 radiation. The glass under and for a short space around the 

 metal is kept warmer than the uncovered portion nearest the 

 window, and hence no deposit in and about the protected portion. 

 To prove that this is the correct explanation, it seemed to me that 

 black absorbent substances would act more efficiently as pro- 

 tectors than bright reflecting ones. Accordingly I arranged a 

 number of bottles containing crude camphor, and attached to 

 them disks and rings of tinfoil, and of black, red, yellow, and 

 white paper. The space kept clear by the black paper was many 

 times larger than that cleared by the tinfoil and the lighter papers, 

 and these all varied in extent of clearance with their colour. To 

 show the protecting influence of white paper on a large scale, one 

 half of a large cylindrical jar containing crude camphor was 

 loosely covered with white paper in the direction of its length, 

 and so placed in the window, the paper side nearest the light. 

 There was an abundant deposit on the exposed half of the glass, 

 which has remained some months, but no trace of a deposit has 

 been found on the covered side. 



I think enough has been stated to prove that the motion of 

 camphor, &c. towards the light is really an effect of heat. The 

 laws which regulate the deposit of dew and hoar frost apply here. 



