536 Royal Society : — 



along with another glass, opaque or nearly so, which gives out light 

 of the ordinary description. They are heated together in the same 

 field of heat, and then viewed in the dark as they cool. During 

 this process the tint of each changes, becoming somewhat redder 

 as the temperature falls. A difference in the temperature of the 

 two glasses might therefore cloak or even reverse the peculiar differ- 

 ence of tint which it is sought to establish, unless this is very 

 marked. This difficulty would be got over if we could by any means 

 compare glasses of different tints at precisely the same temperature 

 in the dark together ; but I have not yet succeeded in contriving an 

 apparatus for this purpose. 



With respect to glasses that whiten the fire when used as screens, 

 these all, without exception, as far as I have tried, give out a red 

 description of light ; and this is peculiarly remarkable in some light- 

 green glasses. 



The following circumstance renders some glasses unfit for experi- 

 ment. They absorb nearly all the red light of low temperature, 

 and it is only when the light rises to a white that a notable propor- 

 tion is allowed to pass. Now if the law be true that the radiation 

 is equal to the absorption, these glasses should, at a low red heat, 

 give out nearly the whole light belonging to their temperature, the 

 same as if they were opaque. It is only therefore when we raise 

 the temperature that we cau expect any result in the way of pecu- 

 liarity of tint ; for it is only then that the glasses, as screens, allow 

 a notable proportion of light to pass, and that of a peculiar cha- 

 racter ; but the glass has now assumed a pasty condition, which 

 renders it unfit to work with. Those glasses, therefore, are to be 

 preferred that allow a considerable proportion of all the kinds of 

 light to pass : for, just as in heat the radiation is most peculiar in 

 rock-salt, which absorbs but little heat ; so in light the best results 

 are obtained by glasses that absorb only a small proportion of the 

 light that falls upon them. As a proof of this, I may mention that 

 difference of tint is very noticeable in the process of spinning threads 

 of coloured glass. The heated red glass thread has, when being 

 spun, a pale green hue, while the green glass thread has, under the 

 same circumstances, a decidedly reddish appearance. These threads 

 do not absorb much light, but what they do absorb is of a peculiar 

 kind. 



It has been mentioned that a screen of glass is peculiarly opaque 

 for heat from glass ; but if the side of the radiating plate furthest 

 from the screen be coated with lamp-black, its heat now passes 

 the glass screen as readily as ordinary heat of that temperature. 

 A similar fact is noticeable with regard to light. A red glass, 

 which, when heated and viewed in the dark, gives out a greenish 

 light, while in the fire scarcely appears to differ in tint from the 

 surrounding coals ; and the same fact holds for all coloured glasses. 

 Ultimately they all appear to lose their colour in the fire as they ap- 

 proach in temperature the coals around them. This may be ex- 

 {)lained thus : — the red glass, for instance, still gives out its greenish 

 ight ; but it passes red light from the coals behind it, in such a 



