PROFESSOR TYNDALL ON CALOEESCENCE. 



21 



Paper. 



Glazed orange-coloured paper 



red- „ 



green- „ 



blue- „ 



black- „ 



white- ,, 



Thin foreign-post .... 



Foolscap 



Thin white blotting-paper . 

 „ whitey-brown „ 



Ordinary brown „ 



Thick brown ,, 



Thick white sand-paper 

 Brown emery „ 



Dead-black „ 



Condition, 



Barely charred. 



Scarcely tinged ; less than the orange. 



Pierced with a small burning ring. 



The same as the last. 



Pierced ; and immediately set ablaze. 



Charred ; not pierced. 



Barely charred ; less than the white. 



Still less charred ; about the same as the orange. 



Scarcely tinged. 



The same ; a good deal of heat seems to get through 

 these two last papers. 



Pierced immediately, a beautiful burning ring expand- 

 ing on all sides. 



Pierced, not so good as the last. 



Pierced with a burning ring. 



The same as the last. 



Pierced, and immediately set ablaze. 



We have here an almost total absence of absorption on the part of the red paper. 

 Even white absorbs more, and is consequently more easily charred. Kubbing the red 

 iodide of mercury over paper, and exposing the reddened surface at the focus, a thermo- 

 graph of the coal points is obtained, which shows itself by the discharge of the colour 

 at the place on which the invisible image falls. Expecting that this change of colour 

 would be immediate, I was at first sui'prised at the time necessary to produce it. We 

 are here reminded of Franklin's experiments on cloths of dijfferent colours, and his 

 conclusion that dark colours are the best absorbers. This conclusion, however, might 

 readily be pushed too far. Franklin's colours were of a special kind, and their deport- 

 ment by no means warrants a general conclusion. The invisible rays of the sun possess, 

 according to Muller, twice the energy of the visible ones. A white substance may 

 absorb the former, while a dark substance — dark because of its absorption of the 

 feeblest portion of the radiation — may not do so. The white powder of alum and 

 the dark powder of iodine, exposed to the action of a source in which the invisible 

 rays greatly surpass the visible in calorific power, exhibit a deportment at direct 

 variance with the popular notion that dark colours are the best absorbers. 



§10. 



In conclusion, I would briefly refer to a few experiments made to determine the 

 calorescence obtainable through glasses of various colours. In the first column of the 

 subjoined Table the colour of the glass is given ; in the second column the effect 

 observed when a brilliant spectrum was regarded through the glass is stated ; and in 



