Prof. Tyndall on Calorescence. 4*^1 



solution of iodine, incandescence was obtained at the invisible 

 focus of the lens on the roof of the Royal Institution. 



Knowing the permeability of good glass to the solar rays, I 

 requested Mr. Mayall to permit me to make a few experiments 

 with his fine photographic lens at Brighton. Though exceed- 

 ingly busy at the time, he in the kindest manner abandoned to 

 my assistant, Mr. Barrett, the use of his apparatus for the three 

 best hours of a bright summer's day. A red heat was obtained 

 at the focus of the lens after the complete withdrawal of the 

 luminous portion of the radiation. 



§ 9. Black paper has been very frequently employed in the 

 foregoing experiments, the action of the invisible rays upon it 

 being most energetic. This suggests that the absorption of those 

 rays is not independent of colour. A red powder is red because 

 of the entrance and absorption of the luminous rays of higher 

 refrangibility than the red, and the ejection of the unabsorbed 

 red light by reflexion at the limiting surfaces of the particles of 

 the red body. This feebleness of absorption of the red rays 

 extends to the rays of greater length beyond the red; and the 

 consequence is that red paper when exposed at the focus of invi- 

 sible rays is scarcely charred, while black paper bursts in a 

 moment into flame. The following Table exhibits the condition 

 of paper of various kinds when exposed at the dark focus of an 

 electric light of moderate intensity. 



Paper. Condition. 



Glazed orange-coloured paper . Barely charred. 



„ red- „ . Scarcely tinged ; less than the orange. 



„ green- „ . Pierced with a small burning ring. 



„ blue- „ . The same as the last. 



„ black- „ . Pierced ; and immediately set ablaze. 



„ white- „ . Charred ; not pierced. 



Thin foreign-post .... Barely charred ; less than the white. 



Foolscap . 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 expanding 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 conse- 

 quently more easily charred. Rubbing the red iodide of mer- 

 cury over paper, and exposing the reddened surface at the focus, 

 a thermograph 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 



Thin white blottingpaper 

 „ whitey-brown „ 



Ordinary brown „ 



Thick brown . . . 

 Thick white sand-paper 

 Brown emery „ 



Dead-black ,, 



