Prof. Tyndall on the History of Cahrescence. 219 



This fact is thus announced in a footnote at page 67 of the 

 Philosophical Transactions for 1862 : — "A layer of bromine, suf- 

 ficiently opake to intercept the entire luminous rays of a gas- 

 flame, is highly diathermanous to its obscure rays. An opake 

 solution of iodine in bisulphide of carbon behaves similarly. The 

 details of these experiments shall be published in due time : they 

 were publicly shown in my lectures many months ago. — June 

 13, 1862." 



Turning to my published lectures on Heat as a Mode of 

 Motion," I find at page 357 one of these experiments described in 

 the following words : — " I cannot use iodine in a solid state, but 

 happily it dissolves in bisulphide of carbon. I have the densely 

 coloured liquid in this glass cell. I throw the parallel electric 

 beam upon the screen ; this solution of iodine completely cuts 

 the light off; but if I bring my pile into the path of the beam, 

 the violence of the needle's motion shows how copious is the trans- 

 mission of the obscure rays." Turning back to page 307 of the 

 same work, I find experiments on smoked rock-salt and black glass 

 described as follows : — " Here is a plate of rock-salt coated so 

 thickly with soot that the light, not only of every gas-lamp in 

 this room, but the electric light itself is cut off by it. I inter- 

 pose the plate of smoked salt in the path of the beam ; the light 

 is intercepted, but this rod enables me to find with my pile the 

 place where the focus fell. I place the pile at this focus ; you 

 see no beam falling on the pile, but the violent action of the 

 needle instantly reveals, to the mind's eye, a focus of heat at the 

 point from which the light has been withdrawn.' - ' 



I would ask the reader to picture any experimenter standing 

 by a focus of invisible rays, the exposure to which, for an instant, 

 of the face of my thermo-electric pile caused the heavy needles 

 of a coarse galvanometer to dash against their stops. Could he 

 escape the temptation to put his hand there ? I did so fifty 

 times, trying moreover to concentrate the radiation by pushing 

 out my lens. The camera of my electric lamp was fur- 

 nished with a cod cave reflector, at the centre of which stood 

 the carbon-points whence issued the electric light. The rays 

 were converged by a lens in front ; and when the points were 

 at their proper elevation, the focus of the lens coincided with 

 that of the mirror. Causing the two foci to coincide, and con- 

 verging the rays as much as possible, I cut off the light by the 

 solution of iodine, and brought in succession my hand, my cheek, 

 pieces of brown paper and of lead-foil into the dark focus. It 

 was simply a substitution of these bodies for the face of my 

 thermo-electric pile. But while the action on the skin was almost 

 intolerable, I obtained neither the charring of the paper nor the 

 fusion of the foil. I knew that a large portion of the radiant heat 



Q2 



