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IV. " On the Nature of the Light emitted by heated Tourma- 

 line." By BALFOUR STEWART, Esq., M.A. Communicated 

 by J. P. GASSIOT, Esq. Received May 22, 1860. 



Some months ago I had the honour of submitting to the Royal 

 Society a paper on the light radiated by heated bodies, in which it 

 was endeavoured to explain the facts recorded by an extension of 

 the theory of exchanges. 



Having mentioned the difficulty which I had in maintaining the 

 various transparent substances at a nearly steady red heat for a 

 sufficient length of time in experiments demanding a dark back- 

 ground, Professor Stokes suggested an apparatus by means of which 

 this difficulty might be overcome ; and it is owing to his kindness in 

 doing so that I have been enabled to lay these results before the 

 Society. 



The apparatus consists of a thick, spherical, cast-iron bomb, about 

 5 inches in external and 3 inches in internal diameter the thickness 

 of the shell being therefore 1 inch. It has a cover removeable at 

 pleasure. There is a small stand in the inside, upon which the sub- 

 stance under examination is placed, and when so placed it is pre- 

 cisely at the centre of the bomb. Two small round holes, opposite 

 to one another, viz. at the two extremities of a diameter, are bored in 

 the substance of the shell. If, therefore, the substance placed upon 

 the stand be transparent, and have parallel surfaces, by placing these 

 surfaces so as to front the holes, we are enabled to see through the 

 substance, and consequently through the bomb. Let the bomb with 

 the substance on the stand be heated to a good red heat, and then 

 withdrawn from the fire and allowed to cool. It is evident that the 

 cooling of the substance on the stand will proceed very slowly, as it 

 is almost completely surrounded with a red-hot enclosure. It is also 

 evident that, by placing the bomb in a dark room, we may view the 

 transparent substance against a dark background. By this method 

 of experimenting, therefore, the difficulty above alluded to is over- 

 come. 



Before describing the experiment performed on tourmaline, it may 

 be well to state what result the theory of exchanges would lead us to 

 expect when this mineral is healed, and we shall perceive at the same 



