PEOFESSOK TTNDALL ON CALOEESCENCE. 23 



follow that there is a gap in the calorescence, the atoms of the platinum vibrating in 

 red and blue periods, and not in intermediate ones. But I wish here to say that 

 further experiments, which I hope shortly to make, are necessary to satisfy my own 

 mind as to the cause of this phenomenon. 



The incandescent thermograph of the coal points being obtained, a very light-red 

 glass introduced between the opaque solution and the platinum reduced the thermo- 

 graph both in size and brilliancy. A second red glass, of deeper colour, rendered the 

 thermograph still smaller and feebler. A dark-red glass reduced it still more — the 

 visible surface being in this case extremely minute, and the heat a dull red merely. 

 When, instead of the coloured glass, a sheet of pure-white glass was introduced, the 

 image of the coal points stamped upon the platinum foil was scarcely diminished in 

 brilliancy. A thick piece of glass of deep ruby-red proved equally transparent; its 

 introduction scarcely changed the vividness of the thermograph. The colouring-matter 

 in this instance was the element gold, not the compound suboxide of copper employed 

 in the other red glasses. Many specimens of gold jelly, prepared by Mr. Faraday 

 for his investigation of the colours of gold, though of a depth approaching to 

 absolute blackness, showed themselves eminently transparent to the obscure heat-rays ; 

 their introduction scarcely dimmed the brilliancy of the thermograph. Hence it would 

 appear that even the metals themselves, in certain stales of aggregation, share that high 

 diathermic power which the elementary metalloids have been found to display. 



I have just said that a sheet of pure-white glass, when interposed in the path of the 

 condensed invisible beam, scarcely dimmed the brilliancy of the thermograph. The 

 intense calorific rays of the electric light pass through such glass with freedom. We 

 here come to a point of considerable practical importance to meteorologists. When 

 such pure-white glass has carbon mixed with it when in a molten condition, the 

 resulting black glass is still eminently transparent to those invisible heat-rays which 

 constitute the greater part of the sun's radiation. I have pieces of glass, to all appear- 

 ance black, which transmit 63 per cent, of the total heat of the electric light; and 

 there is not the slightest doubt that, in thicknesses sufficient to quench entirely the 

 light of the sun, such glass would transmit a large portion of his invisible heat-rays. 

 This is the glass often, if not uniformly, employed in the construction of our black-bulb 

 thermometers, under the impression that the blackening secures the entire absorption 

 of the solar rays. This conclusion is fallacious, and the instruments are correspond- 

 ingly defective. A large portion of the sun's rays pass through such black glass, 

 impinge upon the mercury within the bulb, and are ejected by reflexion. Such rays 

 contribute nothing to the heating of the thermometer. 



When a sheet of common window-glass, apparently transparent, was placed between 

 the iodine solution and the platinum leaf at the focus, the thermograph was more 

 dimmed than by the black glass last referred to. The window-glass here employed, 

 when looked at edgeways, was green ; and this experiment proves how powerfully this 

 green coloui-ing-matter, even in infinitesimal quantity, absorbs the invisible heat-rays. 



