450 Prof. Tyndall on Calorescence. 



posed in the path of the condensed invisible beam, scarcely- 

 dimmed the brilliancy of the thermograph. The intense calo- 

 rific rays of the electric light pass through such glass with free- 

 dom. We here come to a point of considerable practical import- 

 ance 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 appearance 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 uni- 

 formly, employed in the construction of our black-bulb thermo- 

 meters, under the impression that the blackening secures the 

 entire absorption of the solar rays. This conclusion is fallacious, 

 and the instruments are correspondingly 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 thermo- 

 meter. 



When a sheet of common window-glass, apparently transpa- 

 rent, 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 em- 

 ployed, when looked at edgeways, was green ; and this experi- 

 ment proves how powerfully this green colouring-matter, even 

 in infinitesimal quantity, absorbs the invisible heat-rays. Per- 

 fect imperviousness might doubtless be secured by augmenting 

 the quantity of green colouring-matter. It is with glass of this 

 description that the carbon should be mixed in the construction 

 of black-bulb thermometers ; on entering such glass the solar 

 rays would be entirely absorbed, and greater differences than 

 those now observed would probably be found to exist between 

 the black-bulb and the ordinary thermometer. 



In conclusion, it gives me pleasure to mention the intelligence 

 and skill displayed by my assistant, Mr. Barrett, in executing 

 the numerous experiments committed to his care during the 

 progress of this investigation. 



