24 



The Temperature of the Solar Radiation, 



respects under the same conditions as the one usually regis- 

 tered, but the bulb of the second thermometer was covered 

 with a coating of lamp-black and size, so as to present a dead 

 black surface. These two thermometers read the same in the 

 shade, and as ordinary thermometers were accurately inter- 

 comparable. It was immediately apparent, however, that the 

 simple black glass thermometer did not indicate the true solar 

 radiation, the coated bulb always attained a higher tem- 

 perature than the other, and the difference was found to 

 vary with the temperature — the greater the temperature the 

 greater the difference. It was also observed that the coated 

 bulb thermometer was longer reaching its maximum, and 

 was subject to greater variations, and more rapid fluctua- 

 tions than the thermometer with the plain black glass bulb. 

 The following table deduced from observation showed the 

 variation of the differences with the increase of temperature, 

 and prove clearly that the ordinary black glass bulb ther- 

 mometer is not a reliable means of measuring solar radiation : 



Blade Glass Bulb. 



Coated Glass Bulb. 



Correction to be added to 

 Black Glass Bulb. 



Deg. 



Deg. 



Deg. 



70 



77-3 



7-3 



75 



84-4 



9-4 



80 



91-0 



11-0 



85 



96-6 



11-6 



90 



102-0 



120 



95 



107-7 



12-7 



100 



113-5 



13-5 



105 



118-8 



13-8 



110 



124-1 



141 



115 



129-3 



14-3 



120 



134-6 



14-6 



125 



140-0 



150 



130 



145-2 



15-2 



135 



150-5 



15-5 



140 



155-7 



15-7 



These results are exactly what we should expect if we 

 accept Professor Tyndall's statement with respect to the 

 diathermancy of black glass. The greatest heat rays belong 

 to the invisible part of the spectrum, just beyond the least 

 refrangible or red end ; these rays are known to traverse 

 substances perfectly opaque to all visible rays,* and would 

 therefore pass through the black glass without absorption, be 



* Indeed, when every trace of visible rays is sifted out from sunlight, the 

 heat rays collected to a focus can he made to melt platinum — not a trace of 

 light beirg sensible to the eye until the platinum becomes heated. 



