OF RADIANT UEAT THROUGH DIFFERENT BODIES. 63 



screens of difterently coloui-ed glass, iu order to make them pass through 

 one common screen of ahmi. 



Screens from which the 100 rays issue t.t , r 



that are made to fall successively , Number of rays 



on the same slice of alum. transmitted by this shoe. 



Glass, white 27 



— red 27 



— orange 27 



— yellow 27 



— green (apple) 5 



— green (mineral) 3 



— blue 27 



— indigo 27 



— violet 27 



— opake (black) 1 



We see here that the rays emerging from the red, orange, yellow, 

 blue, indigo, and violet are transmitted through the plate of alum in the 

 same proportion as the rays that issue from the white glass. The co- 

 louring matter introduced into the composition of these different kinds 

 of glass has no other eifect than to extinguish part of the calorific sheaf 

 which passed through the white, without perceptibly altering the rela- 

 tions of quantity between the several species of rays of which that sheaf 

 is composed: they act in respect to radiant heat just as brown or black- 

 ish substances dipped in a transparent fluid would act in respect to light. 

 But the case is different with respect to green and opake black ; for 

 these being introduced into the composition of glass, it will stop nearly 

 all the rays that the alum is capable of transmitting. This effect arises 

 from the green or opake colouring matter producing a certain modifi- 

 cation in the diathermancy of the glass, and we havejust seen that this 

 species of calorific colouration is invisible and totally independent of co- 

 loration properly so called, since it exists in bodies possessing the great- 

 est transparency. It is then extremely probable that the black or the 

 green should not be supposed to enter as mere neutrals into this phas- 

 nomenon, which will thenceforth depend on svich or such a property 

 of these colouring materials. I have found, in fact, some green glasses, 

 which produced a much feebler action than others of the same tint but 

 possessing a less brilliant colouration. The green glasses which act most 

 powerfully are of a bluish cast ; from which circumstance it would seem 

 to follow that they contain a considerable quantity of oxide of copper. 

 Whatever may be said of this singular property of green and black 

 opake glasses and the cause by which it is produced, it is nevertheless 

 an indisputable fact which everjj man can easily verify and of which we 

 intend to give some new proofs presently. But it will perhaps be ad- 

 visable previously to adduce the results furnished by several diather- 



