30 M. MELI.OJNI ON THE FREE TRANSMISSION 



Table IV. — Glass {coloured). Common thickness 1"""'85. 



Deviations of Rays 



the galvanometer. transmitted. 



Deep violet 18-62 .53 



Yellowish red (flaked) 18-58 53 



Purple red (flaked) 18-10 51 



Vivid red 16-54- 47 



Pale violet 16-08 45 



Orange red 15*49 44 



Clear blue 15-00 42 



Deep yellow 14-12 40 



Bright yellow 12-08 34 



Golden yellow 11-75 33 



Deep blue 11-60 . 33 



Apple green 9-15 26 



Mineral green 8-20 23 



Very deep blue 6-88 19 



It is sufficient to cast the eye rapidly over the second and third tables 

 to be fully sensible of the truth of the projjosition, that " the capacity 

 which bodies possess of transmitting radiant heat is totally independent 

 of their degree of transparency T 



In fact, the liquid chloride of sulphur of a tolerably deep red brown 

 transmits a considerably greater number of caloric rays than the fat 

 oils of nut, the olive, and colza having a clearer tint; while these 

 oils, although of a very decided yellow colour, are more permeable 

 to radiant heat than several other liquids which are perfectly limpid, 

 such as concentrated sulphuric and nitric acid, aether, alcohol, and water. 

 The case is the same with solid bodies, among which we see sulphate 

 of lime, citric acid, and other very diaphanous substances allow a 

 much smaller quantity of heat to pass than some other bodies coloured 

 or translucid, such as emerald, agate, tourmaline, borax, adularia, and 

 sulphate of barytes. 



But nothing is better calculated to demonstrate that transparency 

 has little or no effect in the transmission of heat than a comparison 

 of the effects obtained by the crystal of alum with those obtained by 

 means of the smoky rock crystal. The table shows that, in respect to 

 these substances as well as the others which we have just mentioned, 

 the capacity to transmit radiant heat is inversely as the capacity of 

 transmitting the rays of light. I was anxious to try how far this in- 

 verse ratio of the calorific to the luminous transmissions might extend, 

 by varying the degrees of thickness so as to give to the light all the ad- 

 vantage and the whole of the loss to the caloric. We submitted to the 

 test a plate of well-polished and perfectly transparent alum only one 

 millimetre and a half in thickness, and a smoky rock crystal the thick- 

 ness of which in the direction of its polished faces was 86 millimetres. 



