and Mechanical Condition on Radiant Heat, 



301 



The plate of rock-salt chosen for this purpose was a very per- 

 fect one. I have never seen one more pellucid. The thickness 

 was 08 of an inch, and its size, compared with the aperture in 

 front of which it was placed, was such as to prevent any part of 

 the rays reflected from its lateral boundaries from mingling with 

 the direct radiation. M. Knoblauch has clearly shown how the 

 absence of caution in this particular may lead to error. The 

 mode of experiment was that usually followed : the source was 

 first permitted to radiate against the pile, and the deflection 

 produced by the total radiation noted. The plate of rock-salt 

 being then interposed, the deflection sank, and from its new 

 value the transmission through the rock-salt was calculated and 

 expressed in hundredths of the total radiation. 



3 III. — Transmission th 



rough Rock-salt fi 



*ora the follow- 



ing substances raised to 



a temperature of 100° C. 



Substance. 



Transmission. 



Radiation. 



Rock-salt . . . . 



. . 67-2 



35-3 



Biniodide of mercury . 



. . 76-3* 



39-7 



Milk of sulphur . . 



. . 76-9* 



40-6 



Common salt . . . 



. . 70-8 



41-3 



Yellow iodide of mercui 



y . . 79-0* 



46-6 



Sulphide of mercury . 



. . 73-1 



46-6 



Iodide of lead . . . 



. . 73-8 



47-3 



Chloride of lead . . 



. . 73-1 



55-4 



Chloride of cadmium . 



. . 73-2 



56-5 



Chloride of barium . . 



. . 70-7* 



58-2 



Chloride of silver (dark 



) . . 74-2 



58-6 



Fluor-spar . . . . 



. . 70-5* 



68-4 



Tersulphide of antimon 

 Carbonate of lime . 



V • • 77-1 

 . . 77'6 



69-4 

 70-2 



Oxysulphide of antimor 

 Sulphide of molybdenu 

 Sulphate of baryta . 

 Chromate of lead 



ly . . 77-6 



m . . 78-4 



. . . 71-3 



. . 71-6 



70-5 

 71-3 



78-4 

 79-2 



Red oxide of lead . . 



. . 74-1 



79-2 



Subchloride of copper 



. . 76-3 



78-6 



Oxide of cobalt . . 



. . 76-5 



79-7 



Red oxide of iron . , 



. . 78-4 



81-0 



Sulphide of copper . 

 Black oxide of iron . 



. . 79-0 

 . . 81-3 



82-3 

 82-7 



Sulphide of iron . . 

 Lampblack . . . 



. . 81-7 

 . . . 84-0 



83-3 

 83-3 



Here we have a transmission varying from 67 per cent, in the 

 case of powdered rock-salt to 84 per cent, in the case of lamp- 



