AND MECHANICAL CONDITION ON RADIANT HEAT. 93 



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 lampblack. The powders employed were fixed by the 

 sulphur cement. The same powders held by electricity, and permitted to radiate 

 through the rock-salt, gave the following transmissions : — 



Table IV. 



Substance. Transmission. 



Rock-salt 62-8 



Chloride of Silver (white) .... 69-7 



Fluor-spar 70'7 



Sulphide of Mercury 71 "0 



Sulphide of Calcium 72-5 



Milk of Sulphur 72-8 



Sulphide of Cadmium 7 3" 3 



Biniodide of Mercury 73*7 



Washed Sulphur 74-0 



Iodide of Lead 74"1 



Sulphate of Lime 74*2 



Sulphide of Zinc 74-4 



Carbonate of Zinc 74*8 



Sulphate of Baryta 75*0 



Common Sugar 75'4 



Sulphide of Copper 76 "5 



Iodide of Copper 76*5 



Red oxide of Iron 76*8 



Chloride of Silver (black) .... 77-3 



Amorphous Phosphorus .... 78'0 



Oxide of Cobalt 78-2 



Sulphide of Iron 78-5 



Black oxide of Iron 79-7 



Black Platinum 89-0 



The transmissions here are lower than when the sulphur cement was employed. 1 

 do not, however, think that the differences are due to the employment of the cement, 

 but to a slight source of disturbance, which was removed in the later experiments. 



For the heat emitted by black platinum rock-salt manifests its maximum power 

 of transmission, which would lead us to ascribe a maximum dissonance between the 

 vibrating periods of rock-salt and of black platinum. It will also be remarked that, 

 as a general rule, the powerful radiator has its heat more copiously transmitted by the 

 rock-salt than the feeble radiator. To render this clear, I have in Table III. appended 

 to the transmission the corresponding total radiation. The only striking exceptions to 



