92 PEOFESSOE TTNDALL ON THE INFLUENCE OF COLOUE 



The plate of rock-salt chosen for this purpose was a very perfect one. I have never 

 seen one more pellucid. The thickness was 0-8 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 per- 

 mitted 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. 



Table III. — Transmission through Kock-salt from the following 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 Mercury . . . 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 Antimony . , . 77"1 69-4 



Carbonate of Lime 77-6 70-2 



Oxysulphide of Antimony . . 77-6 70-5 



Sulphide of Molybdenum . . . 78-4 71-3 



Sulphate of Baryta 71*3 78-4 



Chromate of Lead 71-6 79-2 



Eed 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 79-0 82-3 



Black oxide of Iron 81-3 82-7 



Sulphide of Iron 81-7 83-3 



Lampblack 84-0 83-3 



