302 Prof. Tyndall on the Influence of Colour 



black. 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. Transmissio 



ii. Substance. Transmission. 



Rock-salt 



62-8 



Carbonate of zinc 



. 74-8 



Chloride of silver (white) 



69-7 



Sulphate of baryta 



. 750 



Eluor-spar 



707 



Common sugar . 



. . 75-4 



Sulphide of mercury . 



71-0 



Sulphide of copper 



. 76-5 



Sulphide of calcium . . 



72-5 



Iodide of copper . 



. . 765 



Milk of sulphur . . . 



72-8 



Red oxide of iron 



. 76-8 



Sulphide of cadmium . 



73-3 



Chloride of silver (black) 77*3 



Biniodide of mercury . 



737 



Amorphous phosphorus. 78*0 



Washed sulphur . . . 



74'0 



Oxide of cobalt . . 



. 78-2 



Iodide of lead .... 



74-1 



Sulphide of iron . 



. 78-5 



Sulphate of lime . 



74-2 



Black oxide of iron 



. . 79-7 



Sulphide of zinc . 



74-4 



Black platinum . 



. . 89-0 



The transmissions here are lower than when the sulphur ce- 

 ment was employed. I do not, however, think that the differ- 

 ences are due to the employment of the cement, but to a slight 

 source of disturbance, which was removed in the later expe- 

 riments. 



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 co- 

 piously transmitted by the rock-salt than the feeble radiator. 

 To render this clear, I have in Table III. appended to the trans- 

 mission the corresponding total radiation. The only striking 

 exceptions to this rule exhibited in Table III. are marked with 

 asterisks. This result, I think, is what might fairly be expected ; 

 for the character which enables a molecule of one substance to 

 radiate a greater quantity of heat than another, may also be ex- 

 pected to influence its rate of oscillation. Hence, as a general 

 rule, a greater dissonance will exist between the vibrating periods 

 of good radiators and bad radiators, than between the periods of 

 the members of either class. But the greater the dissonance the 

 less will be the absorption; hence, as regards transmission 

 through rock-salt, we have reason to expect that powerful radia- 

 tors will find a more open door to their emission than feeble ones. 

 This is, as I have said, in general the case. But the rule is not 

 without its exceptions ; and the most striking of these is the case 

 of black platinum, which, though but a moderate radiator, 





