300 



Prof. Tyndall on the Influence of Colour 



electrifying the cubes. I tried this plan, and found it simple 

 and practicable. It was, however, aided by a circumstance 

 which we did not anticipate. The cube being placed upon an 

 insulating stand, the powder was shaken over it, and electrified 

 by a few turns of a machine. It was found that the cube might 

 then be discharged and set upright, the powders clinging to it 

 in this position. The results obtained with this arrangement 

 are recorded in the following Table : — 



Table II. — Radiation from Powders held by Electricity. 



Substance. Radiation, 



Rock-salt 24-5 



Chloride of silver (white). 25*0 



Substance. Radiation. 



Sulphide of calcium . . 49*1 

 Sulphate of baryta . .51*3 



Sugar 52*1 



Red oxide of lead . . 56'5 

 Sulphide of cadmium . 56*9 

 Sulphate of lime . . . 593 

 Chloride of silver (black) 60-0 



Carbonate of zinc , . 



. 620 



Oxide of cobalt . . 



. 625 



Iodide of copper . 



. 63-0 



Red oxide of iron . . 



. 63-8 



Sulphide of iron . . 



. 65-5 



Black oxide of iron , 



. 65-8 



Milk of sulphur . . . 25*8 



Biniodide of mercury . 26*0 



Iodide of lead .... 360 



Sulphide of mercury . . 30*6 



Spongy platinum . . . 31*5 



Washed sulphur (flowers) 323 



Sulphide of zinc . . . 36*1 



Amorphous phosphorus . 38*0 



Chloride of lead . . . 39'0 



Chloride of cadmium. . 40'0 



Fluor-spar .... 48*6 



The agreement as regards relative radiative power between 

 this and the former Table is as good as could under the circum- 

 stances be expected. The experiments have been several times 

 repeated ; and the Table contains the means of the results, which 

 were never widely different from each other. 



The quantity of radiant heat emitted by a body in all states of 

 aggregation having been thus conclusively shown to depend 

 mainly upon its molecular character, the question as to the qua- 

 lity of the heat emitted next arises. In examining this point, I 

 contented myself with testing the heat by its transmission 

 through rock-salt. The choice of this substance involved the so- 

 lution of the still disputed question whether rock-salt is equally 

 pervious to all kinds of rays*. For if it absorbed the radiation 

 from two different bodies in different degrees, it would not only 

 show a difference of quality in the radiations, but also demon- 

 strate its own incapacity to transmit equally rays of all de- 

 scriptions. 



* The last publication on this subject is from the pen of that extremely- 

 able experimenter Professor Knoblauch. After discussing the results of 

 De la Provostaye and Desains, and of Mr. Balfour Stewart, he arrives at 

 a different conclusion — namely, that pure rock-salt is equally pervious to 

 all kinds of heat. — Poggendorff's Annalen, 1863, vol. cxx. p. 177. 



