296 Prof. Tyndall on the Influence of Colour 



were heated with boiling water, and placed in succession at the 

 same fixed distance in front of the thermo-electric pile, which as 

 usual was well defended from air-currents and other extraneous 

 sources of disturbance. Before giving the complete table of 

 results, I will adduce a few of them, which show in a conclusive 

 manner that in solid bodies radiation is molecular rather than 

 mechanical. 



The biniodide of mercury and the red oxide of lead resemble 

 each other physically, both of them being of a brilliant red. 

 Chemically, however, they are very different. Examined in the 

 way indicated, their relative powers as radiators were found to 

 be as follows : — 



Name. Chemical formula. Radiation. 



Biniodide of mercury . (HgP) 397 



Red oxide of lead . . (2PbO, PbO 2 ) 74-1 



Mixed with gum and applied with a cameFs-hair brush to the 

 surfaces of the cube, the radiation from the following two sub- 

 stances fell out thus : — 



Name. Radiation. 



Binoxide of mercury 8O0 



Red oxide of lead 80*0 



Here the influence of the gum entirely masks the difference 

 due to molecular constitution. 



The effect of atomic complexity upon the radiation is well 

 illustrated by the deportment of these two substances. It is 

 further illustrated by the deportment of two different iodides of 

 mercury : — 



Radiation. 

 Biniodide of mercury (Hg P) . . . 39'7 

 Iodide of mercury (Hg 2 1 2 ) . . . 46"6 



Here the addition of a second atom of mercury to the mole- 

 cule of the biniodide raises the radiation 7 per cent. The expe- 

 riment furnishes a kind of physical justification of the practice of 

 chemists in regarding the molecule of yellow iodide of mercury 

 to be Hg 2 1 2 , and not Hg I. 



The peroxide and protoxide of iron gave the following results: — 



Radiation. 



Peroxide of iron 78*4 



Protoxide of iron 81*3 



I did not expect this, the protoxide being a less complex 

 molecule than the peroxide. On examination, however, the 

 protoxide was found to be in part the magnetic oxide. The 

 formula? of the two substances are Fe 2 O 3 and Fe 0, Fe 2 3 , and 

 the anomaly therefore disappears. 



