26 JOURNAL OF THE 
chloride. Mercuric chloride in aqueous solution is slowly 
changed to mercurous when exposed to the light. 
2H¢gCl,+ H,O =-2HgCl4+ 2HC1+0. 
Gold chloride (AuCl,), in contact with organic substances, 
when exposed to light, is changed first to aurous chloride 
(AuCl) and then to metallic gold. 
It is quite well known that aray of light falling upon a 
piece of selenium changes its conducting power for electric- 
ity. This is not a change of valence but has, it would seem, 
its bearing upon the problem as a possible change in vibra- 
tion. The chemical action of light is generally attributed to 
the vibrations set up among the molecules. Rays having the 
shortest wave-lengths and the greatest frequency are most 
active in this respect though all the rays of the visible spec- 
trum have been shown to exert some action. So far as this 
variation in valence is caused by light then the hypothesis of 
a change in vibration necessitating a change in equilibrium 
may well serve as an explanation. 
Heat.—Again these variations are often easily brought 
about by changes of temperature. Thus cupric chloride be- 
comes cuprous chloride. 
CuCl, =CuCl+Cl. 
Mercurous chloride is temporarily changed into mercuric, the 
mercurous re-forming npon cooling. 
2H¢gCl—=H¢g+ HgCl,. 
Phosphorus pentachloride becomes the trichloride. 
PC1,=PCl, + Cl,. 
Arsenic pentoxide becomes trioxide. 
| As,O,= As,0,+O,. 
An interesting series of changes are those in the sulphur 
chlorides. Thus sulphur tetrachloride (SCl,) becomes sul- 
phur dichlorine (SCl,), if warmed above —22°, and this be- 
comes sulphur monochloride (S,Cl,), if heated above 64°. 
This last can be boiled without change. 
These instances might be multiplied but it is not necessary. 
The most plausible explanation offered as to the effect of 
