168 
light *-. The muriate of silver, when exposed to the 
solar rays, begins to be discoloured at the end of a 
few seconds ; after a minute, says M. Senebier, its 
surface is sensibly violet \ and in half an hour, the 
violet is changed to the shade of umber, and then 
suffers no farther change. This change is effected 
entirely by light alone ; for when this muriate is ex- 
posed to heat, or cold, or moisture, in a dry air, or 
in vacuo, it suffers no change, if the light be careful- 
ly excluded. If, however, the light be thrown on 
it by a lens, it is then coloured in an instant. If it 
be covered by one leaf of paper, the discolouration 
does not begin till the expiration of a minute ; if with 
two leaves, three minutes are required ; if with three 
leaves, ten minutes ; and four leaves entirely prevent 
the action of light f. 
432. In effecting this discolouration, it was re- 
marked by Scheele, that the violet ray acted sooner 
than any other J j and Senebier having thrown the 
prismatic rays, in succession, on portions of this mu- 
riate, observed that the violet ray acted in 15 se- 
conds ; the indigo in 23 seconds ; the blue in 29 ; 
the green in 37 ; the yellow in five minutes and a 
half ; the orange in twelve minutes ; and the red in 
twenty minutes ; but the three last species never pro- 
duced the effect so strongly as the others . 
* On Air and Fire, p, 78. et seq. 
t Mem. Phys. Chim. t. iii. p. 199. 
J On Air and Fire, p. 91. 
Mem. Phys. Chim, torn. iii. p. 199. 
