of the Solar Spectrum on Vegetable Colours. 171 
non-argentine compound has lately occurred to me in the ex- 
amination of that interesting salt, the ferrosesquicyanuret of 
potassium, described by Mr. Smee in the Philosophical Maga- 
zine, [S. 3.] No. 109, September 1840, and which he has shown 
how to manufacture in abundance and purity by voltaic ac- 
tion on the common, or yellow ferrocyanuret. In this process 
nascent oxygen is absorbed, hydrogen given off, and the cha- 
racters of the resulting compound in respect of the oxides of 
iron, forming as it does Prussian blue with protosalts of that 
metal, but producing no precipitate with its persalts, indicate 
an excess of electro-negative energy, a disposition to part with 
oxygen, or, which is the same thing, to absorb hydrogen (in 
the presence of moisture), and thereby to return to its pris- 
tine state, under circumstances of moderate solicitation, such 
as the affinity of protoxide of iron (for instance) for an addi- 
tional dose of oxygen, &c. 
203. Paper simply washed with a solution of this salt is 
highly sensitive to the action of light. Prussian blue is de- 
posited (the base being necessarily supplied by the destruction 
of one portion of the acid, and the acid by the decomposition 
of another). After half an hour or an hour’s exposure to sun- 
shine, a very beautiful negative photograph is the result, to 
fix which all that is necessary is to soak it in water, in which 
a little sulphate of soda is dissolved, to ensure the fixity of 
the Prussian blue deposited. While dry, the impression is 
dove-colour or lavender blue, which has a curious and striking 
effect on the greenish yellow ground of the paper produced 
by the saline solution. After washing, the ground colour dis- 
appears, and the photograph becomes bright blue on a white 
ground. If too long exposed it gets ‘* oversunned,” and the 
tint has a brownish or yellowish tendency, which however is 
removed in fixing: but no increase of intensity beyond a cer- 
tain point is obtained by continuance of exposure. 
204. Prismatic examination of this process demonstrates the 
remarkable and valuable fact, that the decomposition of the 
salt and deposit of Prussian blue is due to the action of the 
blue and violet rays, the less refrangible rays below the blue 
having absolutely no influence either to exalt or diminish the 
effect. ‘The limits of action are about + 18°0 and + 61:0, 
fading insensibly both ways. The greatest intensity of ac- 
tion is at + 38. A feebler maximum occurs at + 23. The 
intensity of the impression is much increased by washing with 
acidulated water, still more if it hold in solution a little per- 
salt of iron; but in this case the ground, if not very carefully 
defended from light, is blue. 
205. If a solution of this salt, mixed with perchloride of 
