374 SCIENTIFIC AMUSEMENTS. 



with acetate, carbonate, or borate of sodium, or other analogous 

 fluids capable of allowing the required action to take place. 

 Whether this action is simply a displacement of gold from gold 

 chloride by silver forming metallic gold and silver chloride (as 

 copper is displaced by iron from copper chloride, Expt. 9), or 

 whether the organic matter of the paper, &c., is also involved in 

 the action, seems somewhat uncertain ; but the final result is that 

 the untoned print gradually changes from a reddish hue to some- 

 thing more akin to purple black, owing to the coating over of the 

 silver particles with gold in so fine a state of division as to appear 

 purple or even blue, somewhat like the gold precipitated from 

 solution by phosphorus (Expt. 136). Platinum chloride and various 

 other salts of precious and certain rare metals are capable of giving 

 somewhat analogous but different tones, a similar action pro- 

 bably taking place in each case, so that extremely fine particles of 

 precious metal are deposited over the reddish silver particles. 



Expt. 396. To prepare Iron Prints or Ferrotypes. Although 

 silver salts are the most convenient substances to use for the prepara- 

 tion of negatives by means of the camera, still various other metallic 

 compounds may also be employed for the purpose of obtaining 

 prints from a negative. Thus when compounds of iron of the 

 " ferric " class are exposed to light, especially when in contact with 

 organic matters of certain kinds, they become so acted upon as to 

 form " ferrous " compounds. These latter, when brought into con- 

 tact with ferricyanide of potassium (Expt. 83), produce a blue pre- 

 cipitate of " Turnbull's blue "; but the former do nothing of the 

 kind. Hence if a piece of paper be soaked in a solution of a suit- 

 able ferric salt and be then exposed to light behind a negative for 

 a sufficient time, the chemical change produced will be rendered 

 evident by the production of Turnbull's blue to a greater or lesser 

 extent on soaking the paper in solution of ferricyanide of potas- 

 sium ; so that the picture will appear in the deepest blue where 

 the shadows occurred in the original object photographed, thus 

 representing the least silver deposition in the negative, and con- 

 sequently the most chemical action on the paper prepared with 

 iron ; and, conversely, the lights of the original object will be the 

 palest blues in the print. If a copy of a drawing on tracing paper 

 be thus made, the ground in the developed print will be blue and 

 the lines white ; or similarly, if a print of a piece of lace or a 

 skeleton leaf, &c., be taken (Expt. 392), the ground will be dark 

 blue and the lines faint blue or white. A convenient ferric salt 

 for this purpose is ferric ammonium citrate; by soaking white 

 paper in a solution of this salt to which a little perfectly pure 

 crystallised ferricyanide of potassium has been added, and drying, 



