168 HISTOKY OF PHOTOGEAPHY. 



CHRYSOTYPE. 



A process of an analogous character to that which has just been 

 described, and in which the chloride of gold is an agent, must be next 

 described. This was discovered at the same time as the cyanotype, 

 and has been termed the chrysotype. 



In order to ascertain whether any portion of the iron in the donble animoniacal 

 salt employed had really undergone deoxidation and become reduced to the 

 sitate of protoxide, as supposed. I had recourse to a solution of gold exactly 

 neutralized by carbonate of soda. The protosalts of iron, as is well-known to 

 chemists, precipitate gold in the metallic state. The effect proved exceedingly 

 striking, issuing in a pi-ocess no wise inferior in the almost magical beauty of its 

 effect to the calotype process of Mr. Talbot, which in some respects it nearly 

 resembles ; with this advantage, as a matter of experimental exhil)ition, that 

 the disclosure of the dormant image does not require to be performed in the 

 dark, being not interferred with I)y moderate daylight. As the experiment will 

 probably be repeated by others, I shall here describe it ab initio. Paper is to 

 be washed with a moderately concentrated solution of ammouio-citrate of iron 

 and dried. The strength of the solution should be such as to dry into a good 

 yellow color, not at all brown. In this state it is i-eady to receive a photo- 

 graphic image, which may be impressed on it either from nature in the camera 

 obscura, or from an engraving on a frame in sunshine. The image so impressed, 

 however, is very faint and sometimes hardly perceptible. The moment it is 

 I'emoved from the frame or camera it must be washed over with a neutral 

 solution of gold of such strength as to have about the color of sherry wine. 

 Instantly the picture appears, not, indeed, at once of its full intensity, but dark- 

 ening with great rapidity up to a certain point, depending on the strength of the 

 solution used, etc. At this point nothing can surpass the sharpness and perfec- 

 tion of detail of the resulting photograph. To arrest this process and to fix the 

 picture (so far at least as the further agency of light is concerned), it is to be 

 thrown into water very slightly acidulated with sulphuric acid and well i^oaked, 

 dried, washed with hydrobromate of potash, rinsed, and dried again. 



Such is the outline of a process to which I propose applying the name of 

 chrysotype in order to recall by similarity of structure and termination the 

 calotype process of Mr. Talbot, to which in its general effect it affords so close a 

 parallel. Being very recent, I have not yet (June 10, 1842) obtained a complete 

 command over all its details, but the termination of the session of the society 

 being close at hand I have not thought it advisable to suppress its mention. In 

 point of direct sensibility the chrysotype paper is certainly inferior to the cal- 

 otype, but it is one of the most remarkalile peculiarities of gold as a photo- 

 graphic ingredient that extremely feeble impressions once made by light go on 

 cftericards darkening spontaneously and very slowly, apparently without limits 

 so long as the least vestige of unreduced chloride of gold remains in the paper. 

 To illustrate this curious and (so far as applications go) highly important 

 property, I shall mention incidentally the results of some experiments made 

 during the late fine weather on the habitudes of gold in presence of oxalic acid. 

 It is well known to chemists that this acid heated with solutions of gold precipi- 

 tates the metal in its metallic state. It is upon this property that Berzelius has 

 founded his determination of the atomic weight of gold. Light as well as heat 

 also operates this precipitation, but to render it effectual several conditions are 

 necessary : First, the solution of gold must be neutral, or at most very slightly 

 Acid; second, the oxalic acid must be added in the form of a neutral oxalate; 



