THE WOMBAT. 79 



best and most careful photographers in the world — is that 

 known as the alkaline gold toning bath, the desired tone 

 being obtained in this bath, and the prints, after subsequent 

 washing, being fixed in the customary manner with hypo- 

 sulphite of soda (strictly speaking thiosulphite of soda.) 

 If a print, after the preliminary washing which is given it in 

 order to get rid of free silver, acid, etc., were treated with 

 a weak solution of chloride (or terchloride) of gold, it would 

 gradually become bleached, and the red image would change 

 to blue, because the chloride of gold on coming into contact 

 with the reduced silver forming the image would be decom- 

 posed, and while the metallic gold, being set free, would be 

 attracted to and deposited on the print, the nascent chlorine 

 would attack the image and convert it into chloride of silver 

 again, and, on the print being fixed only the gold would be 

 left, the chloride of silver being soluble in hypo., and we 

 would get a very faint, sickly image ; so in practice it is 

 customary to combine with the chloride of gold some 

 chemical which will take up the chlorine more readily than 

 the silver forming the image will, thus leaving the gold 

 to be deposited on the image, and yet not re-converting the 

 reduced silver into chloride. 



Various salts are used for this purpose, that used 

 generally for gelatine and collodion prints being sulphocy- 

 anide of ammonium, while for albumen prints the various 

 salts of sodium (acetate, carbonate, phosphate, tungstate. 

 bi-borate, borax, &c), and the chloride and carbonate of 

 lime are used, as these, having a greater affinity for the 

 chlorine than the image has, take it up as it is set free from 

 the gold, and prevent it bleaching the print. When the 

 print is immersed in this bath a chemical change is started,. 

 the solution being decomposed in a manner, and the gold, 

 leaving its solution of ter-chloride, is attached to the surface 

 of the silver forming the image. The solution is not com- 

 pletely decomposed, for in many instances it may be made as 

 good and active by the mere addition to it of more ter-chloride 

 of gold, the other part of its composition seeming to remain 

 practically the same as before, but for our purpose and for 

 convenience of illustration we may speak of the action as a 

 species of decomposition. 



The deposited metallic gold is of a blue colour, and as 

 this gradually covers over the red organic compound of 

 silver forming the print, it changes its tint precisely as would 

 be the case were a blue transparent pigment to be laid over 

 the red one, the mixture of red and blue making first purple 

 and then, if the action of the bath be continued for a sufficient 

 length of time, violet. In the latter case a considerable 

 quantity of gold is reduced and deposited on the print, and 

 all other things being equal, the precise shade which the 



