paper, alternating with solutions of the ammpnio-tartrate or ain- 

 monio-citrate of iron,* the latter solutions being last applied, and 

 in more or less excess. I purposely avoid stating proportions, as 

 I have not yet been able to fix upon any which certainly succeed. 

 Paper so prepared and dried takes a negative picture, in a time 

 varying from half an hour to five or six hours, according to the 

 intensity of the light ; and the impression produced varies in 

 apparent force from a faint and hardly perceptible picture, to one 

 of the highest conceivable fulness and richness, both of tint and 

 detail, the colour in this case being a superb velvety brown. This 

 extreme richness of effect is not produced except lead be present, 

 either in the ingredients used, or in the paper itself. It is not, 

 as I originally supposed, due to the presence of free tartaric acid. 

 The pictures in this state are not permanent. They fade in the 

 dark, though with very different degrees of rapidity, some (espe 

 cially if free tartaric or citric acid be present) in a few days, while 

 others remain some weeks unimpaired, and require whole years 

 for their total obliteration. But though entirely faded out in 

 appearance, the picture is only rendered dormant, and may be 

 restored, changing its character from negative to positive, and its 

 colour from brown to black (in the shadows) by the following pro- 

 cess : A bath being prepared by pouring a small quantity of 

 solution of pernitrate of mercury into a large quantity of water, 

 and letting the sub-nitrated precipitate subside, the picture must 

 be immersed in it, (carefully and repeatedly clearing off all air 

 bubbles,) and allowed to remain till the picture (if anywhere 

 visible) is entirely destroyed, or if faded, till it is judged sufficient 

 from previous experience ; a term which is often marked by the 

 appearance of a feeble positive picture, of a bright yellow hue, 

 on the pale yellow ground of the paper. A long time (several 

 weeks) is often required for this, but heat accelerates the action, 

 and it is often complete in a few hours. In this state the picture 

 is to be very thoroughly rinsed and soaked in pure warm water, 

 and then dried. It is then to be well ironed with a smooth iron, 

 heated so as barely not to injure the paper; placing it, for better 

 security against scorching, between smooth clean papers. If then 

 the process has been successful, a perfectly black positive picture 



* So commonly called, and sold as snch ; bat as I am disposed to regard their 

 composition, their chemical names would be ferro-tartrate and ferro-citrate of 

 ammonia. 



