206 Mr. R. Hunt on the Use ofHydriodic Salts 



on which the light acts with the most power become the 

 darkest of the drawing, while the shaded parts are whitened. 



The dark specks which abound in some kinds of paper 

 must be avoided, and the spots made by flies very carefully 

 guarded against. These are of small consequence, indeed are 

 not noticed during the darkening action ; but when the hy- 

 driodic wash is applied they form centres of chemical action, 

 and the bleaching process goes on around them independent 

 of light, deforming the drawing with small rings, which are 

 continually extending their diameters. 



13. The Muriated Solutions. These saline washes may be 

 considerably varied, and combined to an indefinite extent with 

 a continued change of effect, which is singularly interesting. 

 In their application I am invariably "guided by the combi- 

 ning proportion of the salt; for having tried solutions of all 

 strengths, I am at length satisfied no other proportions give 

 such certain results ; consequently I always work with my 

 scale of equivalents at hand. The following is a list of the 

 salts I most frequently use, selected from upwards of seven 

 hundred combinations which I have tried. They are placed 

 in the order of sensitiveness they appear to maintain, when 

 used under as nearly as possible the same circumstances. 



Colour of Picture. 



TV/T . r f Red, changing to black in 



a. Muriate of ammonia ,..< h' 



b. Chloride of sodium .... Ditto. 



c. Muriate of strontia .... Brown, changes but slightly. 



/r ' i Lii f A rich brown inclining to 



A. Mlimte of baryta ....... j pm ,^ ^^ ^fy 



c. Sol. chloruret of lime . Very red. 



f. Sol. chloruret of soda . Red, changes a little. 



g. Iodide of potassium .... Yellow brown. 



7 w , r f Variable, sometimes yellow- 



h. Chlorate of potassa .... 



ihj of a 



i. Phosphate of soda ....... Mouse colour. 



k. Urateofsoda ............ Yellow brown. 



/. Muriate of iron ......... Deep brown, blackens. 



m. Bromide of sodium .... Red brown. 



The change I mention in the colour of the finished pic- 

 ture is that which arises from a fresh exposure to the solar 

 rays ; where no change is mentioned, it is too slight to be worth 

 notice. This phenomenon will, however, occupy our atten- 

 tion presently (38.). In addition to the salts named I some- 

 times use 



