162 APPENDIX. [No. IV. 



paper. The substances that may be employed for this purpose are dilute 

 muriatic acid, chloride of sodium, hydriodio acid, hydriodate of potash, 

 iodic acid, hyposulphites, and sulphocyanate of potash. Before using any 

 of these substances, the drawing ought to be soaked in common water for 

 a few minutes, to remove any excess of the salt of silver; the stoppiag 

 solution is then to be applied with a sponge to every part of the surface 

 equally. 



No particular advantage attends the use of -the muriatic acid, but it 

 wiU be found to stop pretty well when in the proportion of about twenty- 

 four drops of the distilled acid to an ounce of water, but it is not quite 

 permanent. The chloride of sodium, or common salt, is very effectual in 

 stopping any further action of the light, as drawings fixed by this agent 

 have not undergone the slightest alteration from many hours' exposure to 

 the brightest sunshine. When the impressions are very dark, they do not 

 change colour, but lighter drawings become altered to a yellowish brown : 

 the addition of a little sesquichloride of iron corrects this, and gives a 

 pink tinge to them. The solution recommended by Mr. Bird answers very 

 well ; it contains two ounces of salt, and one ounce of the sesquichloride 

 of iron, to the pint of water. The hydriodic acid, and the hydriodate of 

 potash, are also very effective in preventing any further action of the 

 solar rays ; they turn the white parts to a pale yellow, and are very apt, 

 if the solution be too strong, to remove the colour of the dark ground, 

 especially if the drawing has been exposed to the light for only a short 

 time : for this reason, the solution of hydriodate of potash ought not to 

 exceed ten grains to the ounce of water. A solution of iodic acid, fifteen 

 or twenty grains to the ounce, is very excellent for stopping photogenic 

 drawings ; it is particularly applicable to delicate drawings of feathers, 

 when it is desirable not to allow them to remain long in the light; and 

 at the same time the contrast of black and white heightens the effect. 

 Care must be taken not to apply too strong a solution, for that is apt to 

 whiten the dark ground, but it never turns it to any other tint. 



The hyposulphates of potash and soda have been much used for the 

 fixing of drawings, but, if exposed to the sun, they do not appear quite 

 so effective as the common salt, or hydriodate of potash ; they have the 

 advantage, however, of stopping them a darker colour. The sulpho- 

 cyanate of potassa is also found to stop these drawings ; it changes the 

 colour of the ground to a brown, and has no particular advantage. 



The different effects of these several fixing-solutions can be turned 

 to good account by suiting the colour of the drawings to the fancy of the 

 artist, or the nature of the subject ; and a still greater alteration of tint 

 may be produced by varying the duration of time which the Hght is 

 allowed to act upon the paper. 



Many other chemical substances have been tried for fixing the draw- 

 ings, but none attended with success. The following are the principal : — 

 Chlorine, chloride of soda, chloride of Ume, tincture sesquichloride of 

 iron, chloride of manganese, chloride of tin, chlorate of potassa, solution 

 of iodine in water and in alcohol, carbonate of potash, hydrocyanic acid, 

 dichromate of potash, biborate of soda, oxalate of ammonia, fluate of 

 ammonia, benzoate of ammonia, succinate of ammonia, phosphate of soda, 

 gallic acid, arsenite of ammonia, and sulphite of soda. 



Should it from any cause be thought desirable to remove from the 



