^■o. rv.] APPENDIX. 163 



paper the coloTir whicli it acquired by light, this may be performed either 

 by a strong solution of corrosive sublimate, which -will render the paper 

 quite white, or by a strong solution of hydiiodate of potaeh, which gives 

 it a yellow tint. If to the saturated solution of corrosive sublimate a little 

 gum be added, it may be used with a quill pen, either to prevent the 

 action of light, or to make white lines or mai-ks after the action of the 

 solar rays. Drawings may be made with great effect in this way, on 

 paper previously exposed to the sun; and this is by far the best mode of 

 proceeding, when naturalists or others are desirous of circulating a few 

 copies of any delineation among their own friends ; for, as the white parts 

 are exceedingly diaphanous, and the black impervious to light, the draw- 

 ings made by this means are much more distinct than those made by the 

 ordinary described processes. This mode will be found exceedingly valu- 

 able where a few copies of any drawing of machinery are suddenly wanted 

 for estimates of prices or other causes ; and the strongest light will never 

 affect the original drawing. 



By the common method of making photogenic drawings, should any 

 be imperfect or otherwise damaged, it will be better to expose them freely 

 to the action of the sun ; by which means a uniform black ground will be 

 produced, which will be suitable for the use of the corrosive sublimate : 

 and thus any waste wiU be prevented. A thin paper, which should be 

 slightly moistened before use, is most appKcable to this mode of drawing. 

 The photogenic paper may be blackened either by a dilute solution of 

 protosolphate of iron, or by hydrosulphate of ammonia. 



The principal points in every department of the photogenic art have 

 now been described; and if the minutiae which have been detailed are 

 strictiy followed, and the preparation of silver suited to the kind of paper 

 as here laid down, the student in this interesting and new field of science 

 will be enabled not only to prepare his paper, but also to make and fix his 

 drawings with ease and certainty. 



An omission was made in the paper, on Photogenic Drawing, which 

 was inserted in the last Number of the Literary Gazette; for, whilst 

 treating of the ammonio-nitrate of silver, I forgot to mention the pro- 

 portions which were found most suitable : this may seem unimportant in 

 a paper which is not very sensitive, but, as the ease and certainty of its 

 preparation, as well as its cheapness, exceed all other described papers, it 

 possesses a particular claim on our notice. Twenty grains of nilrate of 

 silver are to be dissolved in an ounce of water; then a few drops of 

 ammonia are to be added, which at first throws down a considerable 

 precipitate ; this, by a further addition of ammonia, redissolves, and the 

 solution becomes quite clear, when it will be ready to be applied by a 

 sponge to the paper. The most suitable paper for this preparation of 

 silver is the " double copy." The whole cost of photogenic paper does not 

 exceed, by this process, one penny for a sheet equal in size to large 

 foolscap, which, if bought of vendors, would cost between one shilling and 

 fourpence and four shillings. The cost of all other papers does not exceed 

 twopence-halfpenny the sheet, except that prepared with the bromide of 

 potassium ; which, for the same quantity, would be about one penny more 

 expensive. The preparation of paper suitable for the use of the corrosive 



M 2 



