W. Crookes on the Wax-paper- Photographic Process. 167 



any empyreumatic odor, and must cause no turbidity when 

 mixed with a solution of nitrate of silver, e. g. in making the 

 exciting bath (42). 



The gallic acid should be as nearly white in color as possible. 



Especial care should be taken to have the alcohol good ; i* 

 should be 60° over proof, and of specific gravity 0*83. On evap- 

 orating a few drops on the palm of the hand, no smell should be 

 left behind, nor should it, under the same circumstances, leave 

 any stain on a sheet of white paper. 



25. The hyposulphite of soda will be found one of the articles - 

 most difficult to obtain pure ; there is a large quantity at present 

 in the market, having little else of the salt but the name, and is 

 of course totally unfit for use ; if there be the least doubt about 

 its purity, it should be tested in the following manner : — 



Weigh out accurately 10 grains of nitrate of silver, dissolve 

 this in half an ounce of distilled water ; then add 4 grains of 

 chlorid of sodium' (common salt) also dissolved in water. On 

 mixing these two solutions together, a white curdy precipitate of 

 chlorid of silver will fall down. Next add 22 grains of the hy- 

 posulphite of soda, and allow it to stand for about ten minutes, 

 stirring occasionally with a glass rod. If at the end of that time 

 the chlorid of silver has dissolved, the hyposulphite of soda may 

 be considered as pure. A greater or less amount of residue will 

 indicate roughly the degree of impurity. 



26. The cyanid of potassium is usually met with in the form 

 of hard white lumps ; they will be found quite pure enough. It 

 is very useful in removing stains formed by nitrate of silver on 

 the fingers, &c, but the greatest care must be taken in its em- 

 ployment, as it is a most energetic poison ; its use in cleaning the 

 dishes from silver stains has been pointed out above (10). 



27. The first operation to be performed is to make a slight 

 pencil mark on that side of the photographic paper which is to 

 receive the sensitive coating. If a sheet of Canson's paper be 

 examined in a good light, one of the sides will be found to pre- 

 sent a finely reticulated appearance, while the other will be per- 

 fectly smooth ; this latter is the one that should be marked. 

 Fifty or a hundred sheets may be marked at once, by holding a 

 pile of them firmly by one end, and then bending the packet 

 round, until the loose ends separate one from another like a fan ; 

 generally all the sheets lie in the same direction, therefore it is 

 only necessary to ascertain that the smooth side of one of them 

 is uppermost, and then draw a pencil once or twice along the 

 exposed edges. 



28. The paper has now to be saturated with white wax. The 

 apparatus for this purpose has been previously described (11.) 

 The wax is to be made perfectly liquid, and then the sheets of 

 paper, taken up singly and held by one end, are gradually low- 



