496 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



iblc, the brushing should be continued for a time with clean 

 water, so as to arrest the tendency of the acid solution to 

 make a further change in the ink. S C\June 23, 1870,215. 



SECRET WRITING. 



In former times secret correspondence was conducted by- 

 writing with milk, diluted sulphuric acid, or a solution of co- 

 balt, which, colorless under ordinary circumstances, becomes 

 visible by heating the paper. At the present time, however, 

 a very different class of fluids is used, especially the dilute 

 saline solutions, which are known to the receiver of the letter 

 alone. Without being specially skilled in chemistry, the re- 

 ceiver needs only to know the particular solution in which 

 the paper has been moistened to have it in his power to bring 

 the invisible characters to light. Thus, should the letter be 

 written with a solution of sugar of lead, one of sulphide of 

 potassium will have the desired effect. Or, if nitrate of sil- 

 ver be used, the paper must be dipped in a solution of am- 

 monia. ' In either case a black letter is the result. Any color- 

 less solution, however, which, when mixed with another equal- 

 ly colorless, produces a colored deposit, may be employed for 

 this purpose. The number of substances. therefore availa- 

 ble is very great in the many possible combinations of the 

 kind. The whole subject has recently received renewed at- 

 tention in Germany. and England since the introduction of 

 the "correspondence cards," as it is hoped to secure the priv- 

 acy of an ordinary letter by writing upon them with sympa- 

 thetic ink.- 15 (7, xiv., 210. 



PHOTOGRAPHING ON WOOD FOR ENGRAVING. 



The use of photography in transferring a picture upon 

 wood, 'as preliminary to the work of the wood engraver, is 

 rapidly extending, although the best method of doing this is 

 a secret confined to a few persons. One of the best, ways of 

 accomplishing the desired result is said to be indicated in the 

 following- statement, which we borrow from a London con- 

 temporary : The block on which the picture is to be made is 

 first dampened with water, then whitened with enamel rub- 

 bed from the surface of good enameled visiting cards. Rub 

 gently, removing only the enamel, after which brush the block 

 smooth with a moderately stiff brush from right to left and 



