332 SALTS. 



Copying by the anastatic process, obviously depends 

 upon the absence of oil from the back ground of the 

 picture. The employment of an oil tint, instead of 

 blank paper, for the back ground, is therefore a perfect 

 security against it. Counterfeiting by the photographic 

 process depends on the fact, that the light which falls 

 on a picture is intercepted by the dark letters. If they 

 are printed in a transparent blue, the chemical rays 

 are permitted to pass through the printed as well as the 

 imprinted portions. A copy with the contrasts of the 

 original picture is thereby rendered impossible. By 

 printing with blue ink, on a back ground of some other 

 color, both of the securities against counterfeiting 

 above mentioned, are combined. 



CHEMICAL ANALYSIS. 

 845. DIRECT METHOD. In the process 



Describe anal- 

 ysis by sol- of analysis, advantage is taken of the dis- 

 tinguishing properties of different sub- 

 stances, to effect their detection and separation. They 

 may sometimes be separated by the employment of a 

 solvent which acts upon one, and leaves the other mi- 

 dissolved. The separation of silver from gold in the 

 process of assaying, is a case in point. 

 Describe di- 846. A more common method is to bring 



rect analysis tne wno le substance into solution, and 



oy precipita- 

 tion, afterward separately to precipitate its sev- 

 eral constituents, by agents which have no effect upon 

 the rest. The separation of alumina from lime may 

 serve as an example. A mixture of the two being dis- 



