202 



THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



colourless this substance is, the more perfect is the mirror, because 

 then the luminous rays which have to travel twice through it to 

 return to the eye after being reflected by the polished surface of the 

 quicksilver, are unchanged in tint and only very slightly weakened by 

 this double transmission. 



One word now on the reflecting surface of silvered mirrors this 

 surface, which is not of glass, but a thin sheet of tin amalgam 

 (that is, a mixture of tin and mercury), is placed at the back of the 

 glass in the following manner. On a very smooth stone table, sur- 



FIG. 139. Mirrors of the ancient Egyptians. 



rounded by a gutter, the sheet of tin is spread out, and is then 

 covered with mercury. The glass, well cleaned, is then passed over 

 the stratum of mercury in such a manner as to clear off the surplus 

 liquid metal; then with the help of weights the adhesion of the 

 glass to the amalgam is effected. The process of coating with 

 mercury is injurious to the health of the workpeople, and silvering 

 has been tried, which is produced by pouring on the surface of the 

 glass a composition of nitrate of silver, ammonia and tartaric acid. 



