M. TECHNOLOGY. 541 



methods were devised and brought into practice, among the 

 most successful of which was that of Professor Liebig, con- 

 sisting in the employment of the ammoniacal nitrate of silver 

 combined with aldehyde. Others made use of a solution of 

 nitrate of silver mixed with tartaric acid, and ammonia in 

 slight excess. This liquid extends over the surface of the 

 glass, and is finally decomposed by the action of heat, the 

 silver separating in a metallic form and attaching itself to 

 the surface of the glass, after which it is coated by means of a 

 brush with a red-lead paint, to protect it from injury and 

 against surface emanations. This process was the most satis- 

 factory of all, yet left much to be wished for. The manipula- 

 tions Avere very uncertain, requiring the greatest care, and 

 often failing without any assignable cause. 



At this time the labors of a practical chemist in France, 

 M. Dode, were brought into successful application. He had 

 been experimenting for many years ; and, after trying every 

 method that had been suggested, both with mercury and sil- 

 ver in all their modifications, he finally fixed upon platinum 

 as the metal which met every requirement, in its indestruc- 

 tibility and immunity from alteration or injury by sulphurous 

 acids and other vapors ; and he soon presented specimens of 

 his workmanship that were universally admired. At first he 

 followed the old process of applying the platinum to the back 

 of the glass, and, although the results were very satisfactory, 

 he found an advantage in changing his method and affixing 

 his material to the surface of the glass, so that the reflection 

 should be precisely as in a metallic mirror from the outside 

 surface, and not from the inner, after passing through the 

 thickness of the glass. In this way he was enabled to effect 

 a great economy in the nature and cost of the preparation of 

 the glass base of the mirror, as, with a single surface properly 

 smoothed. and polished off ; it was a matter of no consequence 

 what was the nature of the opposite side, nor what the char- 

 acter of the material and texture of the mass of glass itsel 

 And the reflection being from the upper surface, the image 

 was much sharper and clearer, and more perfect, and gave 

 but a sino-le image instead of the double one, which can al- 

 most always be appreciated in the ordinary glass mirror. 



At the present time this branch of industry has developed 

 to a vast extent, and is conducted on a very large scale at 



