M. TECHNOLOGY. 547 



360 grains of alcohol, and diluting the deep blue solution ob- 

 tained with a hot solution of 90 grains of gum-arabic in 1400 

 grains of water. 



A very deep black varnish for wood stained black, brass, 

 or leather, can be prepared by using a solution of 315 grains 

 of shellac in 1440 grains of alcohol, instead of the solution of 

 gum-arabic, to dilute the above-mentioned aniline-black so- 

 lution. 15 C, 1872, XXIV., 382. 



NICKEL-PLATED TYPE. 



Type, electro-plated with nickel, are not only superior to 

 copper-plated in their resistance to friction and pressure, be- 

 ing ten times as durable as ordinary type, on account of the 

 almost steel-like hardness of their surface, but, by reason of 

 the smoothness of the coating even when the nickel is depos- 

 ited in a very thin film, they render the finest lines more per- 

 fectly, and possess the decided advantage of allowing the use 

 of inks of all colors, w^hile the copper-plated change some of 

 these inks, and are acted upon by others, as vermilion, etc. 

 6 (7, 1872, XLViL, 375. 



MOULDS FOE ELECTEOTYPES. 



The employment of graphite with gypsum and gutta-per- 

 cha moulds for electrotype purposes causes much trouble and 

 loss of time, because the graphite must be rubbed till it as- 

 sumes a metallic lustre, which, though a matter of no great 

 difficulty with smooth, even surfaces, is very troublesome 

 when irreG:ularities occur. Better results are obtained when 

 the gypsum mould, soaked in wax, is thickly covered over 

 "with a mixture of silver nitrate solution supersaturated with 

 ammonia and alcohol, and is then exposed to the action of 

 sulphureted hydrogen, whereby sulphide of silver is formed, 

 which is a good conductor. The free ammonia in the solution 

 causes it to adhere more readily to the wax, leaving, on dry- 

 ing, a uniform, unbroken covering of the silver compound. 



When a somewhat powerful battery of from four or five 

 Daniell's elements is used, the layer of copper quickly diffuses 

 itself over the whole surface, whereas, by employing a feebler 

 current, a softer and less friable copper is obtained. 



A good method of preparing the solution is as follows : 

 Dissolve fifteen grains of lunar caustic in thirty of water, to 



