Proceedings of the Polytechnic Association. ]003 



steel plate, tlie perfectly smooth and ivory-like surface of chalk 

 remaining is covered with a wash of strong size, and when dry is 

 ready to receive a line drawing, similar to that made on wood. The 

 drawing is made with sable-hair brushes of different sizes, wet with 

 an ink composed of glue and lan^pblack witli a little glycerin, or of 

 a solution of borax and shellac with lampblack. After the drawing 

 is completed, the surface is submitted to the action of a rotating brush, ' 

 which removes all the chalk coating not covered by the ink, thus 

 leaving the drawing in relief. To protect this surface, it is coated 

 with a solution of water-glass which hardens it, so that a mould can 

 bo taljen from it in the ordinary manner, from which the electrotype 

 or stereotype, to be used in printing, is made. In this case, the artist 

 is the author of the engraving, and is sure to find the picture repro- 

 duced precisely as he made it. This result is obtained with readiness 

 and at a trifling cost ; and for many kinds of illustration, the process 

 is found to have advantages over ordinary wood engraving. 



Aluminate of Soda. 

 This compound, as produced hj the Cryolite soda manufacturers, 

 is quite free from iron, and is especially serviceable in making white 

 opaque glass for French glass gas-globes, in dyeing, and in soap-mak- 

 ing, where heavy, white soap is required. 



Coal Gas not Injurious to Egots. 

 Dr. Poselger has sliown by repeated experiments that the growth 

 of trees and shrubs is not interfered with by any quantity of coal 

 gas tliat may escape in the soil and find its way to their roots, and 

 consequentl}' tliat the illuminating gas escapijig from mains does not 

 injure the trees growing along the streets and promenades of cities, 

 as many persons had previously supposed. 



Temperature of Salt Solutions. 

 Mr. Spence, of England, has been led by experiment to conclude 

 that the temperature of a solution of any salt is in proportion to its 

 specific gravity. He placed a solution of nitrate of soda (having a boil- 

 ing point of about 250*^ Fahrenheit), in a vessel surrounded by a steam 

 jacket ; into the latter he admitted steam until the whole jacket was 

 heated to about 212"^, steam was then discharged into the solution, 

 when the mercury of a thermometer placed in the solution, slowly 

 and steadily moved upward until it indicated a temperature of 250? 

 Fahrenheit. 



