K. MECHANICS AND ENGINEERING. 395 



is gaining ground that cold at least does not increase the 

 tendency to fracture of iron, and that the greater amount of 

 breakage is probably due, as previously suggested, to the 

 diminished degree of elasticity of the road-bed, in conse- 

 quence of the freezing of the soil. 1 A, March 17, 124. 



ZINC ROOFING. 



A great objection to the use of zinc as a covering for the 

 roofs of houses, in spite of its cheapness, and the ease with 

 which the sheets can be laid on, is found in the ready oxida- 

 tion of the metal during wet weather, as well as in the un- 

 pleasant glare proceeding from it in the sunlight. Both of 

 these defects may, however, be obviated by the application 

 of a certain substance which gives to it a permanent slate 

 color, and at the same time prevents decomposition. This is 

 prepared by heating in a porcelain dish one part, by weight, 

 of copper scales, w T ith a mixture of three parts of hydrochlo- 

 ric acid, and one part of sulphuric acid, and continuing the 

 operation until the red vapors cease to be evolved and the 

 copper is dissolved. After this, sixty-four parts of water are 

 to be added to the green solution, and the whole filtered. 



The sheets of zinc to which the application is to be made 

 are to be thoroughly cleaned, and then immersed for a few 

 moments in this liquid. Afterward they are to be washed 

 with water and allowed to dry in the open air. "When dry 

 they are to be dipped in a solution made by dissolving one 

 part of black pitch and two parts of natural asphaltum in 

 twelve parts of benzine or light coal-oil. After drying, a 

 dull lustre may be imparted to the zinc by rubbing it with 

 cotton or a cotton cloth. 16 C, in., 287. 



METHOD OF WARMING RAILROAD CARS. 



For the purpose of warming and ventilating railway car- 

 riages, it is proposed by Mr. Grandjean, of Paris, to use a res- 

 ervoir of suitable capacity, divided into any number of com- 

 partments, in which are placed sponges saturated with the 

 essence of petroleum, mineral essence, or any other carburet 

 of hydrogen of a volatile nature. In the middle of these 

 compartments spaces are left so as to introduce wicks, which 

 should touch the bottom of the receptacle, the number of the 

 same being as large or as limited as desired ; the upper part 



