292 Sir John Robison on the best Means of 



of gas to atmospheric air shall never be so great as to allow of 

 the flame becoming yellow, as, with this precaution, the com- 

 bustion of the carburettecl hydrogen will be complete, and no 

 deposit of soot will take place on cold bodies when set over the 

 flames ; the proper quantity of gas in the mixture is easily de- 

 termined by the stop-cock belonging to each stove. 



For ordinary culinary purposes, the cylinders may be thirty 

 inches long, and three to four inches diameter, and the wire- 

 cloth for the tops should have about thirty wires to the inch. 

 That which is manufactured for safety-lamps answers well for 

 this purpose. 



Whenever, from accidental injury or decay, a hole takes 

 place in a diaphragm, it is no longer possible to use it ; as, 

 when lighted, the flame passes through the fracture, and com- 

 municates with the jet at the bottom of the cyhnder, which 

 then burns like an ordinary gas-light, and, like it, would black- 

 en the surface of any cold body presented to it. The wire- 

 cloths, if not broken through by violence, will last for months 

 although in daily use ; and, if covered by a layer of coarse 

 sand, or pounded limestone, will continue serviceable for an 

 unlimited period. 



When more intense heat is required than is attainable by 

 the unaided combustion of the mixed gases, recourse may be 

 had to various forms of blow-pipes ; and when a large volume 

 of such flame is to be employed, the current of atmospheric 

 air may be ui'ged by double bellows. A very efiicient appa- 

 ratus on this principle is to be seen in the laboratory of Dr D. 

 B. Reid. 



It is to be regretted, that such applications of gas are not 

 more generally known and introduced into work-shops, as 

 there are numerous processes in the arts in which they would 

 affbrd facilities to the workman which he can scarcely com- 

 mand by any other means. For example, in the hardening of 

 steel-tools, it is well known that a piece of bright steel, when 

 heated to redness in a forge or muffle, is subject to oxydation, 

 and that a black scale remains after hardening, which it is 

 difficult to remove without some injury to the work, as in the 

 case of a screw tap ; whereas, if the same piece of steel be 



