364 Dr. R. Hare on Improvements in the 



in fact, always be nearly double that which existed before the 

 pipe B was closed. 



In order that nearly the whole of the acid shall be expelled 

 from the inferior vessel, the tray must be depressed till it 

 touches the bottom of that vessel. 



The pressure being four atmospheres at commencement, as 

 soon as, by means of a pipe attached to the valve-cock N, an 

 escape of gas is allowed, the acid is forced again upon the 

 zinc, and thus prevents a decline of pressure to any extent 

 sufficient to interfere with the process. 



The gases may be used from a receiver in which they exist, 

 in due proportion, safely by the following means : — 



Two safety-tubes are to be made, not by Hemming's pro- 

 cess exactly, but as follows : 



A copper tube, silver soldered, of which the metal is about 

 the eighth of an inch in thickness, is stuffed with the finest 

 copper wire, great care being taken to have the filaments 

 straight and parallel. The tube is then to be subjected to 

 the wire-drawing apparatus, so as to compress the tube on its 

 contents until the draught becomes so hard, as that it cannot 

 be pushed further without annealing. The stuffed tube thus 

 made is to be cut into segments, in lengths about equal to the 

 diameter, by a fine saw. The surfaces of the sections are to be 

 filed gently with a smooth file. By these means they appear 

 to the naked eye like the superficies of a solid metallic cylinder. 

 Brass caps being fitted on these sections, they are to be inter- 

 posed by soldering, at the distance of a foot or more, into the 

 pipe for supplying the jet. Under these circumstances, the 

 posterior section becoming hot, may allow the flame to retro- 

 cede ; but the anterior section being beyond the reach of any 

 possible combustion and remaining cold, will not allow of the 

 retrocession ; and as soon as the flame passes the first section, 

 the operator, being warned, will of course close the cock, and 

 subject the posterior section to refrigeration before proceeding 

 again. 



But this plan of operating may be rendered still more se- 

 cure by interposing a mercury bottle, or other suitable iron 

 vessel, half-full of oil of turpentine, between the reservoir and 

 safety tubes, as in the arrangement of a Woulfe's bottle. A 

 leaden pipe proceeding from the reservoir is, by a gallows- 

 screw, attached to an iron tube which descends into the bottle, 

 so that its orifice may be near the bottom. The leaden 

 pipe communicating through the safety tubes with the jet- 

 pipe, is attached to the neck of the bottle. Thus the gaseous 

 mixture has to bubble through the oil of turpentine in order 

 to proceed through the safety tubes to the jet-pipe. If, while 



