34 THE POPULAR SCIENCE MONTHLY. 



the opening of the Academy of Sciences by Frederick the Great, at 

 Berlin, on the 23d of January, 1744. With a spark from the sword 

 of one of the court cavaliers present on the occasion, Ludolf ignited 

 sulphuric ether. 



Dr. Watson also made numerous experiments on the ignition of 

 bodies by the electric spark. He fired gunpowder and discharged 

 guns. Causing a spoon containing ether to be held by an electrified 

 person, he ignited the ether by the finger of an unelectrified person. 

 He also noticed that the spark varied in color when the substances be- 

 tween which it passed varied. 



These, and numerous other experiments, may be made with a far 

 simpler " machine " than any hitherto described. It was devised for 

 your benefit by Mr. Cottrell. In the electric machine, as we have 

 learned, the prime conductor is flooded with positive electricity 

 through the discharge of the negative from the points against the 

 excited glass. Your glass tube may be similarly turned to account. 

 A strip of sheet-brass or copper, P, Fig. 30, about five inches long 

 and one inch wide, is sewn on to the edge of the silk pad, jR, employed 

 as a rubber. Through apertures in the strip of metal about twenty 

 pin-points are introduced and soldered to the metal. When the tube 

 is clasped by the amalgam-covered rubber, the metal strip and points 

 quite encircle the tube. 



When a fine wire, w, connects the strip of metal with the knob of 

 a Leyden-jar, by every downward stroke the glass tube is powerfully 

 excited, and hotly following the exciting rubber is the circle of points. 

 From these, against the rod, negative electricity is discharged, the 

 free positive electricity escaping along the wire to the jar, which is 

 rapidly charged. 



Connecting the strip of metal with an insulated metallic knob, 

 placed within a quarter or an eighth of an inch of an uninsulated 

 argand burner, at every downward stroke of the rubber a stream of 

 sparks passes between the knob and burner. If gas be turned on, it 

 is immediately ignited by the stream, of sparks. Blowing out the 

 flame and repeating the experiment, a single stroke of the tube infal- 

 libly ignites the gas. Sulphuric ether, in a spoon which has been 

 previously warmed, is thus ignited : but the ether soon cools by evap- 

 oration ; its vapor is diminished, and it is then less easy to ignite. 

 Bisulphide of carbon may be substituted for the ether, with the cer- 

 tainty that every stroke of the rubber will set it ablaze. 1 The spark 

 thus obtained also fires an electric pistol charged with a mixture of 

 oxygen and hydrogen. The two gases unite with explosion to form 

 water, when an electric spark is passed through them. 



Mr. Cottrell has mounted his glass tube so as to render friction in 

 both directions available. The tube-machine is represented in Fig. 



1 1 am indebted to Dr. Debus for the suggestion of the bisulphide as a substitute for 

 the ether. 



