238 



SCIENTIFIC AMUSEMENTS. 



extremely porous condition, and employing hydrogen gas instead 

 of coal gas in Expt. 283, it will happen under favourable circum- 

 stances that it is not necessary to heat the platinum before bringing 

 it in contact with a mixture of combustible gas and air in order to 

 make it absorb the two gases and cause them to act on one another 

 evolving heat, and so gradually making the platinum hot enough to 

 light the gas jet ; a little instrument was invented by Dobereiner for 

 the purpose of utilising this property as a means of obtaining a light 

 when required, and so superseding matches. This consists of a hy- 

 drogen gas-generator, working on much the same principle as those 

 above described, but somewhat different in construction (fig. 98). 

 A cylindrical glass jar, cc, is provided with 

 a lid which can be removed if required, 

 but ordinarily fits on pretty closely. In the 

 centre of this lid is a perforation through 

 which passes the narrow end of a funnel- 

 shaped glass tube, &, fitted with a valve out- 

 side the lid opened by the trigger e. When 

 open, hydrogen issues out by a horizontal jet 

 and impinges on some spongy platinum in 

 the little metal box, /, thereby causing the 

 platinum to heat, and finally to ignite the 

 hydrogen jet so as to produce a flame. 



The hydrogen is generated thus ; a lump 

 of zinc is supported by a wire inside the 

 funnel tube, and the jar is filled about half 

 or three quarters full with diluted sulphuric 

 acid by taking off the lid and pouring the 

 liquid in. When the lid is replaced, the zinc 

 dips in the acid so that hydrogen is generated and rises up inside 

 the funnel tube ; at first the hydrogen is allowed to escape freely 

 by opening the valve so as to displace all air present ; when pure 

 hydrogen issues from the jet, the valve is closed ; the gas having 

 no vent accumulates in the upper part of the funnel tube and 

 depresses the level of the acid therein, raising it in the outer 

 cylinder ; when the acid is so far depressed that the zinc is un- 

 covered, the action ceases. On opening the valve the pressure of 

 the acid fluid in the outer cylinder forces the hydrogen out at the 

 jet, and the acid rises in the funnel so as to come in contact with 

 the zinc and thus generate more hydrogen. In this way the 

 alternate production of a jet of hydrogen and cessation of produc- 

 tion, are effected automatically by opening and closing the valve. 



Expt. 286. Use of Charcoal as a Deodoriser and Sanitary 

 Agent. Although charcoal does not absorb all gases as freely as 



Fig. 98. Dobereiner 

 Lamp. 



