206 



HYDROGEN. 



(c.) It is a constituent of all animal and vegetable bodies, and is 

 found in almost every part of them. 



(d.) It exists in mineral coal of every variety, and most abundant- 

 ly in the bituminous coal. 



9. Its combining weight, when it is made unity for other bodies, 

 is of course expressed by 1 ; if oxygen be unity, then hydrogen will 

 be .125. These are Dr. Thomson's numbers, but I have already 

 stated the reasons why I prefer making hydrogen unity, as most wri- 

 ters now do. 



10. POLARITY. 



Hydrogen, in the galvanic circuit, resorts to the negative pole, and 

 is therefore considered as electro-positive. 



Self regulating reservoirs, for hydrogen and other gases, are 

 occasionally convenient ; the following are from Dr. Hare, being im- 

 proved upon the original contrivance of Gay Lussac.* 



"Suppose the glass jar 

 without, to contain diluted 

 sulphuric acid ; the invert- 

 ed bell, within the jar, to 

 contain some zinc, support- 

 ed on a tray of copper, sus- 

 pended by wires, of the 

 same metal, from the neck 

 of the bell. The cock be- 

 ing open, when the bell is 

 lowered into the position in 

 which it is represented, the 

 atmospheric air will escape 

 and the acid, entering the 

 cavity of the bell, will, by 

 aid of the zinc, cause hy- 

 drogen gas to be copiously 

 evolved. As soon as the 

 cock is closed, the hydro- 

 gen expels the acid from the cavity of the bell ; and consequently, 

 its contact with the zinc is prevented, until another portion of the 

 gas is withdrawn. As soon as this is done, the acid re-enters the 

 cavity of the bell, and the evolution of hydrogen is renewed, and 

 continued, until again arrested, as in the first instance, by preventing 

 the escape of the gas, and consequently causing it to displace the 

 acid from the interior of the bell, within which the zinc is suspended." 



* Dr. Hare states that he used an apparatus of this kind, at Williamsburgh, Va. 

 before he had heard of that of Gay Lussac. It will be seen farther on, that such a 

 contrivance is admirably adapted for obtaining light, instantaneously, by allowing 

 the jet of flame to flow upon spongy platinum. 



