Combustible Substances. 201 



derive these advantages from them, we must 

 renounce a rigid adherence to arbitrary definitions 

 which nature disclaims. 



1st.. Of HYDROGEN it was necessary to treat so 

 amply as a simple substance, that scarcely any 

 thing remains to be said of its combustible qua- 

 lities. If hydrogen gas is mixed in a receiver 

 with oxygen gas, in the proportion of fifteen 

 parts of the former to seventy-five of the latter, 

 and an electric spark is made to pass through 

 them by means of a wire inserted in the receiver, 

 they will explode, and pure water will be formed. 

 Tliis experiment is generally made in a tub of 

 mercury, with the pneumatic apparatus, as de- 

 scribed in Lect. 25. See PL VI. fig. 19. 



Philosophical fire-works are also made by 

 means of different mixtures of this gas with 

 other aerial fluids. Bladders, with glass tubes 

 and brass cocks affixed to their necks, are filled 

 with the different modifications of this gas ; and 

 when the cocks are turned, the inflammable gas 

 issues out, and may be inflamed by a candle. 

 -Pure hydrogen gas affords a white flame; with 

 a mixture of atmospheric air the flame is blue ; 

 and when mixed with carbonic acid gas in dif- 

 ferent proportions, the different shades from red 

 to purple are produced. 



2d. The appearance of PHOSPHORUS has al- 

 ready been described to be that of a transparent 

 substance, of a colour inclining to yellow, like 

 a clear horn ; it is specifically heavier than water, 



K5 



