IGNITION FROM COMPRESSED AIR. 225 



stroyed by accident, I tried the third, eisht inches long by seven 



lines in diameter, which succeeded enuaily well. 



When the instrument is made to act, and the spunk kiniHes, Appearances. 



we see a bright flash, that fills the capacity of the tube ; and .%- ^'^'S^^ ^'S^^ 

 ,.,.,. , if tat compres- 



inis light IS SO much the more vivju, in proportion as the com- sion be rapid : 



pression is more rapid. If the compression be less powerful, 

 the spunk does not kindle, but we perceive in the upper part of 

 the tube a light vapour, that falls in undulations on the piston. 

 When this has disappeared, if we draw back the piston, the va- ^ Ught va- 

 pour will reappear, as long as there is any air in the tube, pour, if not. 

 These effects may be produced several times in succession, 

 merely by pushing the piston with the hand. This vapour is 

 so thin and diaphanous, that it is not perceptible in a strong 

 light. It requires a sort of twilight to see it well. 



But whence arises this vapour, and what is its nature ? As- whence arises 

 suredly it is not furnished by the materials of the instrument ; this vapour? 

 it can only proceed, therefore, from what it contains, from the 

 atmospheric air. Now, according to the present state of our 

 knowledge, the air contains only nitrogen, oxigen, and a very 

 small portion of carbonic acid j all gasiform substances, which 

 are kept in this state by the great quantity of caloric that pe- 

 netrates them, and are consequently heavier than it*. But in 

 compressing the air contained in the tube, what is the substance 

 that must first give way ? Is it not that which is lightest, the Is it the matter 

 caloric, that general solvent, that principle of fluidity and vola- °^ ^^^^<^o^' 

 tilization, which gives wings even to metals to raise themselves 

 in the air ? Is then the vapour in question caloric, rendered 

 visible by the approximation of its particles, which are com- 

 pressed by the surrounding air, as air becomes visible in passing 

 through liquids ? This idea, which I am far from presenting 



* The air likewise, in its ordinary state, contains twelve grains of jt jj not aqne- 

 water in a cubic foot. Tills small quantity of water, reduced to the ous vapour, 

 proportion of the quantity of air contained in the apparatus, contri- 

 butes nothing to its effect : for the heat produced by the friction could at 

 most reduce it to vapour, and in this state it would not kindle the spunk. 

 If the vapour seen in the tube were water in a state of expansion, when 

 it fell on the surface of the piston it would condense there, and appear 

 in the state cf a liquid. But the surface of the pistoa always remainf 

 dry, though on moving it the vapour appears and disappears jeveral 

 times. 



Vol. XXXIII, No. 153. —November, 1812. Q asa 



