IGNITION BY COMPRESSED AIR. 235 



It is not therefore, the condensation of qir, which pro- The rause is 

 duces the ignition; it is the condensation of the immediate [ion^ofcriorkt 

 cause of heat', sometimes called matter of Iteat, but which, 

 in all the records of Natural Philosophy, is named ^re, 

 igneous Jiuid, or their correspondents in all lanc^uages an- 

 cient and modern; and it has always been considered as an 

 expansible Jiuid, of great power of expansion, when arrived 

 to a great density. 



This is the cause of our phenomenon ; it is produced by as when iron h 

 the same kind of operation, which brings to a red-heat a slip [j^mmeredred- 

 of iron very rapidly hammered; and that cause is the con- 

 densation of^re. That Jluid may be compressed or rarejied 

 in the same manner as air, by mechanical means. Thus in Similar pheno- 

 the air pump, which furnishes both examples at once; at "j^"""'" ^ 

 the same time that the manometer rises or falls, by condens- 

 ing or rarefying the air in the receiver, the thermometer 

 rises or falls in it, by the condensation or rarefaction of the 

 free fire mixed with the air; and both effects are produced 

 by lessening or enlarging the spacem which fixed quantities 

 of the respective fluids are contained. 



The only difleience between the two cases proceeds from A difference in 



that of the permeability of bodies to these fluids. The ves- ^}^ '^^''^^ ^'°^ 

 1 1 • • ,1 . • 1 1 . . , 1 fire permeating 



sels bemg impermeable to air, and made atr-tight, the con- all bodies, 



densation or rarefaction of air may be produced as slotcly 

 as convenient, without changing the effects : whereas no ves- whence rapi- 

 sel being fire-light, the operation requires a great rapidity, '^"^y necessary. 

 If the same number of strokes of a hammer, which, by ra- 

 pidly succeeding each other, bring a slip of iron to incan- 

 descence, were struck at great intervals; or if the piston 

 which, being rapidly moved up to the bottom of the syringe 

 here in view, produces the ignition of the tinder, is moved 

 slowly; these effects are not produced: because the con- 

 densed fre has time to escape through the pores, in the 

 first case of the iron, and in the latter of the barrel. 



This, Sir, is wiiat appears to me the cause of the ignition 

 of tinder in that apparatus, which I beg you will consign 

 in your very xiseful repository, if you think proper. 

 I am, Sir, 

 Your most obedient humble servant, 



DE LUC. 

 XVIL 



