IGNITION FROM COMPRESSED AIR, 



" They who imagine, that electricity kindles the spunk; consider pjectrlcltv 

 these sparks as an incontroveiiible proof of their opinion. I supposed to be 

 think they are mistaken in this case ; yet I must not conceal a [j]^ ^^^^^ °^ 

 fact communicated to me by Mr. Veaa-DeLnunay, which seems 

 to confirm this opinion, of which he is a partisan. Out of 

 twelve times, when he operated with the instrument without 

 any spunk in it, he saw sparks emitted three times. There are 

 strong reasons, however, for suspecting, that electricity is not 

 the cause of the inflammation here. These 1 shall give in the , 

 second part of this paper, concluding the present with an im- 

 portant observation on the construction of pistons. 



If we could find ao elastic substance sufficiently compact to On the con- 

 be turned in a lathe, we should have perfect pistons, that struction of 

 would spring and adapt themselves to the inequalities of the 

 tube, without suffering a bubble of air to escape. I have made Attempt to use 

 some with caoutchouc, softened before the fire, in order to give elastic gum for 

 it a degree of elasticity more obedient to the inequalities of the 

 tube. But on attempting to turn it in a lathe, it bent under 

 the tool. Even the edge of a razor would not take hold of 

 it ; so that the piston remained uneven and almost ragged, and 

 yielded hke soft wax under the fingers. In this imperfect state 

 it so far prevents the air from escaping, that a column of three 

 inches is sufficient to kindle the spunk ; but after a few strokes 

 of the piston the heat dilates it to such a degree, that it cannot 

 be moved without considerable force. If a drop of oil be put 

 on it, it moves easily j but this soon spoils the instrument ; for , 



the oil dissolves the caoutchouc, and forms a varnish, which, as 

 the piston grows hot, makes it adhere still more strongly to the 

 sides of the tube. 



Might not these inconveniences be avoided, by arming the This mioht be 

 piston rod with caoutchouc, and covering this with leather ? co'^^'f^red v/uh 

 If this process succeeded, it might be applied with advantage to 

 all sorts of pumps' 



II. To attain, if possible, a knov/ledge of the principle of What is the 

 inflammation in the pneumatic tinderbox, four things are to be l^^fl^mniation ? 

 considered — the materials of the tube, the matter contained in 

 the tube, the materials of the piston, and the friction. Among 

 the materials of the piston I include the grease, with which it 

 is coated, to make it move more easily, and render it fitter to 

 intercept the passage of the air. 



In 



