TOUCHWOOD TINDKR-BOX. 103 



for suspecting, that electricity is not the cause of the inflammation 

 here. These I shall give in the second ptrt of this paper, con. 

 eluding the present with an important observation on the construe- 

 tion of pistons. 



'* If we could find an elastic substance sufficiently compact to 

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

 spring and adapt themselves to the inequalities of the tube, with- 

 out suflering a bubble of air to t scape. I have made some with 

 caoutchouc, softened before ihe fire, in order to give 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 it ; so that the piston remained 

 uneven Mid almost ra^cd, and yielded like 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 witiiout considerable force. 

 If a drop of oil be put on it, it moves easily ; but this soon spoils 

 the instrument ; for the oil dissolves the caoutchouc, and forms a 

 varnMi. which, as the piston grows hot, makes it adhere still more 

 strongly to the si<le> of the tube. 



" Might not these inconveniences be avoided, by arming the 

 piston rod with caoutchouc, and covering this with leather ? If 

 thi> process -ucceeded, it might be applied with advantage to all 

 sorts of pumps. 



*' To attain, if possible, a knowledge of the principle of inflam- 

 mation in the pneumatic tinder box, four things are to be 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 j)i-ton 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 examining the question whether the spunk be kindled by 

 ekctrity , I consider, 



" 1st, That no part of the instrument is insulated ; and that in- 

 sulation is a necessary condition for producing sensible electricity 

 with any of the machines we know. 1 say machines that we know, 

 because the animal electricity, that manifests itself without insula- 

 tion, is an exception to our mechanical means, and cannot here be 

 taken into consideration. 



u4 



