ELECTBicrrr, magnetism, and gbavitation. 87 



throngh the entire length of the cylinder. There were two 

 pistons fitted air-tight — one at each end, so as to meet pretty- 

 nearly in the centre of the cylinder. At the point of junc- 

 tion a small hole was drilled through the diameter of the 

 cylinder. The first half of this hole was again enlarged and 

 tapped to receive a steel screw, which was made to extend as 

 far as the smaller hole on the opposite side of the bore, and 

 being tapered at its extremity as the screw was turned, 

 entered and closed up the entrance into the smaller orifice, 

 and served as a valve to produce a very minute orifice for 

 exudation or percussion.* The outer bore of this smaller 

 hole was again enlarged externally to f of an inch, and 

 tapped to receive the brass nieek of the bladder or other 

 apparatus to be employed. 



Exp. 11. — This new apparatus was now tried, and gave 

 a magnificent flame by percussion, but with considerably 

 less report than before; 



Exp, 12. — With an attached bladder filled with oxygen gas, 

 on percussion a considerable explosion occurred, but in- 

 stead of bursting the bladder, it caused its instantaneous con- 

 tractionto about f of its original bulk, and each successive per- 

 cussion decreased the amount of contained gas until the ivhole 

 had disappeared. The bladder was luminous throughout- 

 its whole extent, and the light and heat developed in this 

 instance were more than double the amount produced by per- 

 cussion of potassium in atmospheric air. 



It is thus seen that the cause of explosion and report 

 in this instance is not by the sudden expansion in volume of 

 gas, as occurs in (I believe) all other cases of explosion, but 

 by the instantaneous collapse in volume of an amount of oxy- 

 gen gas, attracted firom the surrounding atmosphere — the 

 report being caused by the rush of air to fill up the vacuity 

 so formed. This is said to be the case in the report caused 



* This contrivauce was found to fulfil its intention admirably. 



