62 
CAPTAIN NOBLE AND ME. F. A. ABEL ON FIEED GUNPOWDER. 
contrivances of well-known efficacy for closing the joints, such as jpapier-mdchG wads 
between disks of metal (a method which has been successfully employed with guns), are 
inadmissible, because the destruction of the closing or cementing material used, by 
the heat, would obviously affect the composition of the gas. Every operation con- 
nected with the preparation of the apparatus for an experiment has to be con- 
ducted with the most scrupulous care. Should any of the screws not he perfectly 
home, so that no appreciable amount of gas can escape, the gases, instantly upon their 
generation, will either cut a way out for themselves, escaping with the violence of an 
explosion, or will blow out the part improperly secured, in either case destroying the 
apparatus. 
The effect produced upon the apparatus, when the gas has escaped by cutting a 
passage for itself, is very curious. If, for example, one of the plugs has not been 
sufficiently screwed home, so that the products of combustion escape between the male 
and female threads, the whole of these threads at the point of escape present the 
appearance of being washed away, the metal having been evidently in a state of fusion, 
and carried over the surface of the plug by the rush of the highly heated products. 
Again, the difficulty of opening the vessel after explosion, when large charges have 
been used, is very great. This will be readily understood when the temperature and 
pressure of explosion are considered. The exploding-chamber being filled with products 
intensely heated and under an enormous pressure, there is an expansion of the interior 
surface of the cylinder. Hence small portions of the fluid products become forced in 
between the threads of the screws. These solidify into a substance of intense hardness, 
which cements together the metal surfaces, and, on cooling, the contraction of the 
cylinder puts such a pressure on the screw, that, in attempting to open it, seizure is 
very difficult to avoid. In one or two cases it was found impossible to open the 
cylinder until melted iron had been run round it, so as to cause it to expand. 
This difficulty has been in a great measure avoided, in the more recent experiments, 
by making the screws conical, so that when once started clearance is rapidly given, and 
they are removed with comparative ease. 
2. Measurement of Pressure. 
The apparatus used for the measurement of the tension of the gas was precisely 
similar to that which has been used by the Committee on Explosives, and consists of a 
screw-plug of steel (Plate 14. figs. 4 & 5), which admits of a cylinder of copper or other 
material (A) being placed in the small chamber (B). The entrance to the chamber is 
closed by the movable piston (C), and the admission of the gas is prevented by the use 
of the gas-check, D. When the powder is fired, the gas acts upon the base of the piston 
and compresses the cylinder. The amount of compression of the cylinder serves as an 
index to the force exerted, the relation between the amount of crush and the pressure 
necessary to produce it being previously carefully determined. 
