THE ROYAL ARTILLERY INSTITUTION. 
417 
increment of pressure diminishes as the liquid form is approached, the same 
does not appear to hold good as the temperature is increased. Carbonic 
acid gas, so far as is known, cannot be liquefied above 80° R. With high 
temperatures the pressure may increase very greatly when the density is 
great. It may even approach the law of water pressure, which we know 
increases enormously with a small diminution of bulk. Thus even theoreti¬ 
cally, the pressure may attain an amount which it would be impossible for 
us to restrain with all the appliances at our command. 
It is necessary that we should have clear ideas on this point, as we shall 
see it is the slowness and regularity of the combustion of gunpowder that 
are the elements which make it possible to utilise its enormous pressure and 
keep it under control; and, to be of any practical use to us, even were we 
to know exactly what are*the products of combustion, we must also know 
The Rate of Combustion, 
Robins made a very ingenious experiment to determine whether the 
explosion of gunpowder was instantaneous or not. He says :—* 
ec If part only of the powder is fired, and that successively, then, by laying a 
larger weight before the charge (suppose two or three bullets instead of one) a 
greater quantity of powder will necessarily be fired, since a heavier weight would 
be a longer time in passing through the barrel. Whence it would follow that two 
or three bullets would be impelled with a much greater force than one only. But 
the contrary of this appears by experiment; for, firing two and three bullets laid 
contiguous to each other with the same charge respectively, I have found that their 
velocities were not much different from the. reciprocal of their sub-duplicates of 
matter.” 
Though Robins, in accordance with his deductions of a small initial 
pressure, might have been prepared to accept the result he here states, yet 
others, who knew that his estimate was much too low, could not accept his 
conclusion, seeing that, if gunpowder were burnt in its own space, no possible 
gun could withstand its explosive effects. It is to be remembered, however, 
that Robins made all his experiments with small-arms, where the nature of 
the powder used would make the actual facts of the case approximate so 
closely to his deductions as to defy detection by the rude method he em¬ 
ployed. Any windage would allow a greater escape of gas as the number 
of bullets increased, and, though the pressure might be greater at the 
commencement of the bullets motion, such a loss would give rise to a 
diminished pressure afterwards. Sir W. Armstrong mentions another 
circumstance in connection with this:—t 
“ By using a slower burning powder, less heat and pressure are evolved at first, 
and the waste of heat in the stage of initial pressure being less, more heat remains 
for expansive action. Hence the slower burning powder is weaker at first but 
stronger afterwards, and, although the total quantity of gas be only the same, and 
the pressure not so great at any point, yet the aggregate pressure throughout the 
bore may equal that of the more energetic and more dangerous powder.” 
* “ Eney. Brit.” Gunnery. 
f Address, as President of the Institution of Mechanical Engineers, Newcastle Meeting, 1869, 
