THE ROYAL ARTILLERY INSTITUTION* 
265 
domestic boilers from these causes consists in frequent and thorough 
cleaning out, which is especially necessary where the water supply is 
hard. 
Explosions also occur with household boilers of the ordinary cir¬ 
culating class, unprovided with safety-valves, through the stop-taps of 
the pipes which connect them with an overhead cistern being left 
closed, by accident or negligence, in which case steam pressure must 
speedily accumulate to a dangerous extent, all outlets being closed. 
Accidents with such boilers are particularly liable to occur during 
severe frosts, in consequence of the circulating pipes becoming plugged 
up with ice, whereby the outlet for steam pressure is as completely cut 
off as if the stop-taps were closed. Several accidents due to these two 
causes, some of them attended by fatal results, were recorded last year. 
The obvious and simple method of guarding effectually against ' such 
explosions is to have the boiler fitted with a reliable safety-valve, of the 
most simple form. A valve of pendulous dead weight construction is 
specially recommended for this purpose by the Steam Users' Asso¬ 
ciation, as being inexpensive, certain of action, and free from any 
liability to get out of order. So simple a precaution for avoiding the 
possibility of a frightful accident surely only requires public attention 
being directed to it to secure its general adoption. 
Explosions resulting from the ignition of mixtures of inflammable 
gas and air constitute even a more formidable class than that just des¬ 
cribed ; for the number of explosions in coal mines which occur in a 
year is very considerably greater than that of boiler explosions, while 
the loss of life occasioned by the former is very considerable, and is 
occasionally appalling in its magnitude. Thus, in 1871 there were 
52 fatal explosions, 268 men being killed, and 234 non-fatal explosions, 
by which it is estimated that 368 men were injured; and as examples 
of specially calamitous explosions may be mentioned that at Risca 
Colliery in 1860, when 142 men were killed, and those at Oaks Colliery 
in 1866, and Ferndale Colliery in 1867, when 334 and 178 men were 
killed. 
If marsh-gas, or light carburetted hydrogen, which exists imprisoned 
in coal-beds and escapes into the atmosphere in the pit-working, either 
gradually or sometimes under considerable pressure, becomes mixed 
with the air to such an extent that there are about eighteen volumes of 
the latter to one of the gas, the mixture burns with a pale blue flame, 
which will surround that of a candle contained in such an atmosphere. 
The appearance of such a “ corpse light" round the flame of the 
pitman's candle or lamp-flame is a warning—too generally unheeded— 
of the presence of fire-damp in quantities likely to be dangerous; for if 
the proportion of marsh-gas increases much beyond that above speci¬ 
fied, an explosive atmosphere will be formed, the violent character of 
which increases as the proportion of fire-damp approaches that of one 
volume to ten of air. Marsh-gas requires for its ignition to be brought 
into contact with a body raised to a white heat; fire-damp, or a 
mixture of marsh-gas and air, is therefore not inflamed by a spark or 
red-hot wire, but will explode if brought into contact with flame. The 
