Nitro-Explosives: A Practical Treatise | Page 7

and near the
ground line. The angles and the prominent portions of a building being
the most liable to be struck, the conductors should be carried over and
along these projections, and therefore along the ridges of the roof. The
conductors should be connected to any outside metal on the roofs and
walls, and specially to the foot of rain-water pipes.
All the lightning conductors should be periodically tested, to see that
they are in working condition, at least every three months, according to
Mr Richard Anderson. The object of the test is to determine the
resistance of the earth-connection, and to localise any defective joints
or parts in the conductors. The best system of testing the conductors is
to balance the resistance of each of the earths against the remainder of
the system, from which the state of the earths may be inferred with
sufficient accuracy for all practical purposes.
Captain Bucknill, R.E., has designed an instrument to test resistance
which is based on the Post Office pattern resistance coil, and is capable
of testing to approximate accuracy up to 200 ohms, and to measure
roughly up to 2,000 ohms. Mr R. Anderson's apparatus is also very
handy, consisting of a case containing three Leclanché cells, and a
galvanometer with a "tangent" scale and certain standard resistances.
Some useful articles on the protection of buildings from lightning will
be found in Arms and Explosives, July, August, and September 1892,
and by Mr Anderson, Brit. Assoc., 1878-80.
~Nitro-Glycerine.~--One of the most powerful of modern explosive
agents is nitro-glycerine. It is the explosive contained in dynamite, and
forms the greater part of the various forms of blasting gelatines, such as
gelatine dynamite and gelignite, both of which substances consist of a
mixture of gun-cotton dissolved in nitro-glycerine, with the addition of
varying proportions of wood-pulp and saltpetre, the latter substances
acting as absorbing materials for the viscid gelatine. Nitro-glycerine is
also largely used in the manufacture of smokeless powders, such as

cordite, ballistite, and several others.
Nitro-glycerol, or glycerol tri-nitrate, was discovered by Sobrero in the
year 1847. In a letter written to M. Pelouse, he says, "when glycerol is
poured into a mixture of sulphuric acid of a specific gravity of 1.84,
and of nitric acid of a gravity of 1.5, which has been cooled by a
freezing mixture, that an oily liquid is formed." This liquid is
nitro-glycerol, or nitro-glycerine, which for some years found no
important use in the arts, until the year 1863, when Alfred Nobel first
started a factory in Stockholm for its manufacture upon a large scale;
but on account of some serious accidents taking place, its use did not
become general.
It was not until Nobel conceived the idea (in 1866) of absorbing the
liquid in some absorbent earth, and thus forming the material that is
now known as dynamite, that the use of nitro-glycerine as an explosive
became general.
Among those who improved the manufacture of nitro-glycerine was
Mowbray, who, by using pure glycerine and nitric acid free from
nitrous acid, made very great advances in the manufacture. Mowbray
was probably the first to use compressed air for the purpose of keeping
the liquids well agitated during the process of nitration, which he
conducted in earthenware pots, each containing a charge of 17 lbs. of
the mixed acids and 2 lbs. of glycerol.
A few years later (1872), MM. Boutnny and Faucher, of Vonges,[A]
proposed to prepare nitro-glycerine by mixing the sulphuric acid with
the glycerine, thus forming a sulpho-glyceric acid, which was
afterwards mixed with a mixture of nitric and sulphuric acids. They
claimed for this method of procedure that the final temperature is much
lower. The two mixtures are mixed in the proportions--Glycerine, 100;
nitric acid, 280; and sulphuric acid, 600. They state that the rise of
temperature upon mixing is limited from 10° to 15° C.; but this method
requires a period of twenty-four hours to complete the nitration, which,
considering the danger of keeping the nitro-glycerine in contact with
the mixed acids for so long, probably more than compensates for the
somewhat doubtful advantage of being able to perform the nitration at

such a low temperature. The Boutnny process was in operation for
some time at Pembrey Burrows in Wales, but after a serious explosion
the process was abandoned.
[Footnote A: Comptes Rendus, 75; and Desortiaux, "Traité sur la
Poudre," 684-686.]
Nitro-glycerine is now generally made by adding the glycerine to a
mixture of sulphuric and nitric acids. The sulphuric acid, however,
takes no part in the reaction, but is absolutely necessary to combine
with the water that is formed by the decomposition, and thus to keep up
the strength of the nitric acid, otherwise lower nitrates of glycerine
would be formed that are soluble in water, and which would be