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