CHEMISTRY. 37 
the scale-pans and that of the beam is shown more fully in the succeeding 
fizures. 
Pi. 30, fig. 70, represents the method of hanging the balance beam, which 
in the figure appears cut off at each end, so that ania the two central pieces, 
BB, adumected by the central annular body, are visible. @ is a triangular 
prism of steel, the edge of whose lower acute angle rests on a plate of agate, 
b. It passes at right angles through the middle of the beam, supported at 
both extremities by agate plates. Both of these are carefully laid in the 
same plane. ee is a brass frame inclosin&® the body on which the agate 
plate rests. This frame can be raised or depressed by means of a pin 
connected with the excentric disk, e/. When this frame is raised, it catches 
the very extremities of the knife edge in two opposite notches, and raises 
it with the beam from the agate plate. The figure represents only the 
anterior arrangement of the knife edge and agate plate, the posterior being 
precisely similar. When the knife edge is raised by the frame, ee, the two 
pins, Ah, of an arm, gg, firmly connected with the frame, ee, catch at the 
same time under the scale beam, and support it still more. 
Pl. 30, fig. 71, represents the suspension apparatus of the pans. The plate, 
m, is screwed to the end of the beam, B. Ato, the pan is suspended by 
means of a hook in the ring of the stirrup, into which the plate, m, is 
continued. By means of the screw, n, the length of the beam from the 
knife edge to the point of the suspension may a made perfectly oe for 
both arms. 
Pl. 30, fig. 72, is another arrangement for suspending the pans, after 
Hoss. The most essential part of this arrangement is a steel plate (fig. 
73), provided at one end witha knife edge. This knife edge is divided into 
two parts by a small intersection, widening below. The hook r (fig. 72) 
carries in its middle a fine plate of metal, which, fitting in the intersection 
of the knife edge, renders a lateral displacement of the hook impossible. 
The parallelism of the knife edges or the equal length of the arms may be 
regulated by screws attached at 0, p, and n. 
VIII. MisceLLAnreous APPARATUS FOR SPECIAL INVESTIGATIONS. 
Davy’s Safety Lamp (pl. 31, fig. 59). In mines of different kinds, in coal 
mines especially, a certain gas, carburetted hydrogen, often forms in con- 
siderable quantity, and this, when mixed with air, becomes highly explosive. 
Inflamed as it frequently is by contact with the light used by the miner, it 
produces the most disastrous consequences to life and property. Such 
accidents are prevented by the proper use of the safety lamp. The right 
hand figure exhibits a section of the apparatus, which consists of a frame, 
at the lower part of which is placed alamp, the frame itself being completely 
inclosed by a wire cylinder of very fine copper gauze. On introducing this 
lamp into an explosive mixture, the gas penetrating the interstices of the 
gauze, within the inclosed space, may indeed be set on fire, but the flame 
467 
