To meet the fourth necessity, a plentiful supply of the narcotizing vapor, two stoves have been connected with the chamber, each capable of burning two pounds of charcoal per hour, and giving up the products of the combustion into the chamber.
To make the narcotic effect still more certain, and to keep the chamber at all times lethal, I made an extra provision. At the two points where the tubes from the stoves enter the chamber, I have interposed two strong boxes made of elm, and covered with thin lead. These boxes, which are eighteen inches long and four inches broad, are filled loosely with the porous burned loam known as Verity's patent gas-fuel, an excellent substance for filling a grate where coal-gas is burned instead of fuel. This substance is so porous, it takes up narcotic fluids most readily, holds them in its pores, and gives them up in volumes of vapors when warm gas is passed over it. Into the boxes closed in with this fuel there is a funnel, opening at the top, for supplying the fluid, which can be shut with a stopper; and at the end of the box, standing out at a right angle from it, is a continuous section, in which there is a large tap, for regulating the currents of gas from the stove.
When the stoves are in action, the tap is turned on, and the gases from the stove pass through the boxes over the patent fuel into the chamber. Nothing more is done until just before the time when the animals in the cage are to be introduced. Then ten fluid ounces of an anæsthetic mixture, consisting of equal parts of methylated chloroform and carbon bisulphide, are poured upon the fuel through the openings in the top of the little boxes, the openings being immediately closed. After the animals are in the chamber, ten ounces more of the same mixture are added, and if, after three or four minutes, any of the narcotized animals are still breathing, ten or twenty fluid ounces more are poured in.
In pushing the charged cage into the chamber, there is naturally a very great displacement of gas or vapor within. It was necessary to provide an exit which would save strain on the walls of the chamber, and would let out a little gas without letting in common air. I met the problem by the plan shown in Fig. 2, which exhibits the chamber in section. Two feet from the far end of the chamber there is suspended from the top a light hanging screen, which reaches within four inches of the floor. Behind this screen, and in the roof of the chamber, is a shaft, with a valve opening upward. As the cage is pushed in, this screen is raised from the bottom, and the air, rushing out at the lower part, ascends behind, and escapes by the valve. The screen is so balanced that, when sufficient air has been extruded, its lower end reaches the back or lower end-wall of the chamber. It thus acts as a regulating valve, and, when the pressure is off, it returns to its level, letting any gas at the rear of it return toward the cage.
To enable the operators to introduce the cage quickly, and at the
