These considerations necessitate the determination of the toxicity in the form of a vapor and not by the ordinary method of administration by mouth, through the skin (subcutaneously) or through the blood (intravenously). The simplest method of determining the toxicity of a substance as a vapor would be to place animals in a gas-tight box and introduce a known amount of the substance in the form of vapor. But by this method the concentration is not accurately known unless chemical analyses of the air are made, and then it is found to be much less than that calculated from the amount of substance introduced, because of condensation on the walls of the chamber, or absorption of the substance by the skin and hair of the animal and in some cases, of decomposition of the substance by moisture in the air. Moreover, it is found that the concentration decreases markedly with time. Because of these factors, the figures used for the concentration are more or less guess work. To overcome these difficulties, a chamber is used through which a continuous current of air, containing a known and constant amount of the poisonous vapor, is passed. Such an apparatus is shown in Fig. 116. [Illustration: FIG. 116.—Continuous Flow Gassing Chamber for Animals.] The flask _E_ is a 300 cc. Erlenmeyer flask, with a ground glass stopper. The liquid to be tested is placed in this flask together with a sufficient quantity of glass wool to prevent splashing and the carrying over mechanically of droplets of the liquid. Air is passed through _A_ and _C_ (calcium chloride drying tubes) and the rate measured by the flow meter _D_. The air and gas are mixed in _F_ before passing into the chamber _G_. This chamber is made of plate glass, is of about 100 liters capacity, and is air-tight. The entire flow of air and gas through the box, kept constant at 250 liters per minute, is measured at _H_. The gas is removed through _K_, which is filled with charcoal and soda-lime, in order that little gas may pass into the pump. By weighing the flask _E_, and its contents before and after passing air through it, and knowing the total volume of the mixture passing through the chamber during the same period, the concentration of the substance can readily be calculated. This concentration, as determined by the “loss in weight” method, can be checked by chemical analysis (samples taken at _M_—_M_). The method has been found to give accurate values. The concentration in the chamber reaches its constant level within 30 to 40 seconds after the apparatus is started. With the flow of 250 liters per minute, the difficulties mentioned above are reduced to a point where they are practically negligible.