An interesting physiological test has also been developed. The eye has been found to be very sensitive to chloropicrin. The gas affects the eye in such a way that its closing is practically involuntary. A measurable time elapses between the instant of exposure and the time when the eye closes. Below 1 or 2 parts per million, the average eye withstands the gas without being closed, though considerable blinking may be caused. Above 25 parts, the reaction is so rapid as to render proper timing out of the question. But with concentrations between 2 and 25 parts, the subject will have an overpowering impulse to close his eye within 3 to 30 seconds. The time may be recorded by a stop watch and from the values thus determined a calibration curve may be plotted, using the concentration in parts per million and the time to zero eye reaction. Typical figures are given below. It will be noted that different individuals will vary in their sensitivity, though the order is the same. +--------+---------+---------+ | Conc. | A | B | | p.p.m. | Seconds | Seconds | +--------+---------+---------+ | 20.0 | 4.0 | 5.0 | | 15.0 | 5.4 | 5.4 | | 10.0 | 7.5 | 7.5 | | 7.5 | 9.0 | 10.0 | | 5.0 | 13.0 | 15.0 | | 2.5 | 18.0 | 30.0 | +--------+---------+---------+ [Illustration: FIG. 28.—Calibration Curve of Eyes for Chloropicrin.] PROTECTION Because of the stability of chloropicrin, the question of protection resolves itself into finding an absorbent which is very efficient in removing the gas from air mixtures. Fortunately such an agent was found in the activated charcoal used in the American gas mask. The removal of the gas appears to take place in two stages. In the first, the gas is adsorbed in such a way that the long-continued passage of air does not remove it. In the second, the gas is absorbed, and this, really excess gas, is removed by pure air passing over the charcoal. The relation of these two factors has an important bearing on the quality of charcoal to be used in gas masks. It appears that up to a certain point an increase of the quality is desirable: beyond this, it is of doubtful value. Unlike phosgene, chloropicrin is absorbed equally well at all temperatures. Moisture on the other hand has a very decided effect. It appears that charcoal absorbs roughly equivalent weights of chloropicrin and of water; the presence of water in the charcoal thus displaces an approximately equal amount of chloropicrin. In the study of canisters it has been found that the efficiency time is approximately inversely proportional to the concentration. Formulas have been calculated to express the relation existing between concentration and life of the canister, and also between the rate of flow of the gas and the life.