(1_a_) Chlorine is fairly toxic, though its lethal concentration (2.5 milligrams per liter of air) is very high when compared with some of the later gases developed. This figure is the concentration necessary to kill a dog after an exposure of thirty minutes. Its effects during the first gas attack showed that, with no protection, the gas was very effective. (2_a_) Chlorine is very readily manufactured by the electrolysis of a salt (sodium chloride) solution. The operation is described below. In 100-pound cylinders, the commercial product sold before the War for 5 cents a pound. Therefore on a large scale, it can be manufactured at a very much smaller figure. (3_a_) Chlorine is easily liquefied at the ordinary temperature by compression, a pressure of 16.5 atmospheres being required at 18° C. The liquid which is formed boils at -33.6° C. at ordinary atmospheric pressure, so that it readily vaporizes upon opening the valve of the containing cylinder. Such rapid evaporation inside would cause a considerable cooling of the cylinder, but this is overcome by running the outlet pipe to the bottom of the tank, so that evaporation takes place at the end of the outlet pipe. (4_a_) Chlorine is 2.5 times as heavy as air, and therefore the gas is capable of traveling over a considerable distance before it dissipates into the atmosphere. (5_a_) The only point in which chlorine does not seem to be an ideal gas, is in the fact that it is a reactive substance. This is best seen in the success of the primitive protection adopted by both the British and the French during the days immediately following the first gas attack. At first, however, chlorine proved a very effective weapon. During the first six months of its use, its value was maintained by devising new methods of attack. When these were exhausted, phosgene was added (see next chapter). With the decline in importance of cloud gas attacks, and the development of more deadly gases, chlorine was all but discarded as a true war gas, but remained as a highly important ingredient in the manufacture of other toxic gases. MANUFACTURE IN THE UNITED STATES