62 CONTEMPORARY SCIENCE 



What the Germans expected to accomplish by it I am 

 not sure. Presumably they intended to win the war, and 

 they might conceivably have won it then and there if they 

 had foreseen the tremendous effect of the attack. It is 

 certain that they expected no immediate retaliation, as 

 they had provided no protection for their own men. They 

 made a clear and unobstructed gap in the lines, which 

 was only closed by the Canadians, who rallied on the left 

 and advanced, in part through the gas cloud itself. 



The method first used by the Germans, and retained 

 ever since, is fairly simple, but requires great preparation 

 beforehand. A hole is dug in the bottom of the trench 

 close underneath the parapet, and a gas cylinder is buried 

 in the hole. It is an ordinary cylinder, like that used for 

 oxygen or hydrogen. It is then covered first with a quilt 

 of moss, containing potassium carbonate solution, and 

 then with sand bags. When the attack is to be made the 

 sand bags and protecting cover are taken off the cylinder, 

 and each cylinder is connected with a lead pipe which is 

 bent over the top of the parapet. A sand bag is laid on 

 the nozzle to prevent the back "kick" of the outrushing 

 gas from throwing the pipe back into the trench. Our 

 own methods are practically identical with those first used 

 by the Germans. 



The success of a cloud gas attack depends on thorough 

 preparation beforehand. The attackers must know the 

 country, the layout of the trenches, and the direction and 

 velocity of the wind with certainty. Favorable conditions 

 are limited practically to wind velocities between 12 and 

 4 miles an hour. A wind of more than 12 miles an hour 

 disperses the gas cloud very rapidly. An upward cur- 

 rent of air is the worst foe of gas. The weight of the 

 gas is not an important factor in carrying it along, for it 

 mixes rapidly with air to form the moving "cloud." The 

 time occupied by a gas attack is too short to permit of 

 much diffusion of the gas out of the original mixture. 



The gas attack must be planned very carefully. If the 



