=Height of Gas Cloud.= The height to which gas rises in a gas cloud is not exactly known, but it is believed to be not much more than fifty feet, and then only at a considerable distance from the point of discharge. Moving pictures taken of gas clouds show this to be true. It is also indicated by the fact that pigeons, which are very susceptible to poisonous gas, practically always return to their cages safely when liberated in a gas cloud. This was a good deal of a mystery until it was realized that the pigeon escaped through his rising so quickly above the gas. This of course would be expected when it is known that practically all gases successfully used were two or more times as heavy as air. Such gases rise only by slow diffusion, or when carried upwards by rising currents. The absence of these upward currents at night is one of the reasons why gas attacks are more effective at night than during the day. =Horizontal Spread of Gas.= Another important thing to know in regard to the behavior of the wave of gas is the horizontal spread of a cloud. If gas be emitted from a cylinder the total spread in both directions from that point is from 20° to 30° or an average of 25°. This varies, of course, with the wind. The higher the wind the less the angle, though the variation due to wind is not as great as might be expected. This horizontal spread of the gas cloud was measured experimentally, and the results checked by aeroplane pictures of heavy wave attacks over the enemy line. In the latter case the path of the gas was very closely indicated by the dead vegetation. This vegetation was killed and bleached so that it readily showed up in aeroplane photographs. The visibility of a gas cloud arises from the fact that when a large amount of liquid is suddenly evaporated, the air is cooled and moisture condensed, thereby creating a fog. With gases such as mustard gas and others of slight volatility, a visible cloud is not formed. For purposes of identification of points struck by shell, smoke substances are occasionally added, or a few smoke shell fired with the gas shell. As future battle fields will be dotted everywhere with smoke clouds, a point that will be discussed more fully later, the firing of smoke with gas shell will probably be the rule and not the exception. REQUIREMENTS OF SUCCESSFUL GAS