The increased concentration was obtained chiefly by the reduction in the time occupied by the attack. The first attack of all lasted about one hour and a half. The next attack lasted about three hours. The one in question lasted only thirty minutes, so that if the same amount of gas was used the concentration of the cloud must obviously have been increased six times over that of May twenty-fourth, as there is little doubt that the cylinders had been installed in approximately the same numbers--that is, one to a meter of front. Probably the most important feature of the attack was the introduction of phosgene. Now there never was any actual chemical evidence of the poisons of phosgene in the German gas clouds until some of their cylinders were captured by us when they retreated on the Somme in the beginning of 1917. But unfortunately the peculiar effects of phosgene on our men who were gassed were only too apparent. There were a large number of “delayed” cases--men who thought they were only slightly gassed but who became ill or even died several hours or sometimes a day or so later from heart failure, especially if they had taken any heavy exercise in between. In these cases there was hardly any coughing. What was really wanted was rest, but this was not realised at the time, and many men walked to the dressing stations--sometimes a mile or more--through deep mud and became quite exhausted. One officer of the Durhams had been slightly gassed at the beginning of the attack but felt perfectly all right until about noon, when he became faint and exhausted, though not apparently seriously ill. After lying down he felt better, but in the evening got worse again, and in walking to the ambulance to go to the field dressing station he suddenly collapsed and died. This was fourteen hours after the attack. Another weighty piece of evidence as to the nature of the gas was given by the smell, which to trained observers was quite different from the typical chloride-of-lime smell of chlorine; and by the peculiar effects on the taste of tobacco to men who had smelt the gas. If you take a good smell of dilute phosgene and then smoke a cigarette the tobacco tastes like nothing on earth. Tommy’s nearest description of the taste and smell is “mouldy hay.” This peculiar effect is quite typical of phosgene and is known as the “tobacco reaction.” In the hope of getting samples of the German gas clouds for analysis a large number of gas vacuum bulbs were distributed up and down the line, and selected men were taught how to use them. This was supposed to be done by nipping off the drawn-out end of the gas bulb, whereby the contaminated air would rush in. The end was then to be closed with a hollow stopper containing wax.