It was realised of course that any change in protection would have to include both prussic acid and phosgene; and this is not nearly so easy as it sounds. Phosgene is peculiarly chemically inert for such an active poison, and it was some time before a reasonable protection was found which could be incorporated in a smoke helmet. The substance actually decided upon was a solution of sodium phenate--that is, carbolic acid dissolved in caustic soda, the mixture containing an excess of caustic. This solution is quite capable of dealing with reasonable concentrations of phosgene and would successfully protect against three parts of phosgene to ten thousand of the air, which in the circumstances was quite good enough. The French also altered their protection at the same time and used sodium sulphanilate as the basis of protection against phosgene. The objections against the sodium phenate were that it could not be absorbed into a flannel helmet owing to its destruction of the fabric, and on account of its being strongly caustic it would tend to burn the faces of the men it came in contact with. These difficulties were overcome by making the helmet of two layers of flannelette instead of one layer of flannel, and by mixing with the sodium phenate a large quantity of glycerin. This kept the fabric moist and prevented the caustic from exerting its corrosive action. It was realised from the start that a smoke helmet containing free alkali would deteriorate considerably on exposure to air, and it was found advantageous to provide a breathing tube in the mask so that a man would breathe in through the helmet and out through an outlet valve; in this way the breath, which contains a lot of carbonic acid, would have no bad effect on the chemicals. The use of an outlet valve was also found to have the advantage of keeping the air purer inside the helmet and preventing the stuffy feeling which accompanied the older types of helmet. This additional complication to the helmet was not looked upon favourably at first by the troops, but it was very quickly realised that only a little practice was required to make a man breathe quite normally in the way mentioned above, and that the advantages accruing from the alteration were very great indeed. We found that we could carry on for much longer stretches of time without being fagged out, and more exact trials by the scientists showed that a man’s temperature, pulse and rate of breathing did not increase nearly so rapidly if he used an outlet valve as when breathing out and in through the same material. This is largely due to what is called “dead space,” which means the volume of air in between the lungs and the atmosphere and in which the air is largely composed of breath exhaled from the lungs. The smaller this space the easier it is to breathe.