(2) The temperature of the chloroform is not allowed to fall below 13° centigrade. To ensure that this cannot take place without the knowledge of the administrator, two coloured beads are thrown into the chloroform. At the desired temperature of the chloroform (13°–15°C) the blue bead sinks to the bottom, the red one nearly to the bottom. Below 13°C, the red bead also touches bottom, and when this is observed, the chloroform vial is warmed up in the palm of the hand. At the point 15°C, both beads float, and the warming must then stop, or an undesirable addition to the vapour strength yielded will occur. The face-piece is made of rubber, and must be closely adapted to the face; in its side is seen the expiratory valve. Inspiratory valves are present at each end of the horizontal limb. The great advantage of this instrument, to the author’s mind, is for teaching or demonstration purposes. If the student _sees_ the lever gradually being moved over from ·2 to 2 per cent., then slipped back to about 1·5 per cent. after full anæsthesia has been obtained, he begins to appreciate what it is he is aiming at when giving chloroform by the ordinary open method. [Illustration: FIG. 37.--Schimmelbushch’s mask.] Open Method. The appliances requisite are:-- (1) A mask. Schimmelbushch’s is the best known (Fig. 37): as elsewhere explained, it does not accurately fit the face. (2) Material to stretch on the mask. The best is two layers of domette or one of flannelette: surgical gauze is so light that heavy drops of chloroform are apt to “spark” through it and burn the skin of the face: lint rapidly becomes sodden; the drug drips away from its edge instead of vapourising properly. (3) A good drop bottle, of which many varieties are marketted (Fig. 38): it is essential that it should be capable of producing definite _drops_: the old method of intermittent “douching” of chloroform is to be condemned as violating the first general principle for giving the drug. [Illustration: FIG. 38.--Chloroform drop bottles.] It is not possible by the open method to be mathematically accurate with percentages, but the necessary appliances are simple, easily transported, and practically always at hand. If the student learns to use it, and while doing so _to think in percentages_, he will achieve as good results as or better than he will with percentage instruments. While he may not have in front of his eyes a dial which shows the percentage graphically, observation of the patient will inform him whether the percentage being given should be maintained, raised, or lowered. The only remaining point for him to realise, then, is how in practice such regulations of percentage strength can be achieved by the open method. The strength of the vapour will depend upon three factors:-- (1) Nature of the material used on the mask.