[Illustration: FIG. 20.--Details of the Clarke Expiratory Valve. In the position of the lever marked “open” the valve lifts easily and widely, and the breathing will be purely valvular: in the position marked “half open,” the valve lift is diminished, and the breathing is partly valvular, partly to-and-fro. In the position “closed” re-breathing only is possible.] The key to Teter’s advance is his removal of the inspiratory valve from the mouth of the nitrous oxide bag, and his substitution for Hewitt’s rubber expiratory valve, of a _rigid_ valve, the lift of which can be diminished or entirely abolished, at will (_see_ Fig. 20). By damming, as it were, the flow from the expiratory valve, the administrator can oblige the patient to practise a certain amount of re-breathing, and, if he keeps up a free flow of the gases, he can develop a pressure in the nitrous oxide bag definitely exceeding that of the atmosphere. Teter also introduced into his apparatus a means to warm the gases, and to add a little ether vapour to the mixture when required. [Illustration: FIG. 21.--The Clark gas oxygen machine.] The Clarke machine is similar in principle to the Teter, but makes a strong point of the intimate mixture of the two gases produced in the mixing chamber which occupies the centre of the apparatus (Fig. 21). In both these machines, it will be observed, the two bags for N_{2}O and O_{2} respectively are attached to the stand, and the mixed gases are led to the patient by a pipe of wide bore. When re-breathing occurs, it must therefore be up and down this pipe, but the width of the bore seems to obviate any disadvantage which theoretically might be expected from this form of respiration. [Illustration: FIG. 22.--Marshall’s sight-feed gas-oxygen apparatus. Of the two glass bottles the one to the left is the sight-feed, that to the right the ether chamber.] In the experience of the author and of many other anæsthetists, very good results can be obtained from either of these machines. Sight Feed Machine. In the author’s opinion, machines based upon this principle are likely to take a prominent place in the future of gas-oxygen. Fig. 22 explains the simple mechanism. Each gas is led through a tube dipping into water contained in the sight feed mixing chamber. The ends of the pipes are open, and on the sides of each pipe also are a number of holes. If the pressure at which either gas is delivered is small, bubbles will be seen ascending towards the surface of the water from the upper holes only. The greater the pressure, the further down the pipe does the gas carry before all of it escapes through a hole, and one can therefore get an accurate estimate of the pressure from the number of holes through which the bubbles are seen escaping. Upon the surface of the water, the two gases meet and enter into mixture and are conveyed away by the third pipe which, of course, does not dip into the water.