In the outlet pipe from the oxygen bag is placed a ball syringe of 2 ounces capacity. A valve in the pipe obliges the flow of oxygen to take place in one direction only when the bulb is squeezed, viz. from oxygen bag to inhaler. The remainder of the apparatus is identical with Guy’s original inhaler, except that the bag is of 2-gallon capacity, and is perforated at its base by a =Y=-tube, one limb of the fork bringing in the nitrous oxide, the other the oxygen. Method of Use. A few breaths of pure nitrous oxide gas are usually allowed “on the valve.” Rebreathing is then instituted, and the addition of oxygen begun. The amount required to each type of patient can only be learnt with experience, but the average is one full compression of the bulb every ten seconds. If anæsthesia is not complete at the end of one minute, put the indicator to “valves” again, and allow the patient nearly to empty the bag. Then push back the indicator to “no valves,” and refill the bag with nitrous oxide by opening the cylinder with the foot key. Some four to six compressions of the bulb are made while the nitrous oxide is running in. The time will now have come to add the dose of ethyl chloride if it be judged necessary at all. This will have been placed in the vial before the administration is begun. After emptying the ethyl chloride into the bag of the inhaler, anæsthesia should be complete in twenty-five seconds. The same small doses of ethyl chloride are used as is the case with Guy’s original method. After a little practice under supervision, students at the Dental Hospital learn to use this method safely and well. No example of risk to life has arisen after eight years’ daily experience.[11] CHAPTER XIV. MIXTURE OF CHLOROFORM AND ETHER. The mere addition of ether does not remove all the undesirable features of chloroform anæsthesia. A heart poisoned by excess of CHCl_{3} does not respond to ether stimulation. Nevertheless, CE mixtures of varying proportions have great value. The less lethal drug takes on part of the work of the more dangerous one; it also keeps the respiratory centre active. Viewing mixtures as dilute chloroform, it is also obvious that there will be with them a greater margin of error in dosage, than with the pure drug. Some chemical change takes place when the two drugs are mixed, for heat is evolved; of the nature of this change we are ignorant.