In an apparatus such as we are describing the strength of the ether vapour depends on and is influenced by a variety of factors, among which the following may be specially noted:-- (_a_) _The diameter of the ether chamber._--The larger the diameter the stronger will be the ether vapour. The chamber is generally kept two-thirds full: the less the empty space in the chamber the stronger will be the vapour. (_b_) _The rapidity of the air-current._--The larger the amount of air passing over the ether the more rapid will be the vapouration, with the result that the temperature of the liquid ether will rapidly fall and the strength of the ether vapour in the air will be correspondingly lowered. (_c_) _The temperature of the liquid ether._--The higher the temperature of the liquid ether the stronger will be the percentage of ether vapour in the air. If the ether chamber is placed in a bath of water which is maintained steadily at an adequate temperature and if the rate of the air flow remains constant, a constant strength of ether vapour will be given off. It is well in this connection to remember that the boiling point of ether is low and that a very high percentage of ether vapour is readily obtained if the temperature in the water bath is allowed to rise beyond 80°F. For further information regarding ether percentages _see_ Appendix I and II. Such an instrument as Kelly’s requires at its head a _regulating tap_, movement of which is capable of diverting part of the air stream direct to the patient without coming in contact with the surface of the ether. In this way the maximum strength of vapour may be diluted as and when required. In Fig. 33 is shown Shipway’s instrument. Here the drug is dripped into a chamber, the floor of which is kept warm. The ether volatilises at once and the vapour is carried away by the air stream passing through the chamber. The strength of ether vapour in this instrument is regulated solely by the rate of drip which is in complete control of the administrator. Whichever method of making ether vapour is utilised the instrument should be capable of producing a maximum at least of 15 per cent. to 18 per cent., and means must be provided to reduce this percentage at will. 3. DEVICE FOR WARMING VAPOUR.--It is doubtful if the warming of inspired or insufflated vapour has any appreciable influence on the body temperature, but there is a fairly general consensus of opinion that if the ether vapour is warmed there is less likelihood of irritation of the respiratory mucous membrane. It is to be noted, moreover, that in intratracheal insufflation the natural apparatus for warming the inspired air is put out of action. This warming can be effected by the simple device of carrying the tube through a chamber of hot water after it emerges from the ether chamber.