The following are the chief reasons for resorting to inhalation. 1. Certain agents, as nitrous oxide and oxygen, being permanently in the gaseous form, cannot be administered in any other way. 2. By taking advantage of the immense surface of the air-cells of the lungs for absorption, a more sudden and profound effect may be produced by medicine than it would be safe, or, in some cases, even possible to produce in any other way. It is to this circumstance, and to the rapidity with which certain volatile medicines exhale in the breath, and leave the patient free from their effects, that the power of preventing the pain of surgical operations is due. 3. Many medicines which have a disagreeable taste—as turpentine, creasote, and camphor—are not unpleasant when inhaled in the form of vapour; and the process of digestion is less interfered with than by taking them into the stomach. 4. Medicines, such as benzoic acid, and some of the gum-resins, which are believed to exert a local action on the mucous membrane of the air-passages, may be expected to have a greater effect when inhaled, than when they are taken into the stomach in the same doses, and reach the lungs only through the circulation. 5. Some agents, as chlorine and ammonia, have a local action when inhaled, which they could not exert if exhibited in any other way. In every kind of inhalation, the breathing should be allowed to go on freely, and in the natural way. Medicines may be inhaled either at the ordinary temperature, or with the aid of artificial heat; and in the latter case they may be breathed with the addition of vapour of water, or with only so much of it as is naturally present in the atmosphere. The medicines in use for the prevention of severe pain, are always inhaled at the ordinary temperature. CHLOROFORM.