Basing upon these views as to the action of chloroform, and upon the lessons of practical experience, we may formulate definite rules for giving the drug. General Principles for giving Chloroform. (1) Give chloroform evenly, not spasmodically. (2) Increase the vapour strength of chloroform gradually from zero until 2 per cent. or at most 2½ per cent. is reached at the end of two or three minutes; maintain that strength until full anæsthesia is obtained; thereafter, drop down to 1–1·5 per cent. This will result in a B.P. curve corresponding to the line ABA′ in the diagram (Fig. 35). (3) Be guided chiefly by the patient’s respiration. Chloroform kills by stopping the heart, but in the immense preponderance of cases, evidence of failure of respiration appears in ample time to give warning of approaching circulatory failure. The eye reflexes give confirmatory evidence of the depth of anæsthesia, but the superlatively important thing is to _maintain a free airway, and be sure the patient is using it_. (4) If serious struggling and breath-holding occur, withdraw the anæsthetic until the patient “resumes normal.” Methods of Administration. The logical application of such general principles would be to use an instrument which gives a definite and known percentage of chloroform, variable at the wish of the administrator. Many such machines have been brought forward, and while none of them have obtained general acceptance, a description of the best known instrument will be given, as the reader may as a house-surgeon meet with it, and with a surgeon who wishes it to be used. Vernon Harcourt’s Inhaler. In principle, this is a “draw-over” instrument; the patient’s own inspirations are the motive power. Passing over the surface of the fluid drug, the inspired air picks up from it a known percentage of vapour. The other system available for the construction of percentage chloroform instruments is the “plenum”; in this the vapour is propelled to the patient by a pump. [Illustration: FIG. 36.--Vernon Harcourt’s Percentage Chloroform Inhaler.] In appearance, the inhaler resembles the letter T, with a rubber face-piece attached to the lower end of the vertical limb (_see_ Fig. 36). The T portion itself is made of metal tubing of a definite size in cross section. One end of the horizontal limb admits pure air, the other, air which has passed over chloroform and picked up from it a certain proportion of vapour. The proportion of the total inspired volume of air which passes through each of the ends is regulated by a lever seen at the junction of horizontal and vertical limbs, and the exact percentage of chloroform being inhaled is indicated by a series of numerals marked on the dial over which the lever moves. These figures are correct provided:-- (1) The chloroform receptacle is not shaken (this would greatly increase the percentage).