ON ANESTHETICS. 157 



of the bulb. Tims with a bulb = 527 c.c. a decigramme rider (100 

 milligrammes) placed at the 1, 2, ,3, 4, 5, on the same side as the bulb, 

 will counterpoise the ascensional force of air (at 760 mm. and 18°) 

 containing 1, 2, 3, 4, 5 per cent, of chloroform vapour. The same 

 counterpoise placed at 1, 2, 3, 4, 5, on the opposite side of the beam, 

 will raise the bulb (in air) and give deflections of the index corresponding 

 to 1, 2, 3, 4, 5 per cent, of chloroform vapour. 



Thus by placing the rider at 1, 2, 3 on the left-hand side, the deflec- 

 tions of index corresponding to 1, 2, and 3 per cent, of chloroform are 

 readily determined. 



Finer readings — one-tenth per cent, and less — can be made by 

 means of a more sensitive instrument and a larger bulb, using a null 

 method, the first or coarse adjustment being effected by a rider. The 

 5 per 100 rider alluded to above is suitable to this purpose. 



With a balance giving a swing of 2 '5 mm. per milligramme, and 

 a bulb of 1,054 c.c. capacity, we have 1 per cent. =40 milligrammes, 

 so that the 5 per cent, rider is to be taken = 200 milligrammes; 2'5 mm. 

 s\ving = l milligramme =]/40 = - 025 percent., so that 1 mm. swing = 

 0*01 per cent. Thus on such an instrument we have each 



Per cent. 

 Large division of beam = 1-00 

 .Small division of beam = 0-10 

 Millimetre of swing = 0*01 



With a bulb of 527 c.c. capacity the 5 per cent, rider must be taken 

 = 100 milligrammes. In this case 1 mm. of swing indicates 0'02 per 

 cent. 



With this more sensitive balance — that may be designated as type B 

 —it is possible to observe with great exactness the percentage of 

 delivery. Thus, e.g., with the rider at 2 - l and the index at 0, 

 the percentage is 2'10. With the rider as before and the index swing- 

 ing 10 left-5 right, the percentage is 2 - 05. With the index swinging 

 5 left-10 right the percentage is 2' 15. With the index swinging 5 left' 

 2'5 right the percentage is 2'075. 



For ordinary hospital use type B is inconveniently and unneces- 

 sarily sensitive; the readings to be of any accuracy must be corrected 

 for temperature and pressure. Moreover the system cannot be left 

 swinging while a current of ehloroform-and-air is passing through the 

 balance-case. The beam must be out of action while the current is 

 passing, and the current must be interrupted while an observation of 

 density is taken. Nevertheless, for purposes of exact study as, e.g., 

 where it may be desired to know how closely a given percentage has 

 been observed, type B may occasionally be of service, but in such cases 

 it should be used in series with an instrument of type A, with a two- 

 way tap and short-circuiting tube, so that the delivery to the patient 

 is not interrupted while the current through balance B is turned off. 



The corrections for temperature and pressure are made according to 

 the formula : — 

 log P= 1-8377 + 



