ON ANESTHETICS. 159 



recording pen with the recording surface. The latter is adjustable to 

 the former by means of a fine-pitched screw. The pen itself is filled 

 with a special non-drying ink, so that the actual contact between pen 

 and paper is formed by the ink itself. But the adjustment is a delicate 

 one that requires attention, and I do not like to be troubled with it 

 in the hospital use of the balance. 



At the bulb-volume 546 c.c. the bulb sinks with a rise of tempera- 

 ture of 1° by reason of a diminution of ascensional force equal to the 



546 



weight of 2 _-o or ^ c.c. of air, i.e., 2'6 milligrammes approximately, 



and the index is displaced to the left. Similarly the bulb rises and the 

 index travels to the right with fall of temperature. 



Variations of pressure have a reverse effect. With a bulb of, say, 

 532 c.c. and a rise of pressure from 760 to 770 mm. Hg., the ascen- 

 sional force affecting the bulb is increased by an amount equal to the 



5$2 

 weight of _-„=. or 7 c.c. of air, i.e., 9 milligrammes approximately. 



Obviously the zero or point of departure of the index may wander 

 sensibly above or below the zero of a fixed scale. Thus a rise of 2° 

 with a fall of 10 mm. in relation to a 527 c.c. bulb gives approximately 

 - 5 and — 9 milligrammes. At this capacity the weight value of 1 per 

 cent, chloroform is 20 milligrammes, so that the position of the index 

 under these conditions will be shifted to a position 14/20 or 0'7 to the 

 left of a fixed zero. Obviously, however, the readiest means of correct- 

 ing for this wandering is to slide the scale to the left until its zero cor- 

 responds to the position of the index. 



The Chloroform Vessel. 



The vessel containing liquid chloroform over which the current of 

 air passes into the balance-case is double, consisting of two identical 

 vessels each provided with a two-way tap, allowing the current of air 

 to pass (a) directly or (b) over the surface of the liquid chloroform. 

 With a tap at ' off ' the air passes directly, and no chloroform vapour 

 enters the case. With a tap at ' on ' air passes over the chloroform, 

 and enters the case more or less charged with chloroform vapour. The 

 two vessels are in series, so that the current of air can be directed 

 through one or other or both the vessels. 



Under ordinary circumstances, i.e., with a balance-case of a capacity 

 of 30 litres and an air current of, say, 10 litres per minute passed 

 through only one chloroform vessel, i.e., with its tap turned 'on,' 

 the percentage of chloroform vapour in the case, as shown by the rising 

 bulb, rises gradually but sufficiently rapidly to a maximum value from 

 which it subsequently and slowly declines to a minimum value, at which 

 it remains for an indefinite time. The initial rising percentage is the 

 natural effect of the current of chloroform-laden air at 10 litres per 

 minute pouring into the air-filled reservoir of 30 litres capacity. This 

 rise is such that at the end of the first minute the atmosphere in the case 

 is at about 1 per cent, chloroform, and in the third minute at about 

 2 per cent. The maximum value is between 2 and 3 per cent. The 

 subsequent steady minimal value is at about 1'5 per cent. This natural 



