Appendix I. On methods 



313 



source of power, the gearing being driven in the case of the bath at Cambridge by 

 ^g horse-power motor. 



Into the cylinder fits a rack, this rack can stand vertically on a metal base, from 

 the centre of which rises a hollow tube which is concentric with the bath itself. 

 Attached to this tube are five spring clips, into each of which a tonometer can 

 be fixed. 



The bath is placed on its end and filled almost full of water at about 1 2 C. 

 higher than is desired for the experiment. 



The rack with the tonometers is then put in the water bath. The lid containing 

 the thermometer is screwed on, the bath is then placed horizontally on its bearings 

 and the belt put on the driving wheel. It is then rotated at about 15 revolutions 

 per minute' by the motor. The thermometer is just put in so far that the mercury 

 surface can be read from without the apparatus. A spirit flame playing on the 

 bath is sufficient to keep the apparatus up to the required temperature, should the 

 tendency for the temperature to fall be too great. 



After ten minutes the motor is stopped, the bath is taken off the bearings and 

 allowed to stand vertically on the driving wheel for two minutes, so that the blood 

 may drain down into the small receptacle near the tap of each tonometer. The lid is 

 then unscrewed and the tonometers taken out one by one. It is of course essential 

 that the tonometers be carried vertically and not shaken, otherwise the blood will 

 acquire oxygen from the atmosphere of the tonometer at the reduced temperature. 



If the composition of the blood is required the tonometer is placed vertically with 

 its lower end touching a duster, the tap is gently turned and any air and mercury in 

 the tubing together with a drop of blood are allowed to find their way out ; the tap is 

 then closed and the end placed under the ammonia of a blood gas bottle, and if 

 a known volume of blood is not required, the tap is simply opened again cautiously 

 and a suitable amount of blood is allowed to flow gently out. 



If the delivery tube of the apparatus is to act as a pipette of known volume, the 

 tap is turned in the opposite direction, in which case the amount of blood which is in 

 the pipette will run out by its own weight. Should it fail to 

 do so it may be assisted by placing a rubber tube of large 

 bore in the position shown in the figure and blowing down 

 it ; the air pressure resulting will probably be adequate. It 

 is a risky proceeding to attach a rubber tube to the glass 

 one and then blow. 



Meth<><! 77, which is very useful for work outside the 

 laboratory as well as in it, consists of using a water bath 

 from the side of which project two arms, each of which 

 has a crook at the end which acts as a bearing. The 

 arms are placed at such a distance apart that the bearings 

 catch the tonometer, one close to the cork, the other close to 

 the tap. When the tonometer is immersed in the bath it of 

 course tends to rise, but finding itself caught by the bearings 

 which are above it, is unable to do so. A piece of string is FlG - 



put round it, by pulling the ends of the string alternately the tonometer is rotated. 

 The bath is heated and kept warm from underneath. The tonometer is rotated for 

 ten minutes and then held for a short time in the bath with the cork at the surface 



