Appendix I. On methods 



291 



like an estimation of urea in urine with sodium hypobromite. In theory at all events 

 this was so, in practice very much greater precautions had to be taken ( 1 ) to maintain 

 the temperature of the great mass of colloidal fluids constant, and (2) to get off all 

 the oxygen by an immense amount of shaking. These two processes were of course 

 wholly inimical to one another. Nevertheless the method sufficed in the hands of 

 Haldane to establish the theoretical accuracy of the ferricyanide reaction. 



The differential method of Blood Gas analysis. 



The apparatus consists of a manometer, with a blood gas bottle at the head of 

 each limit. The apparatus is shown in Fig. 136. 



FIG. 136. 



The bottles are similar to those of the apparatus previously described. 



No control apparatus is necessary, the principle of the differential apparatus being 

 that changes in the temperature of the bath, &c. affect the two bottles alike and 

 therefore counterbalance one another. 



The tubing of the manometer should be 1 mm. bore approximately ; the bottles 

 each about 25 c.c. in capacity. They should be of the same size within 1 tenth 

 of a c.c. 



To Jill the apparatus with clove oil. 



This fluid is in many ways ideal, it has however one drawback, namely that the 

 apparatus must be chemically dry before the oil is put in. It must also be chemically 

 clean. It is best, I think, to avoid alcohol and ether in the cleaning. I clean the 

 tubes by filling them with potassium bichromate and sulphuric acid and then plunging 

 them in a bath of the same, which I can warm up. The bichromate is then thoroughly 

 washed out with distilled water and the apparatus is dried by aspirating air (which 

 has passed through sulphuric acid) gently through the manometer in a warm air bath. 



192 



