CONDITIONS PAVOURINQ PEEMENTATION. 23 



heated and drawn out to a narrow extremity, after which the urine 

 is gentlj boiled for about two minutes over a flame, great care being 

 taken to avoid any waste of the fluid by spurting. During the 

 continuance of ebullition the extremity of the vessel is hermeti- 

 cally sealed. Some little practice is required to do this properly — 

 that is, on the one hand to seal the vessel whilst there is a gentle 

 outpouring of steam, and, on the other hand, to do it in such a 

 way that there is no inbending of glass at the sealed extremity. 

 Even a small amount of such inbending is very apt to lead to 

 a minute crack at the next stage. After allowing an interval 

 of I of a minute for the sealed tip to cool a little, the vessel is 

 inverted, and in this position is at once immersed in a can of 

 boiling water prepared, and ready to hand, for this purpose. Here 

 the experimental vessel is left for 8 minutes or more. 



Three purposes are served by this double process of heating. 

 In the first place, it simplifies the experimental conditions to get 

 rid of the air by boiling ; secondly, the speedy closure of the 

 vessel and the prolongation of the heating in a can of boiling 

 water reduces the loss of fluid by boiling to a minimum ; and 

 thirdly, and principally, the inversion of the experimental vessel 

 during the second period of heating brings those upper portions 

 of the internal surface, as well as the outer surface, of the liquor- 

 potassse tube (which, during the boiling over the flame, may only 

 have come into contact with steam at 212° ¥.), into conti- 

 nuous contact with the heated fluid itself *. 



After the urine in the boiled retort has become cool, the liquor 

 potassge is allowed to mix therewith. This is easily brought about 

 by shaking the retort or flask so as to jerk the bent capillary extre- 

 mity of the liquor-potassge tube against its internal surface. The 

 neck of the previously closed tube is thus broken off", and the 

 liquor potassse itself, owing to the comparative vacuum within 

 the experimental vessel, is forced out and mingles with the steri- 

 lized acid urine. 



If six or ten vessels have been prepared in this way charged 

 with the same stock of urine, some one or two of them may be 

 selected for " control " experiments. In these the liquor-potassse 

 tubes are not broken, whilst in all the others they are ; so that when 

 subsequently placed in the incubator together at a temperature 



* The whole of the internal surface of the liquor-potassEe tube is similarly 

 exposed to the influence of its heated and caustic fluid during these different 

 modes of heating. 



