56 



Prof. A. E. Wright. On the Measurement of [Aug. 5 y 



may now be tentatively indicated on the outside of the capillary stem by 

 a light pencil mark. This done, the column of mercury is displaced 

 until its proximal end stands level with the 5-cm. division mark. If 

 the distal end of the mercury column now coincides with the tentative 

 subdivision mark, this last gives the desired 2 '5-cm. division. If it 

 does not coincide, the desired point will be situated half-way between 

 the point now indicated by the proximal extremity of the mercury 

 column and the point indicated by the original trial subdivision 

 mark. 



(/?.) What has been achieved up to this point is a graduation of the 

 capillary stem into five divisions of 5 cm., and a subdivision of the- 

 first of these into two 2*5-cm. divisions. A further process of gradua- 

 tion in terms of 25 cm. is now taken in hand, with a view to finding 

 the points corresponding respectively to 225 and 250 cm. 



(i.) For this purpose a rubber teat is placed upon the upper end of 

 the tube, and a negative pressure having been established, the capillary 

 stem is filled up to the 2 5-cm. mark with mercury, water, or a coloured 

 fluid. It is then filled in succession with eight further 25-cm. volumes, 

 the 25-cm. volumes being in each case spaced off from each other by a- 

 bubble of air. After these air bubbles have risen to the surface in the 

 wide upper portion of the tube, and the separate volumes have here 

 united to form a single body of fluid, a mark is placed on the outside- 

 of the tube to indicate the 225-cm, point. An additional 25-cm. volume 

 of fluid is now introduced, and the point corresponding to 250 cm- 

 is similarly registered. 



These last marks, be it noted, serve only for the provisional gradua 

 tion of the tube. 



(j.) With a view to achieving a more accurate graduation, the 

 portion of the glass tube between the 225 and 250 cm. graduation 

 marks is fused in the blow-pipe flame, and is drawn out into a short 

 thick capillary tube such as will admit of a more accurate calibration. 



(k.) The calibration in terms of 25 cm. is now repeated, and the- 

 points corresponding to 9 and 10 multiples of 25 cm. are now finally 

 marked off on the narrow portion of the tube. 



(I.) A safety chamber is formed on the upper part of the tube, ih& 

 wide end of this last being carried round at right angles to the stem 

 to allow of more convenient manipulation. 



(m.) Lastly, the pencil marks are carried round the whole circuni^ 

 ference of the capillary tube, and they are fixed upon the glass by 

 passing them through the flame. 



(2.) Method of employing the Capillary-diluting Pipette. 



By means of a diluting pipette fitted with a rubber teat any desired 

 dilution of the culture can be obtained very unlaboriously. 



