1902.] the Bactericidal Power of small Samples of Blood. 



55 



Fig. 1. 



measured dilution of the culture (in the case of an ordinary bacterial 

 culture, dilutions of 1,000,000 fold to 10,000,000 fold are appropriate), 

 and, secondly, the transference — with a 

 view to the subsequent enumeration of 

 the colonies which develop — of a series 

 -of measured volumes of the diluted cul- 

 ture to the surface of a solid nutrient 

 medium. 



250 cm m. 



10 



The processes of diluting and measur- 

 ing off the desired volumes of diluted 

 culture can be conveniently and unlabo- 

 riously carried out by means of the dilut- 

 ing pipette figured below. (Fig. 1). 



(1.) Method of making and calibrating the 

 "Diluting Pipette" 



(a.) A piece of glass tubing about 

 15 cm. long is drawn out at one end into 

 .a capillary stem. 



(b.) A standard 5-cm. pipette is fitted 

 with a rubber teat,* and is then filled in 

 up to the calibration mark with mer- 

 cury. The 5 cm. of mercury is now trans- 

 ferred from the capillary pipette to the 

 wide end of the glass tube, and is made 

 to enter the upper portion of the capillary stem. 



(c.) When this has been effected, the points corresponding to the 

 upper and lower ends of the mercury column are marked off' on the 

 outside of the capillary stem with a coloured pencil, preferably one of 

 the oil pencils sold for writing on glass. 



(d.) The mercury column is now displaced downwards until the 

 upper end of the column stands opposite the lower of the calibration 

 marks. . This point is again marked off with the coloured pencil. 



(e.) By a similar procedure, three more marks indicating divisions 

 of similar value are placed on the outside of the capillary stem. 



(/.) This done, the tube is filed and broken off at the lowest mark. 



(g.) It will now be convenient to divide the lowest 5-cm. division 

 into two divisions of 2*5 cm. This can be conveniently effected by the 

 following method of trial and error : — Mercury is drawn up into the 

 tube until the upper limit of the column of metal stands as nearly as 

 possible midway between the orifice and the first 5-cm. division mark. 

 The point corresponding to the proximal end of the mercury column 



* A mechanically controlled teat, such as that made by Mr. A. E. Dean, jy.ii., 

 73, Hatton Grarden, E.C., is a convenient form to employ for this purpose. 



F 2 



