﻿IX, B, 4 Barber: The Pipette Method 355 



be used to remove the medium surrounding the infected filament 

 and to substitute any other during the course of an experiment. 

 If it is found that there is not enough pressure to eject the 

 dose, one may move a Bunsen burner or electric light to a point 

 a few centimeters from the loop. Or a rubber bag containing 

 warm water may be adjusted by means of a bent wire holder 

 to the sleeve of the pressure apparatus, so that the loop in 

 rising will come into contact with it. This is rarely necessary 

 if the loop is cooled sufficiently below room temperature while 

 the capillary is being filled with mercury. The pipette may be 

 so supplied with mercury that it will be under negative pressure 

 at room temperature. In that case, warm water is kept in the 

 cup in place of the cold, and the necessary expansion of 

 the mercury is obtained by immersing the loop in it. But the 

 arrangement first described best meets the main requirement — 

 the possibility of gradually applying pressure and of stopping 

 it quickly. 



If a volatile liquid is to be injected, or if large or repeated 

 doses of the same substance are to be used, one may fill part of 

 the capillary with the substance to be inoculated before making 

 the injecting point, but in most cases one can better regulate the 

 dosage by filling from the point. If very small doses are 

 to be injected, one may keep the top of the mercury column in 

 view after supplying it with the dose. The cell is pierced, pres- 

 sure is applied, and the rising of the mercury column to the tip 

 shows that the dose has come out. Focusing on the mercury 

 column may be facilitated by piercing the cell obliquely with a 

 pipette bent at less than right angles, instead of from directly 

 below. With larger doses, the top of the mercury column is 

 usually below the reach of the lens. Here, one can focus as far 

 down as possible in the lumen of the pipette after its introduc- 

 tion into the cell and stop the pressure on the appearance of 

 the mercury column. If pipettes of the form represented in fig. 

 19, a, are used, much more force is required to expel the mercury 

 than to bring it to the tip, so that one has time to stop the 

 pressure before any mercury can come out. If it is desired to 

 remove cell contents, the retreat of the mercury column indicates 

 that the contents are being drawn into the pipette. 



Small doses may be measured by estimating the cubic contents 

 of the pipette between the top of the mercury column and the 

 tip. Larger ones may be estimated by expelling the dose on 

 the cover glass and measuring the droplet expelled by compar- 



128443 4 



