148 Laboratory Arts 



A stoppered weighing bottle is weighed, the stem of the 

 pipette stroked with a brush full of melted paraffin wax, and 

 a mark scratched through this with a needle at right angles 

 to the axis of the tube. Water is filled into the pipette in the 

 usual way, and emptied from the mark into the weighed 

 bottle, drained five seconds, and the last drop blown into 

 the bottle. The bottle and water are then weighed, and the 

 difference shows the amount of water delivered from the 

 pipette under the conditions of using. The number of grams 

 of water is, of course, divided by the density of water to obtain 

 the volume of the pipette. Should the result not correspond 

 with that desired, the mark is moved up or down the stem, 

 and a second experiment made, and so on until the exact spot 

 is found. Usually three trials will be required before success 

 is reached the first and second giving the relation between 

 length of stem and volume discharged, the differences being, 

 of course, proportional; hence a simple calculation should 

 show where the correct mark is situated, and a third trial 

 should justify this. 



The correct position having been marked, the whole stem 

 and bulb of the pipette is covered with wax, all previous marks 

 being obliterated, the correct mark alone piercing the layer 

 of wax on the stem. The mark should be continued in a circle 

 completely round the stem, and the number of cubic centi- 

 metres delivered by the pipette written, through the wax, upon 

 the bulb. 



Filter paper is then cut of such size as to well cover the 

 various marks a strip i" x J" for the stem, and a square 

 covering the writing for the bulb. These filter paper strips 

 are then saturated with a solution of hydrofluoric acid, applied 

 to the pipette, allowed to stand ten minutes and kept moist 

 with the acid if necessary. The filter paper is then washed off, 

 the glass well washed, and the wax removed by heating and 

 wiping with a dry cloth. The marks should be properly etched 

 on the glass, and appear white, not transparent, as would be the 

 case if the acid were applied in liquid form. 



(b) A burette. Pinch-cock burettes may easily be made 

 and graduated by any student who cares to follow the directions 



