434 SCIENCE PROGRESS 



apparatus can be used in the same manner as the ordinary 

 pyknometer. 



(iii) Bousfield pyknometer (3) consists of a U-tube holding 

 from 70 to 250 ccs. Each arm of the U terminates in a 

 capillary neck expanding to a cup at the top. The ends of the 

 U are connected by a piece of solid glass which serves to 

 attach a platinum wire to hang up the apparatus in the balance- 

 case or the thermostat. A piece of lead hung on the lower 

 part of the U keeps it vertical, and so assists in the adjust- 

 ment. Sometimes the solid piece of glass is replaced by a 

 narrow tube with the idea of facilitating the filling of the 

 apparatus ; there is, however, little difficulty in filling by 

 suitably tilting the apparatus, while the tube acts as a trap 

 for bubbles. Glass stoppers are provided and ground-in glass 

 tubes are sometimes used to assist in filling by suction. This 

 apparatus has the advantage of having a large volume and 

 a relatively large surface so that thermal equilibrium is rapidly 

 attained ; it is also easy to clean. It has the disadvantage 

 of having two ground-glass stoppers with the increased chance 

 of leakage, and of the larger air-space in the cups which may 

 lead to some uncertainty in the vacuum correction. Perhaps 

 it would be worth while to use two sets of stoppers, shallow 

 ones leaving a large air-space in the cups for use in the ther- 

 mostat, and deeper ones almost filling the cups for use during 

 the weighing. 



For accurate work it is advisable to correct to vacuum 

 each of the weighings separately, using the values of the tem- 

 perature and pressure recorded at the time of weighing, rather 

 than to use one of the formulae for reducing the final specific 

 gravity to vacuum ; this entails more arithmetical labour, 

 but gives more reliable results. The method of calculation is 

 shown in the following example : 



Weight of bottle = 317426 grams at i6°C. and 747 mm. pressure. 



Weight of bottle) . ..,-, _. „« , 



full of water C = IDI 68 49 grams at 18*5 C. and 742 mm. pressure. 



Weight of bottle } 

 full of liquid f . „ ~ , 



under examina- f = I S7'4^7 grams at 19-9 C. and 739 mm. pressure. 



tion J 



These three weights (in grams) corrected for errors in the 

 weights used, become : 



317492 ioi"697i 157-4290. 



