SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 31 



centimetres in both cases, and the air which it displaces, at 1'2 milligram per cubic 

 centimetre, weighs I "5 milligram. 



In order to avoid complication, we suppose that the necessary "added weights" 

 have been added to the internal load of the closed hydrometer, and that the dis- 

 placing weight of the hydrometer quoted for each immersion is its true weight in 

 vacuo, and that nothing which can affect the immersion of the instrument in the 

 liquid is immersed in air excepting the exposed portion of the stem itself. This 

 disengages the effect produced by the buoyancy of the stem from that of every 

 other cause. 



Let the weight of the hydrometer so floating at mm. in distilled water be 180 "25 

 grams ; and let its weight when floating also at mm. in the solution be 185 "25 grams; 

 then, neglecting the buoyancy of the stem, the specific gravity of the solution is 

 1 '027739. But the effect of buoyancy is to reduce the effective weight in both cases by 

 1*5 milligram, so that the specific gravity of the solution corrected for buoyancy of 



stem is =1 "027740. Therefore, when the whole of the stem is exposed, its 



180-2485 ^ 



buoyancy affects the resulting specific gravity to the extent of only a unit in the sixth 



decimal place. 



§ 13. Determination of the Weight of the Hydrometer. — For this purpose the 

 hydrometer is placed on the right-hand pan of the balance in an upright position, and 

 is brought to equilibrium with weights and rider on the left-hand pan. The hydrometer 

 is then removed and equilibrium again established by means of standard weights. 

 These are then replaced by the hydrometer and equilibrium re-established by shifting 

 the rider of the counterpoise if it has been disturbed. The hydrometer is again 

 removed and replaced by standard weights until equilibrium is established. In this 

 way four independent weighings by replacement by standard weights are obtained. 

 The temperature of the air is noted, also the temperature of the wet-bulb thermo- 

 meter and the height of the barometer. Three such series of weighings are made 

 on different days when the meteorological conditions are different. Each series is 

 treated by itself. In order to obtain the vacuum correction we require to know the 

 weight of the air displaced by the hydrometer and by the weights respectively. 

 The difference of these two weights, the net buoyancy, is the correction to be added 

 to the apparent weight of the hydrometer. 



We take as an example of the method the determination of the weight in vacuo 

 of hydrometer No. 17. 



1st Determination. 

 5th March 1894. 



Barometer = 740'86 mm. 



Temperature, dry bulb = 6*15°, wet bulb= 5*1° C. 



Whence the vapour tension is 6*03 mm., and the weight of 1 litre of this air is 



1-2288 gram. 



