SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 163 



if we take its specific gravity to be ir35, the volume of air which it displaces is 

 c.c, the weight of which at the observed density is a^ grams, whence the weight 



1 J. oo 



in vacuo of the lead is 



L„ = L + «(. 



If the load L has been so adjusted that at the temperature, T, fixed for the experiments 



the hydrometer floats in distilled water at the top of the stem, then the weight of 



distilled water displaced by the hydrometer when so floating is in vacuo G^ + L^ + A^, 



whence the external volume of the whole instrument is obtained ; let this volume be V, 



then we have 



r^ T A 



where (p is the density of the air expressed in grams per cubic centimetre if A,, is 

 expressed in grams. 



In this equation V, G^,, and L,, are known, therefore 



</.- V2-5 + 11-35J 

 whence 



In any locality (p varies with the weather, but it can always be ascertained by the 



observation of the meteorological elements. The relative humidity of the air in the 



special room required for this work seldom differs much from 50 per cent. ; therefore 



the variations in the density of the air are due almost wholly to variations of the 



barometric pressure. In London, the extreme range of barometric pressure may be 



taken to be between 730 and 770 millimetres, having therefore an amplitude of 40 



millimetres. If we suppose that the barometric pressure was 750 millimetres when 



the instrument was weighed, and that the air then weighed 1 "2 milligram per cubic 



2 

 centimetre, the extreme variations of density to be expected will be =t— x 1*2 = 0'032 



milligram per cubic centimetre. In the case of hydrometer No. 1, when loaded for 

 work in distilled water the volume of air in the instrument was 112"5 c.c, which at 

 1'2 milligram per c.c. would weigh 135 milligrams, and the extreme variations of this 

 weight would be dz4"32 milligrams. 



It is evident, therefore, that the actual weight of the air in the hydrometer at the 

 time of making an experiment is an essential factor in computing the weight of liquid 

 which it displaces. There is therefore an advantage in making such observations when 

 the weather is settled ; the variations of the barometric pressure in the course of a day 

 are then of such an order as to be almost negligible. If, however, a cyclonic depression 

 is passing over the locality, the change of barometric pressure from hour to hour may 

 have to be taken into account. 



The dimensions of this instrument, fig. 4, are : — from lower extremity A to 



