THE DENSITY OF LIQUIDS 437 



the suspension was weighed and the sinker was replaced and 

 again weighed : the mean value of the difference gave the 

 apparent weight of the sinker in the liquid. The density was 

 calculated from the expression 



S-Wi-w+Wi-W f, d\ 



2 V 8-4/ 

 D[ = 



V t 



where S = the mass of the sinker. 



V t = the volume of the sinker at f C. (the temperature 

 of the thermostat). 

 W 1 and W 2 — balance readings with the sinker on. 

 w = balance reading with the sinker off. 

 d = the density of the air. 



The average variation from the mean was 2 or 3 in the 

 sixth place. 



As shown by Lamb and Lee ( 1 ) the displacement or hydro- 

 static method has not the three drawbacks which the pyk- 

 nometer method already mentioned possesses. The liquid 

 can be kept at a constant temperature, as it is possible for it 

 to be stirred. The humidity effects do not interfere, as the 

 float is surrounded by the liquid at constant temperature and 

 so a definite surface condition will result. Thirdly, the load 

 on the balance is small, since the weight of the float is neu- 

 tralised by the buoyant effect of the liquid. To overcome the 

 surface tension effects caused by the wire in the suspension 

 method, a submerged sinker method has been devised by 

 Pisati and Reggiane, who use a sinker of known specific gravity 

 and add distilled water until the sinker hovers in the liquid 

 or add small platinum weights to the sinker until the same 

 state is obtained. Richards and Shipley (1) make tempera- 

 ture the variable factor and obtain results correct within 1 

 in 5,000,000. It is difficult to vary the buoyancy of the sinker 

 by very small amounts (variations as small as *oi milligram 

 being necessary) while slight temperature alterations are also 

 difficult to bring about. Lamb and Lee have devised an ex- 

 tremely accurate apparatus in which they have avoided these 

 difficulties. They placed a piece of soft iron in the bulb of the 

 sinker and by means of an electric current sent through a 

 properly placed external circuit exert an electro-magnetic 



