436 SCIENCE PROGRESS 



the apparatus, (c) The reduction of sensibility of the balance 

 when weighing a relatively heavy load. 



The method of hydrostatic weighing is sometimes used 

 in density determinations. The Mohr-Westphal balance is an 

 example of this method. With this instrument specific gravities 

 can be directly read to the fourth decimal place. The hydro- 

 static method was employed in another manner by Osborne, 

 McKelvey, and Bearce (5) with results in very good agreement 

 with pyknometer determinations. The sinker used was of 

 Jena glass and was 33 cms. long and 1*3 cm. external diameter; 

 it was ballasted with mercury, and before sealing was well 

 annealed at 45o°C. The sinker was suspended, by means of 

 a platinum wire, 0-3 mm. in diameter, in the densimeter tube 

 which was only slightly larger, and was attached to the bottom 

 of one pan of a good balance standing on a shelf above the 

 thermostat. The wire was covered, by electro-deposition, 

 with a layer of dull gold at the point where it entered the 

 liquid. This device was found to ensure the wetting of the 

 suspension and the prevention of slight sticking which other- 

 wise would occur. Platinum black on the fine platinum sus- 

 pension wire has also been used. These devices reduce surface 

 tension effects, but do not completely eliminate them. To 

 the wire was attached a plug which fitted into a hole in the 

 top of the densimeter cover, and the proportions were so 

 arranged that when the balance was arrested the plug filled 

 the hole, while the sinker just rested on the bottom of the 

 densimeter. When the beam was released the sinker was 

 raised off the bottom, and, the plug being removed from the 

 hole, it left the sinker freely swinging. By this means the 

 liquid was only exposed to the air during the actual weighing. 

 The chief difficulties were due to alteration of the concentra- 

 tion of the solution by evaporation and by absorption of water 

 from the air. Also, unless the liquid was first carefully freed 

 from dissolved air, small bubbles formed on the sinker which 

 could not easily be removed. These writers state that the 

 density of alcohol is altered by eight in the fifth place by 

 saturation with air at 25°C. The volume of the sinker was 

 first determined by weighing it in air and then in air-free 

 distilled water ; then by making use of the known densities 

 of the air and of the water, its volume was obtained. Next, 

 the sinker was weighed in the liquid, and, having been removed, 



