2° 



4° 



6° 8° 



10° 



12° 



14° 



16° 



18° 20° 



29 



32 



33 34 



32 



29 



25 



19 



12 4. 



Hydrometers of Total Immersion. 511 



The specific gravity of air-free water at various tempera- 

 tures was calculated from the recent determinations of Max 

 Thiesen, Karl Seheel, and Diesselhorst *, by the use of the 

 interpolation formula 



Y - Y + rA' + X{X ~ 1] A » + *(*-l)Qg-2) A '» 



with the exceptions that at 5° the specific gravity of water was 

 assumed to be 0*9999922, and at temperatures near the point 

 of maximum density it was calculated from the formula 



l- e = {0 58627 + 012(^71(4^. 



If <7y be the density of air-free water, and di that of air- 

 containing water, then 



Temperature ... 0° 

 W(df-di) 25 



Above 20° the difference is not noticeable. 



By air-free water is understood water only exposed to the 

 air during an experiment ; by air-containing, water which 

 had been left in a flask stoppered with cotton- wool for from 

 36 to 48 hours previous to the experiment. 



B. On the Weights used. 



The relative values of the brass weights used were determined 

 at the beginning and at the end of the experiments ; the 

 greatest difference between the two determinations was 0*05 

 milligramme. 



The platinum rings used to increase the weight of the 

 hydrometer were made from pieces of platinum wire of 

 different diameters. Small knots and loops were made on the 

 pieces of wire before they were bent into rings to facilitate 

 identification. The rings were photographed and kept in a box 

 provided with separate compartments. 



It was found impracticable to use rings of less than 10 milli- 

 grammes, but any desired weight, to the nearest milligramme 

 could be obtained by combination. 



* Ann. Phi/s. C/iem. 1897 (ii.) lx. pp. 340 349. 

 f Ibid. 



