TEMPERATURES, SPECIFIC GRAVITIES, AND SALINITIES OF CERTAIN SEAS. 75 



Researches on the Specific Gravity and the Displacement of some Saline Solutions." # 

 It was used by Mr Wilton in the same way as during the Challenger Expedition, i.e. 

 with the help of previously determined constants. These constants are (l) the weight- 

 in vacuo of the hydrometer in grams ; (2) the volume in c.c. of the body of the 

 hydrometer at 0° C. up to the lowest division of the stem ; (3) the rate of expansion 

 of the body per degree C. in c.c. ; (4) the volume per mjn. of the divided stem in c.c. 



Of late years Mr Buchanan has introduced vast refinements into the method of 

 using the instrument on shore for chemical research, and these are described in the 

 monograph referred to above. However, it was not very practicable to apply these 

 delicate methods on board the Scotia, so the original method of use was adhered to 

 during this Expedition. This form of hydrometer has a volume of about 180 c.c. 

 Its divided stem is 100 mm. long and displaces from 0'8 to 1*0 c;c. The maker, 

 before sealing it up, loads it to float at the bottom of the stem in distilled water of 

 from 30° C. to 40° C. (when it is intended for use in aqueous liquids). Its weight 

 in vacuo is once for all determined with the greatest care. A set of weights of such 

 form as can easily be placed on the top of the stem is provided. The heavier weights 

 used by Mr Wilton were made of brass wire, and the lighter ones of aluminium wire. 

 The probable error of each weight was not more than zbO'l mg. With these weights 

 the effective weight of the hydrometer can be increased at will, and they are so adjusted 

 that the successive addition of each weight can increase the total weight by 0*05 gram. 

 The stem is divided into 100 mm., and its displacement is O'l c.c. -per from 10 to 

 12 mm. Thus when the hydrometer is floating in distilled water, or in an aqueous 

 solution, of not very high specific gravity, the addition of O'l gram to the effective 

 weight sinks the stem by from 10 to 12 divisions. 



Hydrometer 25. — This hydrometer was used in* the testing of the density of 

 samples 1 to 199. Its weight in vacuo is 186*2944 grams. The volume of the body 

 up to the lowest division of the stem at 0° C. is 186*8427 c.c. The rate of expansion 

 of the body per degree C. is 0*00567 c.c. The volume per mm. of the stem is 

 0*00887 c.c. These constants were determined in Edinburgh before the start of 

 the Scotia Expedition. 



Let t' be the temperature in degree C. of the sample of water whose density is to 

 be determined. 

 R the stem-reading at the place of immersion. 

 W the weight in vacuo of the hydrometer in grams, 

 w the total weight of the added weights in grams. 



Then ( W + w) is the total effective weight of the hydrometer, and consequently is the 

 weight in grams of the sea-water displaced; while (186*8437 + 0"005G7t' + 0*00887R) 

 c.c. is the volume of the immersed part of the hydrometer and consequently of the 

 displaced sea-water. 



* Trans. Roy. Soc. Edin., vol. xlix, part i, 1912. 



