SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 169 



By subtracting from this the volume of glass and shot ( ^ 24*6 c.c. ; see § 83), the 

 resultant volume, 112'493 c.c, is that of the enclosed air. 



(The internal volume of the stem above the 50-mm. division is here disregarded.) 

 The weight of this volume of air is obtained as follows : — 



Weight of 1 c.c. air under the atmospheric conditions during the 



experiments = 0"001208 gram. 

 AVeight of 112-493 c.c. air = 0-13592 gram. 



These numbers give the amount of the air contained in the hydrometer when it 

 carries an internal load of 95 '6 grams of lead shot. If this load is altered, tlie residual 

 volume of air experiences a corresponding alteration. 



§ 87. Coy^rection for the rion-immersed Portion of Stem. — When the hydrometer is 

 floating at 50 mm. in distilled water, there is a length of stem of 75 mm. in air — 

 namely, 50 mm. to the end of the scale, and 25 mm. to the open end of the stem. 



By line g of the table in § 86, we see that 10 "6 9 mm. of scale are immersed by 0"1 

 gram, and 75 mm. are immersed by 0*7 gram, whence the volume of the non-immersed 

 portion of the stem may be taken as 0"7 c.c. 



By the Archimedean principle the non-immersed portion of the stem displacing this 

 volume of air loses weight equal to that of the air so displaced ; so that, were the air 

 removed from the surface of the liquid, the hydrometer would sink into the liquid and 

 the scale reading would be higher. The value of this difference of scale reading is the 

 weight of air displaced by the non-immersed portion of the stem. 



The weight of 0'7 c.c. of air, under the atmospheric conditions quoted above, is 

 0-00084 gram. 



We have then, for the total weight which immerses the hydrometer to the 50-mm. 

 division : — 



Weight of loaded hydrometer in vacuo =136'11132 grams. 

 Weight of enclosed volume of air = 0-13592 



Added weight to immerse stem to 50 mm. = 0-75471 



137-00195 

 Correction for exposed portion of stem = — 0*00084 



Sum= 137-00111 



This number represents the weight of distilled water displaced by the hydrometer 

 up to the 50-mm. division when immersed in it at 19-5° C. 



The determination of the weight of any experimental solution displaced by the 



hydrometer up to the 50-mm. division is determined in a precisely similar manner. If 



any adjustment of the internal load of the hydrometer is made, its volume must be taken 



into account in estimating the volume of enclosed air. 



§ 88. The degree of accuracy attainable by the use of the hydrometer is best 

 TRANS. ROY. SOC. EDIN., VOL. XLIX., PART I. (NO. 1). 22 



