SPECIFIC GRAVITY AND DISPLACKMENT OF SOME SALINE SOLUTIONS. 165 



enclosed. As mean of the experiments made at 19'5° C, the added external weight 

 required was 075471 gram; so that the total solid weight of the hydrometer was 136"86603 

 grams, which may be taken as the weight of distilled water at the temperature 19*5° 

 displaced by the hydrometer. Dividing this weight by 0*99834, the density of distilled 

 water at 19"5°, we obtain 1 37*093 c.c. as the volume of the water, which is equal to the 

 external volume of the instrument which displaces it. If from the volume so found 

 we deduct the volume of the glass and lead, we find the volume of the air contained 

 in the instrument to be 112*493 c.c. Under the atmospheric conditions prevailing at 

 the date of the experiment, 1 c.c. of air weighed nearly 1*2 milligram, whence we 

 obtain 0*13592 gram for the weight of the enclosed air at the time. The weight of 

 enclosed air is not constant. It is subject to slight variations, principally those of 

 the barometric pressure, and these have to be taken into account. 



§ 84. There remains now only one item to complete the total effective weight of the 

 floating hydrometer, namely, that of the air displaced by the portion of the stem above 

 water when the instrument is in equilibrium with the water. The total effective weight 

 of the hydrometer is diminished by this amount. When it is immersed up to the 

 oO-mm. division, the portion of the instrument not immersed in water is the part of the 

 stem above the 50-mm. division, having a length of 75 millimetres, namely 50 mm. to 

 the upper end of the scale and 25 mm. to the top of the stem. 



The experiments made in distilled water at 19*5° C. with this hydrometer showed 

 that the addition of 0*1 gram to the weight at the top of the stem increased the im- 

 mersion by 10*69 mm., whence we obtain 0*7 c.c. as the volume of the exposed stem ; 

 and the weight of this volume of air is found as above to be 0*00084 gram. This has to 

 be deducted from the sum of the weights of glass, lead, and air. We have then : — 



Weight in vacuo of glass and lead 



,, ,, air enclosed ... 



„ ,, added external weight . 



Giving as total weight .137*00195 



Deducting weight of air displaced by stem .... -0*00084 



We have for the final weight of the loaded hydrometer floating 



at 50 mm. in distilled water at 19*5° C. . . . 137*00111 



136*11132 grams. 

 0*13592 „ 

 0*75471 „ 



Deducting from this the added external weight, 075471 gram, 



we obtain the true weight of the hydrometer alone . 136*24640 



Section XII. — Numerical Details illustrating the Use of the Open Hydrometer. 



§ 85. For this purpose we will consider in detail the items of the experimental 

 determination of the weight of liquid displaced by the instrument which is designated 

 " Hydrometer A " ; it has for some time been in constant use. 



A scheme for recording the items of observation is given in § 86. 



