SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 167 



In order to arrive at the value of the correction for temperature difference in terms 

 of weight, two series of observations are made in the experimental solution at two 

 temperatures one degree higher and lower respectively than the standard temperature 

 selected for the specific gravity determination. The same added weights are used 

 in both cases. 



Under these circumstances the scale readings for the same added weight in the first 

 case must be higher than those in the second ; a series of differences of scale readings for 

 the same added weight is obtained, which in the case of hydrometer A amounted to 5 "0 mm. 

 for each pair of observations ; hence, for a difference of O'l"^ C. in the mean temperature 

 indicated in line I, we arrive at a value of 0"25 m.m. in scale reading, which represents 

 the effect of the alteration of the temperature of the liquid by 0'1° C, and, as has been 

 indicated above, this scale reading can be interpreted in terms of added weight diVt, to 

 be added to or subtracted from the total added weight according as the mean temperature, 

 T, is higher or lower than the selected standard temperature, T. 



We thus obtain the total weight of experimental liquid displaced by the hydrometer 

 when floating in it at the 50-mm. division. 



When the experimental liquid is distilled water, the entries in lines t and v are 

 identical, and the corresponding entry in line w is unity. Before proceeding with the 

 determination of the specific gravity of solutions, a number of series of observations 

 are made with the instrument in distilled water at the selected standard temperature, T, 

 by which we arrive at the total weight of the instrument when floating and immersed 

 in this liquid up to 50 mm. on the stem. When we are using the closed hydro- 

 meter this number is a constant. When the experiments in distilled water are being 

 made with the open hydrometer, the weight of air actually present in it at each experi- 

 ment is ascertained and taken into account in the computation of the whole displacing 

 weight of the hydrometer in the given conditions. But whether the hydrometer is 

 open or closed, the mass of air in it only contributes so much to the total weight of 

 the instrument at the moment. It makes no difference whether the air enclosed in 

 it forms a greater or less proportion of it. 



§ 86. Scheme for Logging the Observations made with the Hydrometer in the 

 JExperimental Liquid at the Selected Standard Tempei-ature, T. 



Line. 

 a 



Symbol. 



Explanation. 





Designation of the hydrometer used. 





b 



W 



Weight in vacuo of the closed hydrometer ; or of the glass and shot in the case 



of the open hydrometer. 



c 





Date when the experiment was made. 





d 





Time of commencement of experiment. 





e 



t 



Initial temperature of liquid. 





/o 





Added weight w in grams :— Scale reading R in millimetres. 





/i 





Wi Ri 





h 





1^2 1^2 





f} 





W>3 Rg 





h 





^4 R4 





h 





W5 Rs 





f} 





Wg Re 





f) 





w, R, 





U 





Wg Rg 





h 





Wg Rg 





