46 



MR J. Y. BUCHANAN ON THE 



Table C — continued. 



Weight required to immerse hydro- 



r 





3-7034 



3-7063 



3-7052 



meter to 50-mm. mark at standard 













temjjerature, T, ( = w + dio,. + dw^, 













Total weight of solution displaced by 



s 



• • • 



184-6147 



184-6176 



184-6165 



hydrometer when immersed to 50-nim. 













mark at standard temperature, T, 













Corresponding weight in distilled water 



s 



... 



181-7105 



181-7105 



181-7105 



Specific gravity of solution-^ 



t 



... 



1-015982 



1-015998 



1-015992 



§ 19. Influence of the Meniscus. — It may be here pointed out that the weight 

 which causes the immersion of the hydrometer is its own weight plus that of the liquid 

 meniscus which it carries on the stem above the line of flotation. It is impossible to 

 measure or weigh this exactly ; but we are concerned only with the question if, 

 and to what extent, it can affect the exactness of the determination of the specific 

 gravity of our solutions. 



The weight of the meniscus depends on the surface-tension of the liquid, and this 

 has been determined for distilled water and for a few saline solutions. The results are 

 given in Tables Nos. 124 to 129 of the Smithsonian collection of Physical Tables, of 

 which a new edition has been recently published.* In these tables only one salt 

 belonging to either of the enneads MR or MROg is included, namely, chloride of 

 potassium. For the solutions of the remaining seventeen salts there are no experi- 

 mental data. We are therefore unable to state exactly the effect which would be 

 produced on the specific gravities of their solutions if the weights of the meniscuses of 

 the distilled water and of the solution had been taken into account. Nevertheless, it 

 is worth while to study the influence of the meniscus on the specific gravity of the 

 solutions of the salts quoted in the table, because light will thereby be thrown on 

 the probable extent of its influence on the specific gravity of the solutions of the 

 salts of the two enneads. 



When we determine the specific gravity of a solution, we first immerse the hydro- 

 meter in distilled water, and add (if necessary) small weights to the top of the stem 

 until it floats at a certain division on the stem, say 50 ; and we call the total displacing 

 weight H. We then immerse the hydrometer in the solution, having the same 

 temperature as the distilled water had, and add weights to the top of the stem until 

 the instrument floats in the solution at the same division, 50 ; and we call the total 



TT/ 



displacing weight H'. Then the specific gravity of the solution is S = -^ • Here we 



have not taken account either of the meniscus of the distilled water or of that of the 

 solution. But each of these meniscuses exerts the pressure of its own weight and 

 contributes to the displacing weight of the instrument when it floats at 50 in the 

 distilled water and the solution respectively. Let h = the weight of the meniscus of 



* Smithsonian Physical Tables^ 5tli revised edition, 19 lU. 



