SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 171 



The table in § 90 includes three series obtained with hydrometer A, and two obtained 

 with hydrometer B. The values of the mean specific gravity (S) furnished by each 

 series, and its probable error (±^"o), expressed in units of the sixth decimal place, are 

 collected in the following table : — 



\ Hydrometer. 



j 



s. 



±ro. 



A 



1-409752 



3-1 





1-409746 



3-5 





1-409746 



3-6 



B 



1-409727 



4-5 





1-409753 



6-4 



It will be seen that the uncertainty of the means of each series lies entirely in the 

 sixth decimal place. The mean of the five means tabulated is 1*409744, and its 

 probable error is ±3 '16 in the sixth decimal place. 



§ 89. In order to reap the full benefit of the precision of which the hydrometric 

 method is capable, the operations must be carried out with attention to every pre- 

 caution, and the experimental data must be recorded according to strict method. 



Scrupulous cleanliness is of the first importance, and the operations must be carried 

 out with attention to all the precautions usually observed in laboratories from which 

 exact work is expected to proceed. 



It is important that the room in which the observations are made should have a 

 north light and be entirely under the control of the experimenter, who is its only 

 occupant. This is essential, because the management of the temperature of the room, 

 which must be that which the experimenter has found by his own experience to be the 

 one which maintains the experimental liquid constantly at the selected standard 

 temperature while the observations are being made, is the most important element of 

 success and the most diflicult of achievement. The conditions are similar to those 

 which have to be observed in the room in which gas analysis is made by Bunsen's 

 original method, only they are rather more stringent. For myself, when I begin 

 hydrometric observations I always lock the door, a practice which I adopted on board 

 the Challenge?' and have adhered to ever since. 



The conclusion arrived at from the discussion on temperature conditions which 

 is given in Section IV. on the closed hydrometer applies with equal force in the 

 use of the open hydrometer. An interesting diff"erence occurs in experiments on 

 strong solutions, since they have a lower specific heat, which may fall as low as 0*5, 

 as in the case of most concentrated solutions of CaCl2, so that the thermal mobility 

 of these solutions is greater, and this condition may be met by allowing a somewhat 

 increased margin of difierence between air and solution temperature when the 

 compensating luminous flame is used. 



