174 Dr. A. G. Levy. Estimation of Specific Gravity [Nov. 25, 



in value, and may be taken in each case, for purposes of calculation, as 

 2*75 milligrammes per mm., the actual figures given by some other 

 observers varying slightly.* 



I further find, by experiment, that the surface tension of a mixture 

 of the two fluids of any specific gravity between 1 -000 and 1 -080 is, 

 for practical purposes, the same as that of the individual fluids. 



The numerical difference between the values of the surface tensions 

 of water and a chloroform and benzol mixture is therefore 4*55 milli- 

 grammes (7 "3 — 2 - 75), and this, when multiplied by the circumference 

 (expressed in mm.) of the hydrometer stem, is equal to the weight in 

 milligrammes of a column of the mixture of the same diameter as the 

 stem in question, and of a length which equals the extent to which 

 the stem is exposed below the specific-gravity mark which should be 

 the proper reading of the hydrometer. 



This length may be calculated according to the simplified formula 

 A = 2T/nv, where h is height, T is surface tension, r is the radius of the 

 stem, and w is the specific gravity of the fluid. 



Having calculated this height in the case of a hydrometer immersed 

 in a chloroform and benzol mixture of sp. gr. l'OOO, the division of 

 this by the average length of the first divisions of the scale gives the 

 theoretical error (at this specific gravity) of the hydrometer, expressed 

 in scale units. 



Example— In the case of Hydrometer No. 4, 



h = 



2 x 4-55 



no 1 "565 x 1 

 5-8147 



5-8147 



0-398 



- 14-6 



As each division represents 0*001, the error = 0"0146. 

 In the following table, the calculated errors are contrasted with the 

 observed errors of Table I. 



Hydrometer. 

 1 



2 

 3 



4 



Table II. 



Observed error. 



0-002 

 0-003 

 0*095 

 0-010 



Calculated error. 

 0-0035 

 0-0056 

 0-0123 

 0-0146 



The not inconsiderable disparity between the two columns was, for 

 a time, an unsolved problem, until I found by the following experi- 



* Chloroform. 



Temp. 16°-6 C. 2'75 Quincke 

 „ 12°-5 C. 2-813 Bede. 



Benzol. 

 Temp. 15° C. 2"87 



Schiff. 



•76 Bede. 



