26Ô JAKOB SCHETELIG. 



and from falling off when the hydrometer is moved up and 

 down in the Hquid. In order to hang the weights on, I raise 

 the hydrometer by the aid of the stirrer, so that the stem with 

 the spiral comes above the edge of the cylinder (see PI. 23, 

 fig. 2). 



I now add weights until the hydrometer remains poised in 

 the middle of the water. The meaning I wish to convey by 

 this term is as follows. 



However well the contents of the bath and cylinder are 

 stirred, there will always be a difference in temperature, in the 

 cylinder, between the upper and the lower strata, which it is 

 difficult to reduce to less than 0'1° C, when the temperature of 

 the laboratory is not constant. I continue to add weights until 

 the hydrometer plus the weights, though lighter than the water 

 near the bottom, becomes heavier than the water at the surface. 

 It remains poised in the middle of the cyhnder. 



I now merely note down which spiral and which weights 

 I have employed, as also the temperatures of the bottom and 

 the surface, read off exactly at the moment when the hydro- 

 meter is poised. The mean of these temperatures I consider to 

 be the true temperature of the sample at the moment of obser- 

 vation. Thus all dates necessary are found. 



In this manner it is possible with practice to make four 

 determinations in an hour; and when the accuracy attained by 

 this method is taken into consideration, it may safely be asserted 

 that the rapidity of the method is satisfactory. It would certainly 

 be impossible to make four pycnometer-determinations in the 

 same time, and attain the same degree of accuracy. 



In the method itself there is no source of error. If it is 

 certain that the water samples have not changed in any way 

 during the time they have been kept, their true specific gravity 

 at the given temperature can be determined with the hydro- 

 meter of total immersion. The noticeable change in the specific 

 gravity by the glass of the bottle being dissolved in the water- 



