376 APPENDIX 



THE CONNECTION BETWEEN VARIOUS HYDROMETER SCALES AND THE 

 TRUE SPECIFIC GRAVITIES OF LIQUIDS. 



In England, for technical purposes, Twaddell's hydrometer is often 

 employed. This applies to liquids heavier than water. 



The relation between degrees Twaddell and true specific gravity 

 is such that T. corresponds to a specific gravity of 1, while the 

 general formula is 



100 



or n = 200 (d _ 1} 



where d true specific gravity 

 and n = degrees Twaddell. 



Twaddell's hydrometers are, perhaps, based upon a more rational 

 system than most of the other hydrometers. The determinations 

 should be made at 15'5. 



In France, Baume's hydrometers are in general use. For liquids 

 heavier than water a hydrometer which sinks to in pure water 

 and to 10 in a 10 per cent solution of common salt, both at 17'5 C , is 

 employed and a uniform scale engraved on the stem. 1 



For liquids lighter than water, an instrument which sinks to the 

 zero point in a solution of one part of common salt in nine parts of 

 water, and to a point marked 10 in pure water, is constructed and the 

 graduation extended as before. 



i Baume hydrometers are purely empiric, and have nothing to re- 

 commend them ; it is desirable that they should be abolished, but 

 inasmuch as they are still extensively used in France and, to some 

 extent, in America, it may be advisable to give the connection between 

 their readings and true specific gravity. 



The following formulae connect together these values : 



For liquids heavier For liquids lighter 



than water. than water. 



At 12-5 C. d = U5 ' 88 J U5 ' 88 



145-88 - n 135-88 + n 



At 15 C. d = U6 ' 3 d = K6 ' 3 



14:6-3 - n 136-3 



At 17-5 C. d = U6 ' 78 d = U6 ' 78 - 



146-78 - n 136-78 + n 



Other hydrometers are employed for special purposes. The prin- 

 cipal ones in use are given in the following table : 



1 Most of the modern Baume's instruments are so constructed that in water 

 they read at 15 C. and in sulphuric acid of specific gravity 1-8427 they read 66. 



