APPENDIX 



» 



Specific Gravities. — The most rational method of expressing 

 the specific gravity of a solid or liquid is in terms of water — 

 i.e., by a number which expresses the ratio of the weight of 

 any volume of the solid or liquid to that of an equal volume 

 of water at a specified temperature. 



This plan is always adopted for solids, but for liquids for 

 technical purposes various empiric scales are employed. 



In England Twaddle's hydrometers are often used for 

 liquids heavier than water. 



These are so constructed that the relationship between true 

 specific gravity and degrees Twaddle is 



-^+ 100 

 . . d= \^^ ,orn = 200(d-l), 



where d = true specific gravity and n = degrees Twaddle. 



The determinations are assumed to be made at 15" 5° C. 

 (60° R). 



Other hydrometers, based upon purely arbitrary and empiric 

 systems, are also in use in various branches of industry, and it 

 is to be regretted that they do not give way to a more rational 

 method of expression of density. 



Thus Baume's hydrometer, for liquids heavier than water, 

 is so constructed that it sinks to 0° in pure water and to 10° 

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

 the scale is continued in a uniform manner down the stem. 



285 



