GENERAL MEETING. 27 
hand, while hydrometers may be steadied so that the instrument 
may be read with a reasonable degree of precision, as is shown in 
its constant use in the determination of the degree of saturation of 
the water in the steam-boiler, and in other instances. 
To use the hydrometer for the determination’ of the specific 
gravities of solids I take advantage of the fact that, when a body 
floats in a liquid in which it is wholly immersed, the specific gravi- 
ties of the liquid and the solid are the same, and we have simply to 
determine the value for one of them. 
The process is carried out by taking a dense solution, dropping 
in it the solid to be determined, (which must be light enough to 
float on the surface,) and then diluting slowly with water until the 
solid floats immersed, stirring the mixture constantly. The solid 
is now removed and the hydrometer inserted and read. For the 
determination of the specific gravities of the bituminous coals and 
lignites a thick solution of cane sugar was used, while for the 
heavier anthracite concentrated sulphuric acid, diluted with dilute 
sulphuric acid, was employed. The increase in temperature in the 
latter case causes no appreciable error if the reading is quickly 
taken. The following results were obtained by the method des- 
cribed, the specific gravity of each specimen having first been de- 
termined by Jolly’s balance: 
By Jolly’s balance. By mixture. 
PTI LOLA CLE Mots sure eee eS 1.5640 1,560 
Bituminous ‘coal’ 22 bs 2 pat eureh 1,3008 1,310 
Bituminous) coals sxe) (yee woes 153000 1,300 
(AIS (CIDE ne SIS oa AE ne 1,2790 1,285 
Canney coala@ipniform) 222-2 ==. 22 b 1,1550 1,155 
Cannelicoali acest eee ie el a 1,1292 1,120 
VRSp SES) Leela — 1,0909 1,090 
Mr. Durron remarked that the same principle had recently been 
successfully applied to the separation of the component minerals of 
crystalline rocks. A sample is powdered and then placed in a very 
heavy liquid (a solution of mercuric iodide and potassium iodide), 
the density of which is gradually diminished, until the particles of 
the heaviest mineral sink to the bottom. A repetition of the process 
eliminates each mineral in turn. 
