On Crystallography, 347 



I( is in this Way that the tables of specific gravities pub- 

 lished bv several authors have been made up. OF these the 

 tabic of M. Brisson is the amplest and the most correct. 



We employ in these experiments the hydrostatic balance. 

 The body to be examined is suspended by means of a horse- 

 hair to a small hook under one of the scales, which pro- 

 cures the facility of plunging this body into water in order 

 to weigh it. 



Nicholson has suggested in these experiments the use of 

 an areometer of tinned iron, represented in fig. 75, and the 

 static of which, B, is a brass wire, which has at its ex- 

 tremity a small cistern A. This stalk is marked in the 

 middle by a line h made with a tile. The lower part keeps 

 suspended an inverted cone EG, concave at its base, and 

 balanced within by a piece of lead*. The weight of the 

 instrument ought to be such that, when we plunge it in 

 water and leave it to itself, a part of the tube floats above. 

 The cistern at the top of the stalk, and which has the form 

 of a spherical shell, is fixed to it by means <jf a small tube 

 of tinned iron into which this stalk enters. Generally 

 tliere is a second cistern somewhat larger, which is placed 

 above the first, into the concave part, of which it is fitfed, 

 in consequence of its convexity. We may thus move 

 this second cistern, either to remove more easily the weights 

 with which it is charged, or to make some change in their 

 arrangement. This instrument, which is very cheap, and 

 easily carried from place to place; is extremely useful to 

 mineralogists. An example will best explain the method 

 of using it. _ , 



If any doubts are entertained whether a transparent 

 stone of a blue colour belongs to the stony substance 

 commonly called oriental sapphire, or to the variety of 

 quartz which is called water sapphire : — Take distilled wa- 

 ter at a given temperature ; Birisson has adopted that of 

 14° of Reaumur, which answers to 17*3° of the centigrade 

 thermometer, as the medium in our climate. Having 

 plunged the areometer in this water, charge the upper 

 cistern A, until the scratch b marked on the stalk descends 

 to the level of the water. This is called levelling the 



♦ In several areometers this cotie has a fixed position, by means of brass 

 wires which ktep it atti^cheti to the instrument- But M. Gillet" with good 

 reason prefers giving it its liberty, by Euspending it with a hook, as is re- 

 presented in the figure. In this way the axis of the instrument always 

 takes a vertical direction, otherwise »t would lean more to one side thaa 

 to the other, when the initrument would be iu ctiuilibrium round its centre 

 <»f oscillation. 



areometer. 



