16 GENERAL CHARACTERS OF PRECIOUS STONES 



The use of the counterpoise, as described above, has the effect of neutralising any error 

 in the zero point of the balance. Unless special accuracy is required the counterpoise may 

 be dispensed with, especially when a good balance with the zero accurately adjusted is 

 available. The stone is then weighed in the usual way by placing it in one scale-pan and 

 the necessary weights in the other. Let its weight thus determined be p, and its weight 

 when immersed in water be q ; then the loss of weight will he p — q, and the specific 



gravity is fl! = — £- — . 



^ ^ p-q 



In the above determination two weighings only are necessary, whereas, when a counter- 

 poise is used, three must be made. When, however, a series of determinations are made with 

 one counterpoise two weighings suffice, and either method is equally expeditious. For the 

 purpose of determining a stone or distinguishing it from another, the value of its specific 

 gravity, obtained without the use of a counterpoise, is generally sufficiently accurate. The 

 following is a numerical example of this method. A garnet (cinnamon-stone) weighed in 

 air 4"375 grams (p), and in water, 3'168 grams {q) ; the loss in weight is therefore 

 p — q = 4'375 — 3"168 = 1'207 grams. Hence the specific- gravity is 



p - q~ 1-207 ~ '^^'^■ 



Since determinations of specific gravity are not, under ordinary conditions, made with 

 water at the temperature of 4° C, it is necessary, when accuracy is required, to reduce the 

 i-esults of calculation to this standard. For the practical purpose of the jeweller, however, 

 the direct observation is quite sufficient. 



3. Method with WestphaVs Balance. — While methods 1 and 2, described above, are 

 susceptible, as has been shown, of extreme accuracy, they have a disadvantage in the 

 amount of time which must be expended on careful weighing. The method now under 

 consideration combines a degree of accuracy sufficient for all practical purposes with a 

 considerable economy in time ; it is therefore valuable to the practical jeweller. The 

 balance used in this method is named after its maker, the mechanician Westphal, of Celle, 

 in Hanover. It enables a specific gravity to be readily and quickly determined, correct to 

 the second place of decimals, or under favourable conditions to the third place. It has the 

 advantage of cheapness over the hydrostatic balance, and, moreover, can be used for other 

 purposes, as will be shown later. 



Westphal's balance with accessories for determining the specific gravity of solids, and 

 therefore of precious stones, is illustrated in Fig. 5. In principle it is really a simplification 

 of the hydrostatic balance with a counterpoise, the left-hand scale-pan and counterpoise of 

 the latter being replaced in Westphal's balance by a counterpoise fixed to the beam. It 

 consists of a beam, abc, to which is fixed a knife-edge of hardened steel at h. This knife- 

 edge, which is directed downwards, is suppoiled by, and turns on, a grooved steel plate 

 fixed to the curved brass piece de, itself supported by the brass column *'. The latter 

 slides in the tube hh, and can be fixed at any convenient height by means of the screw g. 

 Passing through the disc I, attached to the foot A;, of the instrument, is a levelling screw m,. 

 directly beneath the end of the balance beam; by means of this the column yean be 

 adjusted so as to be accurately vertical. 



The beam carries at one end (the left in Fig. 5), a heavy weight of brass, which takes 

 the place of the counterpoise and left-hand scale-pan in the ordinary hydrostatic balance. At 

 the same end, a, of the beam is a pointer which, when the beam is horizontal, indicates zero 

 on the scale attached to the piece de. At the opposite end, c, of the beam (on the right in 

 Fig. 5) is fixed a knife-edge with its edge directed upwards. On this rests a hook, from 



