CATALOGTTE OF GEMS. 



485 



and be scratched b}" the next nuin])er of the scido idxn'e it, its hardness 

 is somewhere })etween the two units. 



The property of hardness is an essential in gem stones, since upon 

 it depend its capabilities of receiving- and retaining a high degree 

 of polish and of resisting wear from abrasion. It is also often an 

 available method of identifying a gem. In testing cut stones, however, 

 care nuist be taken not to disfigure them, and, if possible, the girdle, 

 or the part hid by the mount, should be used as the test surface. 

 Furthermore, the term "hardness" must not be confused with tough- 

 ness or difficulty of breakage. A very hard stone may be a very 

 brittle or fragile one. Hardness means simply 

 liability or nonliabilit}^ to scratch. 



Specific (jVimHy. — This is the density of a l)ody 

 compared with that of distilled water at a cer- 

 tain temperature (commonly at 60° F.). 



The determination of specilic gravity is in 

 principle very simple. The sul)stance is at first 

 weighed in air, then in water; divide the weight 

 in air by the loss of weight in water, and the 

 quotient is the specific gravit}'. Thus if a gem 

 w^eighs 5 grams in air and only 3 in water, it is 

 evident that it has displaced 2 grams of water, 

 and its specific gravity is 2.5. 



In order to get absolute results, very delicate 

 balances have to be used and man}' precautions 

 taken, but for ordinary work such care is not 

 necessary. 



A very convenient specific-gravity apparatus 

 that is sufficientl}^ close for ordinary use is the 

 Nicholson hydrometer (fig. 3). This consists of 

 a float having a descending hook, to which is 

 hung a pan to hold the substance weighed in 

 water. A wire stem supports a cup on which the weights and the 

 substance weighed in air are placed. A mark on the stem shows the 

 point at which the whole apparatus will float in a vessel of water when 

 a certain known weight (called the balance weight) is placed in the 

 weight cup. The specimen under examination must not exceed in 

 weight that of the balance weight, this being the limit of the appara- 

 tus. Suppose the limit to be 100 grams. Then to find the specific 

 gravity of a substance, place it on the weight pan and add weights 

 until the instrument sinks to the mark. The difference between the 

 added weight and 100 is the weight of the specimen in air. Then 

 place the specimen in the lower pan; as much more weight on the 

 weight pan will now be required as corresponds to the weight of a 

 bulk of water equal to that of the specimen. The difference of weight 



Fig. 3. 



NICHOLSON IIYDRO.METEK. 



