16 



measurement with the microscope. As I have not access to the 

 paper on the subject his manner of preparing the prills for 

 measurement is unknown to me. Harkort and Plattner in 

 making scales for the determination of the weight of gold and 

 silver prills weighed prills corresponding to the larger divisions 

 of the scale, and from their weight calculated the weight for 

 the smaller divisions. These prills were taken direct from the 

 cupel, and at the point of contact are flattened, but as the 

 amount of flattening is not always the same and hardly varies 

 in extent with the size of the prill, and as the converging 'lines 

 on the scale cannot be very sharply defined, this method is not 

 capable of the same accuracy as where the almost perfect 

 spheres are measured with the microscope. 



No other flux seems to possess advantages equal to those of 

 boracic acid for obtaining a sphere of gold. Borax and other 

 fluxes are so fluid when hot that the gold is very liable to alloy 

 with the platinum wire ; this rarely occurs with boracic acid, 

 on account of its great viscosity, even when white hot. Boracic 

 acid is also easily soluble in water, so that the gold spheres 

 can be separated without loss of time. 



The following rules and figures may be useful to anyone 

 wishing to adopt the system here described : — 



1. The weight of a sphere increases as the cube of the 

 diameter. 



2. The weight of a sphere of any substance of which the 

 specific gravity is known is obtained by multiplying the weight 

 of a unit sphere of water by the specific gravity of the sub- 

 stance and the cube of the diameter. 



Constants for use with gramme weights — 

 1 weight of a sphere of water 001 mm. in diameter= 

 0-0000000005236 of a gramme. 



2. "Weight of a sphere of gold 0*01 mm. in diameter= 



0-0000000102102 of a gramme. 



3. "Weight of a sphere of gold 00x mm. in diameter, x 3 x 



0-0000000102102 of a gramme. 



4. If 20 grammes of ore are taken for assay the number of 



grains of gold per ton is found by x 3 x 0-008004 in which 

 x=the diameter of the sphere of gold in hundredths of a 

 millimetre. 

 Constants for use with grain weights — 



1. "Weight of a sphere of water 0*001 inch in diameter= 



00000001324 of a grain. 



2. "Weight of a sphere of gold 0-001 inch in diameter= 



0-000002582 of a grain. 



3. "Weight of a sphere of gold 0-OOx inch in diameter, x 3 x 



0000002582 of a grain. 



4. Two hundred grains of ore being taken for assay the number 



