690 POPULAR SCIENCE MONTHLY. 



consisted of a small stone mortar with a stone pestle. But there was 

 a development in milling methods, and the next consisted of a block 

 of rock, with a slight depression in the top for the ore, which was 

 pulverized by means of a heavy rubbing stone. A still later develop- 

 ment, possibly belonging to the time of the Ptolemies, 330-322 B.C., 

 was a mill somewhat like the arastra which may still be seen in Cali- 

 fornia and Mexico. It consisted of a block of rock several feet in 

 diameter and circular in outline, in the top of which was a shallow 

 depression to receive the ore. The pestle consisted of a large spherical 

 or cylindrical block of the same kind of rock. The finely broken ore 

 was placed in the depression, and the movable block was rolled around 

 over it until it was reduced to the desired fineness. The mill was 

 essentially a huge shallow stone mortar in which the pestle was rolled 

 by slaves. 



The powdered ore from the mortars was sprinkled upon the tops 

 of sloping stone tables over which water was kept, running. By this 

 means the lighter rock powder was carried away and the gold remained 

 behind. The supply of water was secured by the construction of large 

 cisterns or reservoirs on the slopes of the mountain. In the upper ends 

 of the tables there were tanks for the immediate supply, and the 

 operator dipped the water out and poured it over the sloping surface. 

 When a quantity of gold had accumulated on the table it was removed, 

 dried and placed in a covered clay crucible and heated for five days 

 in a charcoal fire blown constantly by a rude bellows, or by mouth 

 blowpipes. No fluxes were used in the earliest times, but it was not 

 long before these primitive metallurgists learnt the use of lead and 

 common desert salts in refining their gold. Diodorus Siculus — second 

 century B.C. — mentions the use of lead by the Egyptians in the refining 

 of gold, but it is believed that it had been in use many centuries before 

 his time. 



Pictures and sculptures of very early date show the refiners of gold 

 (and silver) sitting before the fire with blowpipes of very large size, 

 but otherwise not unlike those in use at the present time. The main 

 part of the pipe was probably of wood, but a metal tip concentrated and 

 directed the current of air. Others show the workman blowing the 

 fire with a bellows consisting of two leather bags furnished with metal 

 nozzles. The bellows was distended by pulling a cord attached to the 

 upper side of the bag. When at work, the operator stood with one foot 

 on each bag and held the cords in his hands. Then by a swaying 

 movement he threw his weight first on one foot and then on the other 

 and at the same time pulled the cord attached to the bag from which 

 the pressure was removed, raising its upper wall and causing it to take 

 in air and become distended. The pressure of the foot forced the air 

 through the nozzle of the bellows into the fire. When a large amount 

 of metal was to be worked, an open fire of charcoal was used and a 



