A MEXICAN ARASTRA.
"The patio process was invented in 1557 by Bartolomé de Medina, and is so called because a patio, or yard, is required for its operation. The ore is crushed and ground fine in arastras. An arastra is a mill where an animal, generally a mule, walks in a circle and turns a millstone that rolls upon a floor, on which the material to be ground is placed. We have seen arastras at work several times since we came into Mexico; and they are not unknown in the south-western part of the United States.
"If there is any gold in the ore, fifty or sixty per cent. of it may be saved by putting silver or copper amalgam into the arastras. Some of the Mexican ores must be roasted to remove certain chemicals which they contain, but this is not the case with all of them. The paste from the arastras is spread in heaps on the floor of the patio; after it has hardened somewhat by the evaporation of a part of the water it contains, it receives a quantity of salt, which is in proportion to the amount of silver in the ore. Then it is mixed by men with shovels and by the tread of horses or mules, and a day or two later a mixture of copper vitriol and salt is added.
"Then follows more treading and mixing; then quicksilver is spread over the mass and trodden in, and the next day there is another mixing and treading. These performances are repeated on alternate days, quicksilver being added one day and the mass being trodden the next, until the treading has been repeated seven or eight times. The quicksilver unites with the silver and forms an amalgam; the formation is carefully watched, and when it has reached the proper condition the amalgam is gathered up into hide or canvas bags. Some of the quicksilver is squeezed out, and the rest is driven off by evaporation and condensed in a pipe that runs into a tub of water."
"There's a good deal more," said Fred, "but I'm afraid if we say too much about the process we shall lead our young friends at home to skip the whole story. So we've made it short."
"You've said quite enough," replied the Doctor, "to give a general idea of what the patio process is. Anybody who wants to know more can look it up in books on mining, or in cyclopædias."
CARRYING ORE TO THE REDUCTION-WORKS.