The Patio and Cazo Process. 11 



had. A wheel of this kind with a diameter of six meters will 

 carry two mullers for 24 hours without stopping, as fast as four 

 mules will, that cannot work for more than eight hours a day. * 

 At Chihuahua such a wheel runs both the arrastra and the 

 stamps. When overshot water-wheels are used, the power is 

 transmitted by spur gearing on the upper part of the central 

 shaft. 



In some few cases an overshot water-wheel is used to run a 

 number of arrastras. The power is transmitted by wooden gear- 

 ings. When the arrastra is new, or when a new bottom has 

 been put in, reoajado, it is turned either empty or with a few 

 cargoes of tailings, jales, or low grade ores, tierras cle labor, so 

 as to make the stones even and fill up the cracks — if the stones 

 have been simply laid together — with material of but little value. 



A good deal of importance is attached to the use of the pro- 

 per quantity of water, and to the times as well as the way in 

 which it is added. When a new bottom has been put in, one 

 muller is attached to the arm, and it is set to work grinding up 

 with water the residues of the washing of a torta, to smooth down 

 the pavement and to fill up any cracks. This is continued for 

 one day. The next day another muller is attached; the third day 

 another. On the fourth day poor ores are charged; at the end of 

 four or five days, the fourth muller is attached, and the usual 

 work is then commenced. From one-half to two-thirds of the 

 total quantity of ore to be treated is added at first. If there is any 

 free gold or silver in the ore, a little mercury is added at the 

 start in order to catch it. The quantity of gold contained in 

 most Mexican ores is so small, that if it was not separated in 

 some w r ay in the treatment, it would be absorbed in the silver, 

 and its separation by a parting process would hardly pay, so 

 that it would be lost ; but by adding mercury, especially that 

 w r hich has already been through the arrastra, much of it is col- 

 lected. When the ores contain a very considerable quantity of 

 native gold or silver, it is desirable to collect as much as possible 

 with mercury in the arrastra ; and if no other minerals are 

 associated with it, the whole or the greater part of the treat- 

 ment, as at Chihuahua, is comprised in its treatment here. 



^Report of the U. S. Mining Commission, 1872, p. 436. 



