492 SCIENCE PROGRESS. 



and ground in mills with mercury. In addition the ore was 

 frequently passed over or through baths of mercury (as is 

 still done in many Australian mills) and concentrated 

 in various ways. Amalgamated copper plates over which 

 the pulp flowed were tried, but in Western America were 

 at first almost everywhere rejected, 1 probably owing to the 

 great depth of the stream of ore and water made to flow 

 over them. When this mistake began to be rectified some 

 twenty-five years ago the value of the plates was soon 

 recognised in California. 



In the extraction of gold from crushed ore by means of 

 amalgamated copper plates, the pulp- is led over their 

 surface in a very thin stream, not more than a quarter of 

 an inch deep. The plates are slightly inclined, wider than 

 the screens from which the pulp issues, and from six to 

 twelve feet long. The pulp does not run down in a regular 

 stream, but in a series of little wavelets which tumble over 

 and over and are supposed to bring every part of the pulp 

 in succession in contact with the amalgamated surface. The 

 catching powers of the plates are thus supposed to be prac- 

 tically independent of the tendency of the particles of gold 

 or amalgam to sink to the bottom of the stream. This 

 theory is not accepted by the Austrian school, and it is 

 certain that native gold is caught more easily in proportion 

 as it contains less silver, so that when the particles of metal 

 consist of an alloy largely consisting of silver, and are 

 therefore of comparatively low density, the yield on the 

 plates is generally poor. In any case, however, the 

 amalgamated plate should theoretically be better adapted 

 for its work than the Tyrolean mill and other machines 

 using mercury baths, owing to the slight depth of the 

 pulp on the plates and the short distance through which 

 the gold particles are compelled to settle before reaching 

 a catching surface. The plates are wiped down with 

 rubber or brushes about once a day and the gold sepa- 

 rated in the usual way from the excess amalgam thus 

 collected. 



1 See Nevada and California Processes of Silver and Gold Extraction, 

 p. 6 1, by G. Kiistel, San Francisco, 1863. 



