SCIENCE. 



119 



THE REDUCTION OF CHLORIDE ORES. 



For the benefit of those not familiar with the processes of 

 reducing gold and silver ores, a brief explanation of what 

 is meant by " free milling," an expression so often used by 

 mining men, may not be out of place. In separating, by 

 amalgamation, the precious metals from gangue or waste 

 rock with which they are almost always associated, it is 

 necessary to the success of the process to present the parti- 

 cles of gold or silver contained in the ore to the mercury 

 with which they are to be alloyed, in such form that the 

 latter can seize upon them readily. If these metals always 

 occurred in nature in their pure metallic state, this would 

 be a very easy matter. In free milling gold ores it is fre- 

 quently only necessary to place the quicksilver beneath the 

 stamps of the battery in which the ore is crushed, and upon 

 an inclined copper plate over which the pulp is carried by 

 water after it leaves the battery. The stamps, by reducing 

 the rock to fine particles, release the minute scales and 

 crystals of gold, which are readily taken up by the quick- 

 silver, while the rock, for which the mercury has no affinity, 

 is carried away as " tailings." 



But silver rarely occurs in a native or pure metallic state. 

 It is usually mixed with clorine, lead, iron, sulphur, man- 

 ganese, copper, antimony and other base metals, and is 

 found in the form of chloride of silver, argentiferous galena, 

 in which the silver is in the form of a sulphide, and in 

 many other compounds, for most of which quicksilver has 

 no more affinity than it has for the common rock of the gan- 

 gue. In most cases, therefore, if the silver ore was simply 

 crushed and brought into contact with an amalgamating 

 surface, little or none of the metal would be caught by the 

 quicksilver and saved. Mercury has a strong affinity for 

 metallic silver, stronger even than that of chloride, so that 

 ff chloride of silver and quicksilver are brought together 

 the mercury will seize the silver, forming an amalgam, and 

 the chlorine which is released will escape as gas or unite 

 with some other substance which presents itself and for 

 which it has an affinity ; but sulphur will not give up sil- 

 ver, with which it is chemically mixed, to mercury, unless 

 the sulphur has first been driven off by fire. This process 

 of converting chloride of silver into an amalgam is not an 

 instantaneous one like the amalgamation of free gold, 

 but requires several hours to be perfected, and it is 

 hastened by the presence of other chemicals, such as sul- 

 phate of copper, sulphuric acid, and cyanide of potassium, 

 the action of which it is unnecessary to explain here. 



In order to reduce silver ores by amalgamation, it is 

 necessary, as will be understood from the above explana- 

 tion, to have the particles of metal either in a pure or 

 chloritic state. When they are found in nature in either of 

 these conditions they need no special treatment before being 

 put into the mill, and the treatment of them is called "raw 

 amalgamation." The process employed is to crush the ore 

 to a fine pulp, and then transfer it to a large round iron tub, 

 where it is agitated for several hours in hot water with 

 quicksilver, some or all the chemicals I have named being 

 added with common salt to promote the union of the mer- 

 cury and the silver. If the silver in the ore is in the form 

 of a sulphide, as it frequently is, and the amalgamation pro- 

 cess of reduction is to be employed, the ores have to be 

 roasted with common salt for several hours after they are 

 crushed. Without explaining in full the chemical reactions, 

 I may simply say that the heat volatilizes the sulphur mixed 

 with the silver, and separates the salt into its constituents 

 of chlorine and sodium, the first of which unites with the 

 silver from which the sulphur has been driven off, and forms 

 a chloride which is then ready for the amalgamating pan. 

 The desulphurization and chlorination of an ore is an ex- 

 pensive process, and greatly increases the cost of reduction. 



When such metals as lead, zinc, or copper are present in 

 ores in large quantities, it is usually cheaper to reduce them 

 by smelting, and by that process the lead and copper are 

 generally saved and add to the value of the product. 

 Almost any ore can be reduced by fire, if it is mixed in 

 small proportions with ether smelting ores. In large smelt- 

 ing establishments like those at Denver, Omaha, and 

 Newark, N. J., where great varieties of ores are purchased, 

 even free milling rock can be used to advantage ; but the 

 reduction of most free milling ores by fire, without mixture 

 with others, would be ruinously expensive if not physically 

 mpracticable. 



ON CURRENTS PRODUCED BY FRICTION BE- 

 TWEEN CONDUCTING SUBSTANCES AND ON 

 A NEW FORM OF TELEPHONE RECEIVER.* 



In a communication to the Royal Society of Edinburgh 

 of date January 6, 1879, I showed that "electric currents 

 were produced by the mere friction between conducting 

 substances." The existence of these currents can be easily 

 demonstrated either by a telephone or a Thomson's galvano- 

 meter. I have since found that these currents are, for all 

 pairs of metals which I have yet tried, in the same direction 

 as the thermo-electric current got by heating the junction 

 of the same two metals. They are also approximately at 

 least, stronger in proportion as the metals rubbed are far 

 apart on the thermo-electric scale — the strongest current, as 

 far as I have yet observed, being got by rubbing antimony 

 and bismuth together. These observations clearly point to 

 a thermo-electric origin for the currents ; but it is possible 

 that they may be due partly to the currents suggested by 

 Sir William Thomson as the cause of friction, and partly, 

 also, to contact force between films of air or oxide adhering 

 to the surfaces of the metals. 



Having ascertained that these friction-currents are of 

 some strength and fairly constant, I proceeded to make 

 several kinds of machines for producing currents on this 

 principle. One of them consists of a cylinder of antimony, 

 which can be rotated rapidly, while a plate of bismuth is 

 pressed hard against it by a stiff spring. When this ma- 

 chine is included in the same circuit with a microphone and 

 a Bell telephone, the current got from it is quite sufficient 

 to serve for the transmission of musical sounds and also 

 loud speaking. The transmitter, which I have found most 

 serviceable in my experiments, is made by screwing two 

 small cubes of gas-carbon to a violin, and placing between 

 them a long stick of carbon pointed at both ends, the points 

 being made to rest in conical holes in the carbon cubes. 

 The looseness of the contact is regulated by a paper spring. 

 This forms an excellent and handy transmitter for all kinds 

 of musical sounds, and also serves very well for transmit- 

 ting speech. 



Seeing that friction between metals clearly produces a 

 current, it seemed natural to inquire if the converse held 

 good, that is, if a current from a battery sent across the 

 junction of two metals affected the friction of the one upon 

 the other. I have tested for this in a variety of ways, and 

 the results obtained leave me in doubt whether to attribute 

 them to variations in the friction, or to actual sticking por- 

 duced by fusion of the points of contact through which the 

 current passes. The most noticable effect is produced when 

 one of the rubbing bodies is a mere point, and the other a 

 smooth surface of metal. This led me to make a modifica- 

 tion of the loud speaking telephone of Mr. Edison, in 

 order to get audible indications of changes of friction pro- 

 duced by the passing of a variable current. It consists of 

 a cylinder of bismuth accurately turned and revolving on 

 centres. The rubber-point is made of a sewing-needle with 

 its point bent at right angles, and its other end attached to 

 the centre of the mica disk of a phonograph mouthpiece. 

 It is evident that this is only a loose contact, which can be 

 perpetually changed. When this apparatus is included in 

 the circuit with the violin-microphone and three or four 

 Bunsen cells, the violin sounds, as was to be expected, are 

 heard proceeding from the loose contact, even when the 

 cylinder is not rotated. They are increased, however, in a 

 remarkable degree by rotating the cylinder slowly, so much 

 so that a tune played on the violin can, with proper care, 

 be distinctly heard all over an ordinary room. 



With regard to the explanation of this effect, it is evident 

 that electrolysis can in no sense come into play, as is sup- 

 posed to be the case in Edison's instrument. I am inclined 

 to look for the explanation rather in the direction of the 

 Trevelyan rocker, although the circumstances are consid- 

 erably different in the two cases. In the rocker we have the 

 heat passing from a mass of hot metal through two points 

 of support to a cold block, whereas, in the other case, the 

 heat is only intense at the points of contact, the rest of the 

 metals being comparatively unaffected. The variations n 

 the current produced by the transmitting microphone must 



* Abstract of a paper read before the Royal Society of Edinburgh by 

 James Blyth, M. A., F.R.S.E.,on May 3. 1880. 



