chemistry in mining - ACS Publications


chemistry in mining - ACS Publicationspubs.acs.org/doi/pdf/10.1021/ed008p1523by MW Deming - ‎1931We ordinarily think o...

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CHEMISTRY IN MINING MERYL W. DEMINO, UNIVERSITY OF NEVADA, RENO,NEVADA

Chemistry i s intimately related to mining. Some of its incidental applications, such as treatment of water and timber, fire-fighting, lighting, etc., are described and the r8le pluyed by chemistry is pointed out.

. . . . . . We ordinarily think of chemistry in mining as concerned solely with the treatment of the ores. This highly important branch of mining called metallurgy has been discussed and fully treated in many books and articles. However, the incidental applications of chemistry to the winning of ores are scattered throughout the literature. It was thought that a resume of the latter would perhaps be of interest to students of chemistry and mining. Deposition of Ores The majority of ores were originally deposited as sulfides (e. g., FeSz), arsenides (e. g., CoAsz), or complex sulfarsenides (e. E., Cu3AsS4= 3Cu& AS&) and sulfantimonides (e. g., CusSb& = 4Cu S,SbzSa). These are known as primary ores. If these ores lie between the surface and the ground-water level they are subject to the oxidizing and dissolving effect of rain water and the oxygen and carbon dioxide contained in it. As a result they are taken into solution as sulfates, chlorides, bicarbonates, etc., and deposited in the lower levels as sulfides, oxides, carbonates, etc. Such deposits are known as secondary ores. Secondary ores of iron, the oxides and carbonates, are much more important than the sulfide. With copper especially, the distinction between the two types of ore is very important. The sulfide a t the suriace is easily oxidized to the sulfate, which is again reduced by the lower sulfides to form rich ore bodies a t the lower water levels. Such secondary enrichment produces the rich deposits of cuprite (CuzO) and native copper. Similarly, galena (PbS) and zincblend (ZnS) are the primary ores, while cerussite (PbCOa), anglesite (PbSOJ, smithsouite (ZnC08), and calamine (2Zn0,H20.SiOz)are secondary ores. Action of Aii and Water on the Ore Body As soon as the ore body is opened up by mining and exposed to air and moisture, rapid changes take place. If silicate minerals are present, these reactions often cause a sharp change in volume and resultant swelling of the ground and increased mining difficulties and costs. In mining sulfide ores much heat is generated by oxidation and (especially in abandoned workings) by friction due to caving in of large sulfide bodies. Serious fires are often started in this manner and burn for years. A manager of a large copper company states that ninety per cent of their fires are due to such spontaneous combustion. 1523

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If the sulfide ore is exposed too long before treatment it becomes coated with a layer of the oxide. As the flotation process is particularly adapted to sulfides this prevents a high recovery of ore.

Water Underground waters in contact with air and broken ore become mineralized very rapidly. In copper mines, especially, the water soon attains quantities of copper sulfate and sulfuric acid. The presence of the former is very destructive to iron equipment because of the solution of the iron and precipitation of the copper. Miners' shoenails, mine rails, pumps and pipe lines are soon destroyed. The acid water (an average of sixteen different mine waters gave four hundred ninety-two parts per million of sulfuric acid) hastens the rusting of all iron objects. Rusting may be considered from the electrochemical point of view as the action of an iron-hydrogen cell, i. e., Fe, Fe+f, H+, Hz. The iron passes into solution as the ferrous salt and hydrogen ion is discharged. It is evident that a high concentration of the latter would hasten the action of the cell. The simplest method to combat acidity is the neutralization of the water with lime. If this is not feasible pumps should be lined with lead, wood, or porcelain, and fitted with bronze plungers. Lead linings are difficultto apply, wear rapidly, and loosen easily. A wood lining, if carefully fitted and swelled with pure water is best, but because of its thickness requires larger and more expensive castings. Cement linings are particularly useful in high concentrations of sulfuric acid (i. e., seventy-five grains per gallon). Column pipes in permanent mine lines should be of a resistant steel alloy or lined with lead or wood. If ores are being leached with acid, all pipesmust be lined with lead. In some cases rusting is reduced by inserting a carbon anode, using the iron as the cathode and connecting a battery. This current tends to plate out iron and reverse the action of the rust cell. If sulfates and chlorides are absent, the iron is often pickled and rendered passive to oxidation. This process consists in cleaning and polishing the metal, followed by dipping in a solution of an oxidizing agent such as nitric acid, sodium dichromate, etc. This treatment renders the iron resistant to ordinary corroding agents. Often the oxidizing agent is incorporated in a paint which is applied to the iron. However, the passivity is not permanent and if the equipment is used underground the protection is soon removed by scratching and the presence of sulfates or chlorides in the water. Another method to decrease the action of the rust cell is the casting of zinc on the iron. Zinc, being more reactive than iron, tends to go into solution forcing the iron to plate out. The result is a retardation of the rusting.

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In a limestone region there is the usual bother of calcium carbonate deposited in the pipes. The combined action of water and the carbon dioxide dissolved in it take the limestone into solution as soluble calcium bicarbonate. The action is easily reversed and carbon dioxide is given off, precipitating the calcium carbonate. To remove this temporary hardness of water lime may be added to neutralize the bicarbonate, forming the carbonate, or the water may be heated, driving off the carbon dioxide and precipitating the carbonate. In hot mines, the water upon cooling may deposit iron oxide in the pipes. This heat may result from the depth of the mine (an average of thirty-five mines gave an increase of 1°F. per 64 feet depth), or from the oxidation of the sulfide ores (water a t 92'F. flowed from an abandoned stope while water from other parts of the mine was a t 6OoF.) Occasionally it is desirable to trace the course of underground water. Fluorescein (CS~Hl2O5), a weak acid, is added and imparts a yellowish brown color which a t a concentration of 1 to 40,000,000 can be detected by the eye. It is neither absorbed by clays nor affected by hydrogen sulfide or sulfurous acid. Other coloring materials used are eosine, fuchsine, and metbylene blue. . Timber For support of mines timber is usually the cheapest and most available material. However, as decay soon sets in and greatly shortens the life of the wood, chemical treatment is resorted to. The standard wood preservative is coal-tar creosote--the fraction between the light oils and pitch being used. The treatment often includes heating under pressure to increase the penetration of the creosote. When first treated the lighter oils are a fire hazard but upon standing they soon evaporate and the wood is no more combustible than untreated timber. Zinc chloride is the standard water-soluble preservative. It is cheap, clean, has no odor, and affords no fire hazard. It is, however, soluble in water and thus easily leached out, besides adding to the weight of the wood. Sodium fluoride and mercuric chloride are used to some extent but are poisonous and more expensive. In some copper mines, the timbers are soaked in the mine water containing copper sulfate. This affords some protection but any iron or steel in contact with i t will be attacked. Gunite, a mixture of portland cement and sand, is often applied under pressure to timbers. It has a high density, is very impervious and is not cracked by hot fires. This treatment, combined with a preservative, is the ideal preparation for lumber to be used in mine work. Fire-resistant paint containing thirty-three per cent magnesium silicate finds some use. Ordinary whitewash is also fire-resistant.

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Lighting Candles were once widely used in mining. Their light is flickering and easily put out by the exhaust from air drills. Although the initial cost is low, they are more expensive per unit of light than electricity or acetylene. The men tend to waste them by throwing away those half used. Furthermore they consume as much oxygen as a man, besides giving off noxious gases and causing explosions in gassy mines. For these reasons they have been replaced by safety lamps and acetylene lamps. The Davy safety lamp was put into service in 1816 in England. Its fundamental part is the gauze which dissipates the heat of the flame so that the impinging gases (e. g., methane) are not heated to explosion temperature. I t has the advantages that it can he used in a mine atmosphere containing an explosive mixture and, as it will not hum in an atmosphere greatly deficient in oxygen, i t warns the men of this danger. Many states require mine bosses to carry one of these lamps. The acetylene lamp is easily the best as regards economy, ease of handling, and amount of light. The acetylene is generated by the action of water on calcium carbide, CaCz 2Hz0 = CzHz Ca(OH)I. The carbide is manufactured in large electric furnaces by the action of lime on coke, CaO 3C = CaCl CO. A lamp the size of an ordinary fruit jar bums for 8-10 hours, the cap size for 2-3 hours. I t consumes only one-fifth the oxygen used by a candle and costs approximately 2 cents per 10-hour shift. Both candle and acetylene lamp have the drawback that they will burn in an atmosphere containing sufficient carbon monoxide to be fatal. If the carbide is not pure small quantities of hydrogen sulfide and phosphine may be generated. The latter is spontaneously inflammable. Most carbide on the market today is manufactured from purified materials, removing this objection. Care must he taken that men do not throw supposedly spent carbide into wet places where any unspent will generate acetylene, forming an explosive mixture with the air. The relighting of lamps that are filled and sealed before being given to the men is accomplished by built-in igniters. Drops of phosphorus may he placed on a strip coated with paraffin. By drawing this tape over a saw-toothed scratcher the phosphorus is ignited. Drops of fulminate on a strip may be ignited by the blow of a small hammer. The safest and surest consists of a small covered vessel of methyl alcohol placed inside the lamp. A piece of spongy platinum when lowered into the vapor becomes heated by absorption and oxidation of the alcohol until the vapor takes fire. A still simpler method is the insertion of a small flint wheel that can be turned by hand to give sparks. Of course, electricity is the ideal. It gives a brilliant steady light, is not affected by drafts, consumes no oxygen, and does not pollute the air. Among its drawbacks are-expensive lamp installation, lamps easily

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broken, not portable, and short circuits especially in wet places. Most of these objections are overcome in the portable electric light, using a battery of two acid or alkaline cells. In gassy mines, the circuit must be arranged so that it is broken simultaneously with the breaking of the bulbs. Otherwise the glowing filaments would ignite the explosive mixture in the air. Such portable lights have the advantages of being clean, safe around explosives, require no attention, and in the long run are cheaper than acetylene. Sanitation The disposal of excreta is an important problem for underground work where a water system is not feasible. The latrines must contain chemicals to disinfect and render the contents liquid. Lime is not a strong disinfectant. Chloride of lime will kill disease germs, but does not affect the hook worm (a common nuisance in mines). Cresol or tricresol in concentrations of 1:19 is efficient. A one per cent solution of caustic soda will kill all disease germs, but a three per cent solution is necessary to destroy the eggs of the hook worm. Explosives Explosives are necessary to break rock and ore. Here we have a violent chemical reaction producing a great force in a small volume. It is very costly to break ore without explosives, as in drilling by hand or machine. Such work may cost from ten cents to ninety cents a foot. If a small circular hole is drilled and filled with dynamite, the explosive does the rest. For this reason they are used whenever possible. Modem straight dynamite is composed of nitroglycerin, sodium nitrate, wood meal, and an anti-acid. Many varieties are produced by adding or substituting ammonium nitrate, nitrocotton, sulfur, nitrostarch, chlorates, etc. The great problem involved in the use of explosives in mining is created by the gases liberated. They all give carbon monoxide. Low-freezing dynamites produce as high as 47% of the gas by volume. Straight nitroglycerin gives 2634% and gelatin dynamite about 3%. A 30% straight dynamite gives the following gases: carbon monoxide-28.4%, carbon dioxide-20.6%, methaneO.7%, hydrogen sulfide--27.4%. If the dynamite is bnmed instead of exploded, nitrogen dioxide, a very poisonous gas, is also produced. In general, if maximum work is performed the fumes are least harmful. Where there are dangerous amounts of inflammable gas or dust a short flame explosive must be used. (This is especially important in coal mines due to the large amounts of coal dust always present in the air.) These contain ammonium nitrate as the chief ingredient and the reduction of temperature is obtained by the water of crystallization of the salts present. The great problem is t o invent an explosive that liberates only nitrogen and carbon dioxide, which are harm-

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less except insofar they dilute the oxygen of the air, or to invent an explosive that aids mine air. One step in the latter direction is the introduction of liquid oxygen explosives. Shortly after Linde produced liquid air in 1895 he tried the cornhination of liquid oxygen and a combustible material. Because of the scarcity of nitrates Germany developed this method during the late war. The liquid oxygen is absorbed in charcoal and crude oil, paraffin or any liquid hydrocarbon is added to aid detonation. The mixture is detonated in the usual way. The advantages claimed for this method are: lower cost, no danger in transportation of high explosives, elimination of misfires (a wait of thirty to forty minutes and all oxygen has evaporated) no danger from storage and no poisonous gases liberated. Among its disadvantages are: must be fired within fifteen minutes because of evaporation of the oxygen, a limited number of simultaneous shots, and the necessity of a liquid air plant. To meet the latter objection a portable apparatus has been developed which has a capacity of three to five liters per hour. A Chile copper mine has been using this type of explosive to blast 20,000 tons of ore a day. However, a recent disastrous premature explosion has caused the government to shut down the liquid oxygen plant. (This is undoubtedly largely due to their wish to aid the use of nitrates.) A new explosive of somewhat the same type is solidified carbon dioxide, which is finding some use in coal mines. To fire most explosives, detonators are necessary to give the required shock. Practically all have, as the essential ingredient, mercury fulminate. It explodes thus: HgzCzNzOz= Hg 2CO Nz. Potassium chlorate may be added in concentrations up to 20%. This gives a more efficient mixture due to oxidation of the carhon monoxide to carbon dioxide and the resultant increase in total energy. Also, the chlorate is an endothermic compound and gives up its energy of formation. In general, salts of the oxy-halogen acids give efficient mixtures. Likewise, the oxides and nitrates of the heavy metals are used, provided their decomposition yields the free metal and not a lower oxide.

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Mine fires are especially dangerous to life because of exhaustion of oxygen and generation of carbon monoxide. Many conditions contribute to the fire hazard. Spontaneous combustion occurs (especially in sulfide mines) from oxidation of finely divided ore. Blasting sulfide ores may produce explosive miktures of dusts which burn to give sulfur dioxide, while waste carbide may generate acetylene, or candles and lamps may be set too close to timber. Probably the chief menace in metal mines today arises from the rapid extension in the use of electricity. Carelessness in installation

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or repairing and wires strung on timber cause short circuits, while electric motors placed too close to timber contribute t o the danger of fire. Some type of hand chemical fire extinguisher should be immediately available. The carbon tetrachloride (Pyrene) type is efficient but gives rise to small amounts of phosgene, chlorine, hydrogen chloride, and to a lesser extent, carbon tetrachloride vapor, sulfur dioxide, and carbon monoxide. Its use is not advisable in a confined space with poor ventilation. (The new refrigerant, dichlorodifluoromethane has been found equal to carbon tetrachloride in extinguishing the flames of methane.) The sodium bicarbonate-sulfuric acid type is valuable in hand apparatus as well as mounted on mine trucks which can be rapidly moved to any location. Water is ordinarily the best, except in zinc mines. Either water or the carbon dioxide spray from the sodium bicarbonate type which carries small amounts of sulfuric acid act on zinc to liberate hydrogen which aggravates the fire. Likewise carbon tetrachloride forms zinc chloride and oxide which are combustible. In such cases foamite is used. This consists of a solution of aluminum sulfate and sodium bicarbonate with licorice extract added to stabilize the foam. For small fires sawdust can be used. Although it bums it gives no flame and blankets the burning material, excluding the oxygen of the air. Occasionally fires can be extinguished by flooding the region with an inert gas, such as nitrogen, sulfur dioxide, carbon dioxide, or even steam. This is usually expensive because of the large quantities of gas required. In case of fire it is essential to warn the men immediately. Warning by such means as bells, lights, etc., may fail because of breakdown of circuits. The most efficient warning has proved to be the introduction of stenches into the compressed air streams. Such a chemical must be non-toxic, have a moderate vapor pressure, and an extremely disagreeable odor. Among those that have been used are: butyl and ethyl mercaptan, butyric acid, amyl acetate, amyl thio-ether, phenyl isocyanide, and pyridine. Trinitrotertiarybutylxylene has the most powerful odor (can be detected a t 0.000051 part per million). As a nasal and eye irritant, chloracetophenone is superior, being sensitive to 0.021 and 0.0083 part per million, respectively. (It has been facetiously suggested, by Professor Carpenter, that odor of lilac or roses be sent down regularly to put the men in a happy state of mind. Perhaps an excess of oxygen would obtain more than 100% efficiency. Some rising young mining engineer will undoubtedly develop these newer methods.)

Air A supply of clean fresh air is a major problem ior any mine, especially a deep metal mine. The mine air may contain carbon monoxide, carbon dioxide, sulfur dioxide, hydroxen sulfide, nitrogen dioxide, and dust from

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blasting; carbon dioxide from men, animals, lights, and rotting timber; methane, carbondioxide, and nitrogen as emanations from the strata; carbon monoxide from smoldering fires; and odors from excretions of men and animals. It may also be saturated with water vapor if it has passed through wet workings. Methane-CH4-Firedamp,-Methane is especially encountered in coal mining, where it enters from the coal seams. In times of low barometric pressure the amounts may increase greatly. It has no effect on the human system and no appreciable odor, but harms by displacing oxygen. Because of its combustibility it forms dangerous mixtures with air or dust. The original test for methane was by means of the safety lamp. To any experienced man, the transparent blue flame over and surrounding the brighter part of the flame produced by methane will indicate its presence at a minimum concentration of 1%. The safety lamp has the further advantage that a deficiency of oxygen is indicated, as the light is extinguished in 17% oxygen, although the percentagemay fall to 10% without injury to man. This is especially important in old mine workings. The most accurate test for methane is the determination in a standard gas203 = COz 2H20 analysis apparatus. Here the gas is oxidized, CHI (liquid), over a glowing platinum wire and the diminution of volume is calibrated in per cent methane. Carbon Dioxide--C0-Choke Damp or Mixed with Nitrogen-Black Damp.-Carbon dioxide is rarely found in concentrations to exceed 4-5y0. Such amounts are not injurious for periods of thirty minutes although more than 2% seriously reduces efficiency and 3.5% causes deep breathing. The ordinary breathing of a man uses in one hour all the oxygen in 2.6 cubic feet of air. This amount is doubled for a man at work while a candle or lamp uses about the same, and a horse or mule three to five times this amount. Thus, for the sake of efficiency the carbon dioxide concentration should be kept low by good ventilation. When entering a region that may contain high amounts of this gas besides a deficiency of oxygen, a breathing apparatus must be used. This should include a supply of compressed oxygen and a soda-lime canister to absorb carbon dioxide. Nitrogen Dioxide.-This compound is produced by burning dynamite. Fortunately it is seldom formed in quantities large enough to be dangerous. Its presence may be detected qualitatively or quantitatively by the color reaction with phenol-disdfonic acid. Sulfur Dioxide-SO2 and Hydrogen Sulfide-H,S-Stone Damp.These gases are produced in the blasting of heavy sulfides. Hydrogen sulfide particularly is a very toxic and insidious gas. Alarming symptoms are producedwithin afew minutes in an atmosphere containing 0.05%. Death is caused in two to fifteen minutes by 0.06%. I t is even more acutely

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poisonous than carbon monoxide. Fortunately, it is rarely produced. To test for these gases, they are collected in absorption tubes containing an ammoniacal solution of cadmium chloride. The sulfur dioxide dissolves as the sulfite and the hydrogen sulfide is precipitated as cadmium sulfide. The sample of air should be taken as soon as possible after formation because of their interaction to form sulfur, 2HzS SOZ= 3 s HzO. Soot, smoke vapors, etc.. may be removed in a breathing apparatus by cotton pads and activated charcoal. Dust.-Dust of all kinds, especially silica dust, is harmful to the lungs and nasal passages. Where water is plentiful the ground should be sprinkled regularly. If water is scarce calcium chloride may be distributed throughout the mine. Because of its hygroscopic nature it retains the moisture for a greater length of time and aids in decreasing dust. Carbon Monoxide.-This substance, because of its ease of formation, is the mast dangerous of the poisonous gases. It is formed by all explosives and wherever combustion takes place in a limited supply of oxygen. Man shows distress a t a concentrationof O.l%, while death is likely when 0.2y0 is breathed for four or five hours or 0.4% for one hour. Because of its importance various methods have been devised to detect the gas. Animal Behavior.-The oldest test and one still widely used is the effect produced on mice or canaries. The rate of chemical change in the metabolism of such animals is much more rapid than for man. It is stated (Haldane) that a mouse weighing one-half ounce consumes more than fifteen times the oxygen that a like amount of human flesh consumes. Until recently canaries have been the official test. They have the advantage over white mice in that distress is more clearly shown and collapse is indicated by fall from perch. Both, however, quickly recover when placed in fresh air and can be used many times. Recent work has shown that Japanese waltzing mice are even more sensitive than canaries. In a concentration of approximately 0.1% carbon monoxide the former give a positive reaction after 5 to 10 minutes while the latter are not affected after 75-131 minutes. Even though such animals are able to warn man in time to prevent death, yet the margin of time in 0.25% carbon monoxide between serious response of man and animal is too narrow to be of practical use for avoiding harmful exposure. Chemical Tests.-Iodine pentoxide (Hoolamite), Iz06+5C0= 5 C 0 ~ 12. The iodine liberated can be dissolved in chloroform or carbon disulfide and the color compared with standards, or the carbon dioxide may be caught in barium hydroxide and the excess base titrated with oxalic acid. In the newer types a bulbful of the air to be tested is forced over pumice containing the pentoxide and fuming sulfuric acid. The color produced is compared with standards calibrated in terms of 0.2, 0.3, 0.5, and 1.0% carbon monoxide. The color fades and the same chemicals may be used seven or

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eight times. Hydrogen sulfide, hydrogen chloride, ethane, and gasoline vapor must be removed with activated charcoal. CO H,O = 2HC1 COz Pd. Palladium Chloride.-PdClz This is the newest and perhaps the best of the chemical tests. Palladium chloride in a water-acetone solution is contained in a hermetically sealed, thin-walled, cotton-covered glass tube. When a test is to be made the glass is crushed and the cotton wet with the solution is exposed to the air. The free palladium colors it a light brown to black according to the amount of carbon monoxide in the air. Comparison color charts are calibrated from 1 to 10 parts of carbon monoxide per 10,000of air. Tests have shown that this method will give semi-quantitative results in the range 2-10 parts per 10,000 of air. Unfortunately, other reducing agents as ethane, hydrogen sulfide, or gasoline vapor give similar results. But as none of them are desirable in the air, the test is very valuable. Hopcalite.-When carbon monoxide mixed with oxygen of the air is passed over this catalyst the former is oxidized to carbon dioxide, liberating energy. If the catalyst surrounds the hot junctions of a thermocouple the resultant electric current may be employed to give a warning. Usually a bell is rung by a relay when the concentration reaches 4 parts per 10,000 of air. If time and skill are available, by far the best test for any of these gases is a quantitative determination in a standard gas analysis apparatus. Portable outfits, such as the Orsat, which analyze for carbon monoxide and dioxide, oxygen and methane, can he obtained. In many mines a regular schedule of such determinations is carried out. used In studies of mine and tunnel ventilation sulfur trioxide-SOa-is to determine the flow of air, direction of currents, and rate of mixing. An SOa) on pumice stone apparatus containing fuming sulfuric acid (HzSOa is attached to a ~ b h e syringe r bulb. A squeeze liberates sulfur trioxide which combines with droplets of moisture to form a white cloud of sulfuric acid. Acknowledgment

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I wish to thank Mr. Jay A . Carpenter, Professor qf Mining, Meckey School of Mines, Uniuersilj, of Nermoda, for his aery kind and kelfifd suggestions concerning the @[email protected] of lhis article.

General References 1. 2.

PEELE, "Mining Engineers' Handbook," 1st edition, John Wiley & Sons, Inc., New York City, 1918. List of Publications of the U. S. Bureau of Mines, 1910-1931: (1) Bulletins, (2) Technical Papers, (3) Miners Circulars, (4) Report of Investigations, (5) Information Circulars. N o man e w r arrived at the top without a climb.--J. G . JONES