PLAIN DIRECTIONS FOR TESTING MINERALS.

Waikato Times, Volume I, Issue 34, 18 July 1872, Page 2

PLAIN DIRECTIONS FOR TESTING MINERALS.

{Continued from No 28, July 4.) GENERAL OBSERVATIOKS. Before leaving the subject of mundio, with whicht our last notice concluded, it will be well to impress upon the readers the fact that iron pyrites, or mundio proper, which is an all but valueless mineral found in vast abundance in many lodes or veins (and from its color and the great specific gravity of stone or .gangua containing it in large proportions, frequently mistaken by ignorant persons for gold) is so similar in appearance to copper"pyrites, which is a very valuable ore, that they may- easily be mistaken, even by practical miners for each other. But to distinguish them readily and, save the trouble of testing by the blowpipe it is only necessary to keep thisau mind, via., that iron pyrites is almost as nard as quartz, and cannot bo scratched with a knife, whereas copper pyrites can easily be scratched. Iron pyrites is also heavier than copper pyrites, the specific gravity of the first being about 5, white the latter is only about 4 ; or in other words, while iron pyrites losses only one-fiith its weight when weighed in water, copper pyrites losses nearly one-fourth. This specific gravity test, however, from the intimate manner in which ..mundic is blended with tho quartz or other stone in which it is found, is not in general readily applicable to pyrites. It may also be well to mention here that the testing appliances referred to in our last issue, viz., blowpipe, platinum wire, borax, cyanide potassium, and other chemicals, crucibles, Ac, are easily procurable in Sydney, and that their cost is very trifling. . On testing minerals by their specific gravity. By specific gravity is meant the comparative weight of equal bulks. Water is taken as the standard of comparison -, the., special gravity of a mineral is a number showing how many times it is, bulk for bulk, heavier than water.

The specific gravity of water is called 1, of gold 19, implying tliat if equal bulks of gold and water were taken, the gold would weigh 19 times as heavy as the water. To put it in otliei words—a piece of gold that weighed an< ounce in air, would weigh nearly 19dwts. in water; while the specific gravity of quartz, for instance, being much less than gold, a piece of that mineral that weighed an ounce in the air would weigh but about 12£dwts. in water. The specific gravity of a mineral is to be found by weighing it first in air in the usual manner, and then observing how much of its weight it loses, when suspended from the arm or pan of a balance, and allowed to hang freely in water. If a piece of quartz weighing 26 grains is attached by a hair or thin cotton to the scales—and weighed wliile hanging in water—it will be found to weigh only 15 grains.; it thus lose* 10 grains, or 10-26ths of its entire weight. Similarly gold would lose 1-19 th of its weight. Mineral® differ very widely in the proportion of weight which they lose in water, but the same mineral invariably loses the same proportion, for instance: — Quartz loses 10-2Gths of its weight; topaz 10-35ths; sapphire KMOfchs; zircon 10-45ths; tin ore 10-70ths. These proportions depend upon the specific gravity of these minerals. The specific gravity of water is called 1, of quart/. 2.G, of topaz 3.5, of sapphire 4.0, of gold 19; signifying among other facts that quartz loseß 10-20ths of its weight in water, topaz 10-45ths sapphire, gold, 1-19 th, In determining how much a mineral loses in water, a very delicate balance is required when the weight in air is under 10 grain*; but for portions weighing heavier than this, a common balance turning readily to a grain, may be U3ed for practical purposes. The mineral must be sound throughout, and free from any pores, or cracks, and its surface should be rubbed over with water immersing it, to prevent bubbles of air adhering-, which would falsify the result. In careful trials rain water should be used. A trial of specific gravity can have no value, unless it is made on a pure portion of a mineral, quite free from any adhering foreign matter. A rough estimate of specific gravity can be formed from the feeling of pressure in shaking a mass loosely in the palm of the hand ; ra this way it can be judged whether the specific gravity is high or low. The rule for finding the specific gravity is to divide the weight of the mineral in air, by its loss of .weight, in water : Example—A piece of quartz .weighed 3398 grains in air, and 862. grains in, water, Here the loss of weight is 536, and the weight in air divided by this number is 2-6, which is the specific gravity of quartz.

Specific Melting Gravity. . ; Heat. | Gold 19.362 2282 deg. Silrer ... ... 10.470 . ... - 1832 Platinum ...' 22.060 ■ . 3082 Tin ... 7.291 512 Copper .. ... 8.915 ... ••• 1922 Antimony ... 6:732 ... ... 810 Lead .- ..: 11.446« ... ~.l. 629/ Bismuth ... 9.756 ... .... 512-" Iron 7.758 2912 Zinc ... ... : 7 : 190 - , 793

Specific Gravity and Melting Heat of Metals,