_Action of Hot Acids_.--This very important test is never, like the above, applicable upon the field, but applied when home is reached. From the body of the mineral as pure and clean as possible a portion is chipped, about the size of a small pea; this is wrapped in a piece of stiff wrapping paper, and after placing it in contact with a solid body, crushed finally by a blow from the hammer. A pinch of the powder so obtained is taken up on the point of a penknife, and transferred into a test tube. Two or more of these should be provided, about six inches long. They may be obtained in the apparatus shop for a trifle. Some hydrochloric, or, as it is generally called, muriatic acid, is poured upon it to the depth of about three quarters of an inch; the tube is then placed in some boiling water heated over a lamp in a tinned or other vessel, and allowed to boil for from ten to fifteen minutes; the tube is then removed and its contents allowed to cool, and then examined. If the powder has all disappeared, we term the mineral "soluble;" if more or less is dissolved, "partly soluble;" if none, "insoluble;" and if the contents of the tube are of a solid transparent mass like jelly, "gelatinous;" while if transparent gelatinous flakes are left, it is so termed. As this method of distinguishment is always applicable, it is very important, and its detail and result should be carefully noticed. Care should be taken that only a small portion of the mineral is used, and also but little acid; the action should be observed, and is frequently a characteristic, in the case with calcspar, which effervesces while dissolving. The acid used is hydrochloric at first, and then, if the mineral cannot he recognized, the same treatment may be repeated using nitric acid. Both of these acids should be at hand and two ounces are generally sufficient.