_The Comparative Hardness_.--The next characteristic of the mineral to be ascertained is the comparative hardness. In mineralogy there is a scale fixed for comparison, from 1 to 10, 10 being the hardest, the diamond, and Number 1 the soft soapstone. These and the intermediate minerals fixed upon the scale are generally inaccessible to those who may use the contents of this paper, and I will give some more familiar materials for comparison. 8, 9, and 10 are the topaz, sapphire, and diamond respectively, and as these and minerals of similar hardness will probably not be found in any of the localities of which I make mention, we need not become accustomed to them for the present. 7 is of sufficient hardness to scratch glass, and is also not to be cut with the file before mentioned, which is used for these determinations. 6 is of the hardness of ordinary French glass. 5 is about the hardness of horse-shoe or similar iron; 4 of the brown stone (sandstone) of which the fronts of many city buildings, etc., are built; 3 of marble; 2 of alabaster; and 1 as French chalk, or so soft as to be readily cut with the finger nail. The method of using and applying these comparisons is by having the above matters at hand, and compare them by the relative ease with which they can be cut by running the edge of the file over their surface. One will soon become familiar with the scale, and it may of course then be discarded. As it is one of the most important characteristics of some of the minerals, it should be carefully executed, and the result carefully considered. It is of course inapplicable under those conditions with minerals that are in very small crystals or in a fibrous condition.