If a mineral scratches calcite and is scratched by fluorite, we say its hardness is between 3 and 4, perhaps 3.5; if it neither scratches nor is scratched by orthoclase, its hardness is 6; and so on. There are very few minerals harder than quartz, and hence the first seven members of the scale are sufficient for all ordinary purposes; and these are all included in the series of specimens accompanying this Guide. Although it is desirable to be acquainted with the scale of hardness, and to understand how to use it, still the student will learn, after a little practice, that almost as good results may be obtained much more conveniently by the use of his thumb-nail and a good knife-blade or file. Talc and gypsum are easily scratched with the nail; calcite and fluorite yield easily to the knife or file, apatite with more difficulty; while orthoclase is near the limit of the hardness of ordinary steel, and quartz is entirely beyond it. 4. _Specific Gravity._—The specific gravity of a mineral, by which we mean its weight as compared with the weight of an equal volume of water, is determined by weighing it first in air and then in water, and dividing the weight in air by the difference of the two weights. Minerals exhibit a wide range in specific gravity; from petroleum, which floats on water, to gold, which is nearly twenty times heavier than water. Although this is one of the most important properties of minerals, yet, being more difficult to measure than hardness, it is less valuable as an aid in distinguishing species. One can with practice, however, estimate the density of a mineral pretty closely by lifting it in the hand. 5. _Lustre._—Of all the properties of minerals depending on their relations to light the most important is lustre, by which we mean the quality of the light reflected by a mineral as determined by the character or minute structure of its surface. Two kinds of lustre, the _metallic_ and _vitreous_, are of especial importance; in fact all other kinds are merely varieties of these. The metallic lustre is the lustre of all true metals, as copper and tin, and characterizes nearly all minerals in which metallic elements predominate. The vitreous lustre is best exemplified in glass, but belongs to most minerals composed chiefly of non-metallic elements. Metallic minerals are always opaque, but vitreous minerals are often transparent. Other kinds of lustre are the _adamantine_ (the lustre of diamond), _resinous_, _pearly_, and _silky_. When a mineral has no lustre, like chalk, it is said to be dull. It should be made clear to children that lustre and color are entirely distinct and independent. Thus, iron, copper, gold, silver, and lead are all metallic; while white or colorless quartz, black tourmaline, green beryl, red garnet, etc., are all vitreous. Generally speaking, any color may occur with any lustre.