but, practically, crystallized masses throw themselves into one of the three groups we have been examining to-day; and appear either as Needles, as Folia, or as Knots; when they are in needles (or fibers), they make the stones or rocks formed out of them “Fibrous;” when they are in folia, they make them “Foliated;” when they are in knots (or grains), “Granular.”  Fibrous rocks are comparatively rare, in mass; but fibrous minerals are innumerable; and it is often a question which really no one but a young lady could possibly settle, whether one should call the fibers composing them “threads” or “needles.”  Here is amianthus, for instance, which is quite as fine and soft as any cotton thread you ever sewed with; and here is sulphide of bismuth, with sharper points and brighter luster than your finest needles have; and fastened in white webs of quartz more delicate than your finest lace; and here is sulphide of antimony, which looks like mere purple wool, but it is all of purple needle crystals; and here is red oxide of copper (you must not breathe on it as you look, or you may blow some of the films of it off the stone), which is simply a woven tissue of scarlet silk.  However, these finer thread-forms are comparatively rare, while the bolder and needle-like crystals occur constantly; so that, I believe, “Needle-crystal” is the best word (the grand one is, “Acicular crystal,” but Sibyl will tell you it is all the same, only less easily understood; and therefore more scientific).  Then the Leaf-crystals, as I said, form an immense mass of foliated rocks; and the Granular crystals, which are of many kinds, form essentially granular, or granitic and porphyritic rocks; and it is always a point of more interest to me (and I think will ultimately be to you), to consider the causes which force a given mineral to take any one of these three general forms, than what the peculiar geometrical limitations are, belonging to its own crystals. [Footnote:  Note iv.] It is more interesting to me, for instance, to try and find out why the red oxide of copper, usually crystallizing in cubes or octahedrons, makes itself exquisitely, out of its cubes, into this red silk in one particular Cornish mine, than what are the absolutely necessary angles of the octahedron, which is its common form.  At all events, that mathematical part of crystallography is quite beyond girls’ strength; but these questions of the various tempers and manners of crystals are not only comprehensible by you, but full of the most curious teaching for you.  For in the fulfillment, to the best of their power, of their adopted form under given circumstances, there are conditions entirely resembling those of human virtue; and indeed expressible under no term so proper as that of the Virtue, or Courage of crystals;—­which, if you are not afraid of the crystals making you ashamed of yourselves, we will by to get some notion of, to-morrow.  But it will be a bye-lecture, and more about yourselves than the minerals.  Don’t come unless you like.