gible thing to be a Phegopteris dryopteris, and in most cases goes away perfectly satisfied. Occasionally, however, it does occur to him that he is just as wise as he was before, and not one whit more so. These are not imaginary cases. It was from being several times in the position of the non-botanical member that led me to reflect that the function of a definition is to define. Now, who is to blame for this extreme haziness of intercourse, Linnæus or his friend? Perhaps both of them.
In the face of these experiences, it is difficult to answer the seemingly simple question, "What is mica?" To say that it is a unisilicate in which the predominant protoxide is potash and the predominant sesquioxide is alumina, is to say something that is fairly unintelligible to those who are not chemists, and something which even to those who are chemists gives only a bit of classification and partial composition, but in reality explains little about the mineral itself. Any answer that we can give is only satisfactory until we learn to push the question a step further. Gautama well expresses the difficuly when he says in the Light of Asia:
"Shall any gazer see with mortal eyes,
Or any searcher know by mortal mind,
Veil after veil will lift—but there must be
Veil upon veil behind."
But this is a difficulty which besets us on all sides when we question any of the thousand and odd minerals described in Dana, or for that matter when we put questions to Nature in any direction.
In the case of minerals we know enough to perceive that there is much yet unexplained which lies well within the domain of the knowable. But it is as difficult for the mineralogist as for the botanist to give even fair descriptions of the objects of his study, for he so soon runs against his brick wall when he comes to talk about either the physical or chemical properties of minerals. The processes of crystallization are as profound a mystery as the life process itself. We are much in the position of the zoologists of the last century, who named and labeled their specimens without knowing the significance of their relationship.
The name mica is not that of a single mineral, but is a family cognomen, which includes a number of varieties. With the outward attributes of the family we are all more or less familiar, for under the common name of isinglass it forms a small part of the stock in trade of every householder. The family is one of some importance in the mineralogical hierarchy. All are shining members, and are alike in splitting into extremely thin leaves or plates; in being more or less transparent; in being highly elastic; and in having certain ingredients in common. There are seven