The splitting strength of mica

Abstract Two well-ground and polished plane glass plates if put together, adhere. This phenomenon is called optical contact and is very much used in the manu­facture of optical instruments, since the boundary between the two glass surfaces shows practically no reflection of light, when the gap between the two glass surfaces is exceedingly small in comparison with the wave-length of visible light. An analogous phenomenon can be easily observed in mica. Two freshly split mica foils if put together adhere again with an appreciable force. It is interesting to see if the restoration is perfect, i. e., if we need to apply the same force to split two mica sheets placed in optical contact as to split a fresh one. In the present work it will be shown that the splitting strength is a constant for mica ; that we can determine a surface energy of mica which is independent of the shape of the mica sheets chosen and that mica placed in optical contact is totally restored. A description will also be given of some electrical phenomena obtained by splitting mica in a high vacuum. It appears that the cleavage surface of mica is covered with electrical charges ; further, that the strength of mica in a high vacuum is more than in air and is also a fairly well-defined constant. The present observations are connected with the well-known work of A. Joffé on the tensile strength of rock salt. In the case of rock salt the surface was either dissolved to avoid the effect of imperfections of the surface or the surface action was avoided altogether as in the experiment with the rock salt sphere.