In the course of my study of the dispersion phenomena of the felspar group in the Mineralogical Laboratory of the University of Cambridge, Professor Lewis advised me to study the remarkable properties of sanidine from the Eifel, which were first noticed by Des Cloizeaux, and he very kindly placed the mineral sections in the Museum at my disposal. In order to be certain of the material used, application was made to Professor A. Lacroix of Paris, who with the courtesy and generosity characteristic of his countrymen was good enough to send some fragments of the very crystals used by Des Cloizeanx in his classical researches.

In the present paper the dispersion of the refractive indices of monoelinie felspars from four different localities is given, likewise that of the optic axial angle and of the optic axes in some of them.

The method of investigation and instruments for these studies have been already mentioned in my previous paper. As the refractive indices and dispersion of the glass hemisphere of the total-reflectometer have not so far been given, I shall in the first place state the way in which they have been determined and their values and the degree of accuracy attainable, for the dispersion of the glass hemisphere differs to au appreciable degree from that of other instruments of the same kind.

Eight years ago (in 1907) Mr. F. N. Ashcroft, who for many years has devoted much of his leisure to bringing together choice specimens of the species included in the zeolitic group of minerals, turned his attention to the specimens occurring in the cavities of the basalt of Co. Antrim. He was fortunate enough to secure the services of Mr. Robert Bell, of Belfast, ill his quest, and between them they have searched many of the quarries and other places likely to yield good specimens of these minerals. Some of the early specimens obtained through Mr. Bell, which, however, did not include any chabazite, were exhibited by Mr. Ashcroft at the anniversary meeting of the Society, on November 12, 1907.

It is impossible to determine when this natrolite occurrence was first discovered, but it has been known at any race locally for many years, and specimens of the mineral may frequently be seen in collections, where, however, they are usually labelled as from the Giant's Causeway. This error in localization is largely accounted tbr by the fact that the guides at the Giant's Causeway regularly offer specimens of the mineral for sale to visitors, together with other so-called Giant's Causeway minerals, fossils, and flint implements.

During the summer of 1913 the Royal Ontario Museum of Mineralogy secured a nmnber of samples of ruby-silver from the collection of a deceased mining engineer who had been employed in the Cobalt district. Judging from their association it is presumed that they were obtained from the O'Brien mine. The crystals for the most part are less than two millimeters in length and very few exceed a millimeter in diameter. They are light ruby-red in colour and exceedingly brilliant, and casual inspection suggested that they were proustite. As this mineral had not been described from the Cobalt region, it seemed desirable to confirm this supposition by chemical analysis and crystallographic measurement.

A Word of explanatiou as to the nature of valency-volumes : The existence of the law of valency-volumes involves the conclusion that each chemical atom in a crystal has its own special domain throughout which its influence is predominant as compared with that of any other atom. According to this conception the entire space occupied by the crystal can be regarded as portioned out among its constituent atoms, each atom having a cell allotted to it the boundaries of which consist of points at which the influence of some one or more neighbouring atoms is equal to that of the selected atom; thus no interstices are left between the cells.

Lemme. Dans tout triangle, l'arc x (fig. 1), qui mené du sommet partage la base en deux segments α et β, est donné par

Measurements for torbernite have been given by numerous observers; the results are, however, of rather early date, and althoug the values for the ratio a:c are generally in fair agreement with the results here stated, the degree of accuracy of these measurements is not always indicated. Moreover, sevelml minor difficulties arise, add it has been suggested that some of the early descriptions refer to the allied species, zeunerite ; both species are optically uniaxial and possess almost the same axial ratios; they can, however, be distinguished by means of their refractive indices.

The crystals here described were sent by Dr. J. E. Stead, F.R.S., for crystallographic determination. They came from a 'bear' or mass of metal weighing about 800 tons that had solidified at the bottom of one of the blast-furnaces of Messrs. Cochrane & Co., Ltd., at Ormesby, near Middlesborough. They are present only in sparing amount in drusy cavities, and are absent in the mass of metal itself.