zontal component of the earth's magnetic force (which alone might make a difference of 2 units in the value of J), that he has not reduced the indications of the thermometer he employed to the air thermometer (a reduction which might make a difference of 4 units), and that he used in his experiments very powerful currents and very feeble resistances (a procedure which must necessarily have been attended with some slight errors), no great weight will be laid upon this small difference; the previously startling discrepancy is removed. It can in two ways be shown that W. Weber, as we have maintained, found the absolute value of Jacobi's unit of resistance about 8 per cent. too small. Bosscha, in 1856*, determined according to Ohm's method the electromotive force of a Daniell's element in absolute electromagnetic measure. His measurements were based on a standard of resistance the absolute value of which, 0-607 x 1010 20 (millim.), was obtained by comparison with the above-mentioned copy by W. Weber of Jacobi's unit. He found the absolute electromotive force of a Daniell's element, in the mean out of several measurements, =10.258 × 1010 millim.3 milligr.*). sec.2 This result is proportional to the resistance taken as the basis of the measurement; the error committed in measuring this resistance enters into the derived value of the electromotive force. From a long series of absolute measurements of the electromotive forces of the Daniell element, the details of which shall be related in another place, I have found that the lowest value of the electromotive force of the Daniell element in absolute electromagnetic measure is 10.96 x 1010 (millim.3 milligr.*), sec.2 that the absolute value of the electromotive force of a Daniell's element of the form usually employed is and that the highest value of the electromotive force of a Daniell's element amounts to Which form of Daniell's element Bosscha used, unfortunately he does not state; but we may assume as extremely probable that he made use of the form ordinarily employed, to which, according to my measurements, belongs the absolute electromotive force 11-30 × 1010 (millim. milligr. sec.2 This value is greater, in the ratio of 1.1016 to 1.0000, than that deduced by Bosscha. Now, supposing that Bosscha has carried out his measurements free from error (a supposition which of course cannot be rigorously correct), then the absolute value of the resistance taken by him as the basis of his measurements, i. e. the absolute value found by W. Weber for Jacobi's unit, would be 10.16 per cent. too little. This calculation of the error is based on two somewhat uncertain assumptions, briefly indicated above. On this account it is a great advantage that an error in W. Weber's determination of the absolute resistance of Jacobi's unit, in the same direction and of the same order of magnitude, can be deduced in quite another way. According to W. Siemens the ratio of Jacobi's resistance-unit to Siemens's is =0.6618. From our numerous and multifariously varied measurements the absolute value of the Siemens unit is 0-9550 × 100 (millim.). Accord X ingly the absolute value of Jacobi's resistance-unit would be, ; while M. from our measurements, 0·6320 × 1010 (millim.) sec. Wilh. Weber found only 0.598 × 101o value about 6 per cent. less than that found by us. -that is, a Hence the absolute measurement by M. W. Weber of Jacobi's resistance-unit has turned out certainly from 6 to 10 per cent. too little. [To be continued.] XIX. Crystallographic Notes. By W. J. LEWIS, M.A., Fellow of Oriel College, Oxford*. DR. R. HUGO MÜLLER had the goodness, some time ago, to send me some crystals of the isomerous compounds Quercite and Inosite, which he had obtained from new sourcesthe former from the leaves of the dwarf-palm (Chamaerops humilis), and the latter from cochineal. * Communicated by the Crystallological Society, having been read October 26, 1877. Quercite. The crystallography of Quercite has been already determined by Sénarmont (Rammelsberg's Die neuesten Forschungen in der Krystall-Chemie); but it was a matter of interest to determine whether any difference either in habit or angles could be found in the crystals obtained from the new source. The crystals were found to show the same hemimorphous habit (fig. 1) observed by Sénarmont; and but a slight change has been made in the elements, which may probably be explained by the fact that the crystals obtained by Dr. Müller were very perfect. The crystal is positive; the optic axes lie in the plane of symmetry; the mean line lies between c and 9, and makes an angle of about 20° with the normal to g, the dispersion (inclinée) being considerable, v>p. The angles of the optic axes in air for the red and blue rays were found to be 55° 17′ and 58° 20′ respectively. The forms observed are a {100}, m {110}, c {001},ƒ{011}, g{101} (fig. 2). The faces of the prism are striated parallel to their intersection with a; and there is a good cleavage parallel to g{101}. The following are the elements and principal angles observed and calculated. (100, 101)=35° 32′2, (101, 001)=33° 30′·8, Inosite. This substance crystallizes in colourless, much striated prisms, attached by one end to the mass of the substance. The striations on the planes lying in the prism-zone rendered it impossible, even in the most delicate needles, to get reliable measurements of the angles in this zone. The prisms were terminated by four small planes, {101}, {101}, and {012}, of which the former was most largely developed, sometimes even to the exclusion of the other planes. The crystals were extremely friable, and lost a portion of their water very readily-properties which rendered the examination difficult and prevented the determination of their optical character. The opposite faces in the zones were in all cases considerably displaced, so that there was always a divergence from the zone and from 180° in the sum of the angles between them. The following elements and measurements can therefore only be regarded as approximate. Fig. 3. Fig. 4. The system is oblique. The forms are b{010}, m{1 10}, p{210}, {4 10}, l{101}, t{Ĩ 0 1}, k{012} (fig. 4). (100, 101)=40° 51′, (101,001)=28° 273′, Jordanite. On a crystal of blende from the Binnenthal in the British Museum two small crystals, the one of Jordanite, the other of Binnite, are implanted. The former, on measurement, was found to be a combination of the forms {001}, {119}, {113}, {225}, {112}, {110}, {013}, {025}, {012}, {023}, {011}, {312}, {311}, {310}. Of these the forms {225}, {0 2 5}, {0 2 3}, {312}, {310} are, I believe, new. The middle index in these symbols corresponds to the brachydiagonal usually denoted by the letter y and the parameter b. This arrangement is not to be confounded with that of Professor vom Rath, in which b corresponds to the makrodiagonal and a to the brachydiagonal. The angles between some of these planes observed and calculated from the elements, c: 0=65° 0′; 10: 10=50° 49′, given by Prof. vom Rath are: |