These angles were measured on three different crystals similar

to fig. 1, and excepting for the angles between the prismatic planes, the values closely agreed on all. The planes +1 were very perfect and the angles between them agreed to a minute. The planes -1 were not so perfect, but the angles which they formed are accurate as given above within a few minutes. The planes i and i were strongly striated parallel to the vertical axis and the angles made by them with other planes could not be measured with any accuracy when the intersection edge was parallel to the direction of the striation. The same was also true of the angles made by the planes I, under the same circumstances, although no striation was visible and the reflected image was frequently well defined. When, however, the intersection-edge was at right angles or greatly inclined to the striæ, the

1.

2

iť

angles could be measured within a few minutes, and were found to be very constant. The planes 17 on all the crystals examined were very imperfect and generally only rudimentary.

The crystals of the bitartrate of casia cleave with great readiness parallel to the plane i, with less readiness, but still easily, parallel to it, giving in each case brilliant planes of cleavage at right angles to each other. No evidence of cleavage parallel to the basal section could be detected, the crystals when broken or split in this direction always giving a conchoidal fracture.

Among the crystals of this salt kindly submitted to our examination by Mr. Allen, two very different types of forms were easily distinguished, which, as we are informed, were the result of wholly different crystallizations. In fig. 1 we have both the positive and negative sphenoids (which form together the fundamental octahedron), the planes of the first being distinguished from those of the last only by being uniformly much more developed and having a greater brilliancy. In another variety of this same type of forms, represented by fig. 2, we have only the

positive sphenoid. Crystals were also observed intermediate between figs. 2 and 1 with the planes of the negative sphenoid in different degrees of development. The crystals of the variety represented by fig. 2 contained a small amount of rubidium; but this isomorphous admixture did not perceptibly alter the angles. We measured on three different crystals

These crystals were very perfect and comparatively large, measuring about 7 millimeters long by 5 millimeters wide in the direction of the brachydiagonal. As with the first variety, no accurate measurements could be obtained of the angles between the prismatic planes.

2.

4.

3.

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3

The second type of crystals is represented by the figures 3 and 4. On these forms we have the planes of a left-handed sphenoid, -43, which are not found on crystals of the first type, and are here so largely developed as to give a very different character to the crystal. Planes of the corresponding positive sphenoid were not discovered, although a large number of crystals were examined. These planes were very dull and rough, even on the smallest crystals, and could not therefore be determined with absolute precision. A reflected image was obtained by attaching to them small plates of mica, and the angles were thus approximatively measured, but the results cannot be relied upon within two or three degrees. The values obtained

No other probable parameters of these planes would even approximatively satisfy these values. The crystals represented by fig. 3 differ materially from those represented by fig. 4, and were obtained by a different crystallization. All of the first have the planes +1, which could not be detected on those of the last. On three separate crystals of the form fig. 3, the angle +1 on I measured 139° 15', the same as on the crystals of the first type.

It is evident, then, from this examination that the bitartrate of cæsia forms two different types of crystals, which present respectively a right-handed and left-handed hemihedrism. Either of these hemihedral forms may appear without the other, as in figs. 2 and 4, or they may be united on the same crystal, as in fig. 3. It would be interesting to examine in this connection the optical properties of the salt, but we had not sufficient material for the purpose. The crystals were all proved by spectroscopic examination to be pure bitartrate of casia, with the exception of those like fig. 2, which, as already stated, contained a small amount of rubidium. Nothing is known in regard to the conditions of the crystallization, which would to any degree explain the formation of the two different types of forms. The most obvious hypothesis is that they are connected in some way with the two opposite modifications of tartaric acid; but there is no evidence that any other than the ordinary variety of tartaric acid was used in the preparation of the salt.

2. Bitartrate of Rubidia, HO, RbO, C,H,O,..-This salt resembles very closely the last, with which it is isomorphous. The crystals examined were all similar in character, about 5 millimeters long by 2 millimeters wide, and very perfect. They belong to the trimetric system and have the axial relations, a b c 0·695: 1: 0·726

5.

The planes observed, with the exception of -1, are represented on fig. 5. They are the same as on the last, with the exception of the negative sphenoid -43. Of this no trace could be discovered. The planes -1, moreover, were at best very small and generally wholly absent. The angles measured or calculated are as follows:

AM. JOUR. SCI.-SECOND SERIES, VOL. XXXVII, No. 109.-JAN.,

1864.

Four different crystals were measured, and the angles on all closely agreed with the exception of the angles between the prismatic planes. To these the same remarks apply as to those of the crystals of the bitartrate of cæsia. As is shown by the fig ure, the planes 17 are more largely developed on the crystals of the rubidium than those of the cæsium salt, and in this as well as in the other figures, we have endeavored to preserve as nearly as possible the general habitus of the crystals, as well as the relative dimensions of their planes.

The cleavage of the crystals of the bitartrate of rubidia is in all respects similar to that of the cæsium salt, and the same is true of the crystals formed by an isomorphous mixture of the two substances. Moreover, the planes i and ii are similarly striated on both.

3. Bitartrate of Potassa.-We add for the sake of comparison the elements of the crystalline form of the ordinary bitartrate of potassa as determined by Schabus ("Rammelsberg's Krystallographische Chemie," page 304). His results, reduced to the system of notation used in this article, are

a:b:c-0-7372: 1:0-7115,
Y=125° 46',

Z=103° 38'.

X=100° 20', For the most part, the same planes occur as on the crystals above described, but the planes of the brachydome are more numerous and more developed. Moreover, the planes i instead of being striated vertically, as on these crystals, are striated horizontally, and corresponding to this striation the most perfect cleavage is parallel to the basal section. Cleavages can also be obtained parallel to i and ii, but they are less perfect, the last being the most difficult of the three. It will be remembered that the crystals of the bitartrates of cæsia and rubidia could not be cleaved parallel to the horizontal section, and hence," although the dimensions of the form are not widely different, the difference of structure is so great that the potash salt can hardly be regarded as isomorphous with the other two.

The crystals described in this article were prepared, in the Sheffield Laboratory of Yale College, by Mr. O. D. Allen, from the Hebron lepidolite, and we are indebted to his kindness for submitting them to our examination. They have an additional interest from the fact that in his hands they have furnished the means of separating perfectly the two new metals, and of determining with great accuracy the chemical equivalent of cæsium. Cambridge, November 25th, 1863.

ART. IX.-Geographical Notices. No. XIX.

SPEKE AND GRANT'S EXPLORATION OF THE SOURCES OF THE NILE.

THE great event of the year 1862, in geographical exploration, has been the reported discovery of the sources of the Nile by the perseverance and boldness of two English officers, Capt. J. H. Speke, and his associate Capt. J. A. Grant.

A telegraphic despatch from Alexandria to London brought, in May, the brief announcement, "The Nile is settled;" shortly afterward the journal of the travellers was communicated to the Royal Geographical Society, and finally, on the 17th of June last, the explorers themselves arrived at Southampton. A meeting of the Society just mentioned, under whose auspices the expedition had been sent out, was immediately called, and in it Capt. Speke made a statement full of interesting particulars in regard to the route he had followed and the discoveries he had made.

Those of our readers who have followed the progress of African exploration will remember that in 1858, Capt. Speke (then travelling in company with Capt. Burton) discovered the head of a great, fresh-water lake lying close on 3° south lat., and at an elevation of about 4000 feet above the sea line, which he at once conjectured, from its size and position, as well as from all which the natives told him of its extent, to be a principal source of the river Nile. This lake was called by the natives Nyanza, a term signifying Water, Lake, Pond, or River, to which the English discoverer added the name of his sovereign, christening it Victoria Nyanza. Being prevented at that time from putting his conjecture to the proof, Capt. Speke returned to England, and with the patronage of the London Geographical Society and the British Government, went forth in 1860, on a new expedition, having for his chief object the determination of this specific question. Reaching the coast of East Africa about the first of October, 1860, Messrs. Speke and Grant made their way to the southern point of the Nyanza, and thence going northward they traced one of the principal affluents of the Nile from its source in the lake to its union with the great river itself. This result has been heralded everywhere, in general terms, but having received Capt. Speke's own Report of the journey we prefer to place its details on record here. Their sagacity, perseverance, bravery and success elicit universal commendation. We understand that a volume may be expected from the explorers at an early day, from the press of Wm. Blackwood, Edinburgh.

' v. Proceedings Roy. Geog. Soc., Lond., vii, 212-217.