134 Progress in Chemistry. [January, 



unfortunately, it is in many points an inadequate" production : for ease of 

 reference it will, however, be expedient to follow the order in which the speci- 

 mens are therein described. 



A fine large crystal of diamond (No. 1172) affords an example of the pecu- 

 liarly brilliant lustre which distinguishes even the natural surfaces of this stone, 

 and permits any one who appreciates its character to single out a rough 

 diamond from all other stones. The specific gravity of the diamond (3-53) 

 does not enable us to distinguish it from glass or white topaz with certainty, 

 though it separates it from white quartz, which is only 2^65, and white beryl, 

 which is only 2-71. There is a black diamond (No. 1173) in the collection, 

 which displays on its facets the characteristic lustre of the stone. A colourless 

 diamond of pure water, and about T \ of an inch across, is a splendid brilliant 

 from the Hope collection. A lustrous yellow diamond of the same size should 

 also be noticed. Of other coloured diamonds there are two green specimens, 

 one of a pale blue hue, one of a curious and very rare puce tint. 



It will not be necessary to dwell upon the peculiarities of any of the many 

 forms of silica represented in the collection. Almost all the ordinary and most 

 of the rare varieties may be here seen mounted in rings, and in some cases set 

 off by borders of diamonds. One amethyst (1187) is remarkable for containing 

 several cavities partially filled with fluid. Some of the translucent specimens 

 known as cafs-eye, of a yellow or brown colour, are good examples of the 

 peculiar " chatoyant " effect produced by regularly disposed asbestos filaments 

 penetrating the quartz. Several of the specimens catalogued under the heading 

 Silica belong, however, to far more recherche species. One, for instance 

 (No. 1188), is probably a pink topaz, and not an amethyst; another, called 

 Burnt Quartz (No. 1194), appears to be a splendid chrysoberyl, the dispersive 

 power of the stone being far too high for silica. Burnt quartz is the name 

 given (after strong heating) to a dark variety of rock crystal from Portugal and 

 Brazil, which assumes, after having been submitted to a high temperature, a 

 sherry-brown or red colour. 



Of opals there are more than a dozen in the Townshend collection ; many of 

 them exhibiting the most superb play of prismatic colours. One is pale grey, 

 and shows a fine blue iridescence. 



The sapphires and rubies include a series of fine specimens of the highly- 

 prized deep velvet blue, and the pigeons'-blood red colours. There are also 

 some violet and salmon-coloured corundums of great rarity and beauty. A 

 golden-yellow one, of great brilliancy (No. 1312) is included with the topazes. 

 The star-rubies and star-stones are well represented. These stones somewhat 

 resemble the cat's-eye, but they show a six-rayed star, which is best seen when 

 the gem, cut across its principal axis, and left with its top en cabochon, or 

 rounded, is viewed in sunlight or in the focus of a condensing lens. In the 

 case of these star stones, the internal reflection is due, not to intruding sub- 

 stances, but the mode in which the crystal has been built up of symmetrically 

 disposed but 1 not perfectly regular layers. 



The turquoise comes next in order. The true turquoise is a phosphate and 

 hydrate of aluminium coloured by a little phosphate of copper. The false, or 

 bone turquoise, is merely fossil ivory tinted with phosphate of iron, not phos- 

 phate of copper, as stated in the catalogue. 



Several of the most beautiful varieties of garnet are seen to perfection in the 

 Townshend series. Two of the specimens (1306, 1307) are wrongly labelled 

 "jacynth," and included under the heading Zircon in the catalogue. One of 

 the so-called garnets, of an amethystine colour, seems likely to belong to 

 another species. 



A fine inscribed emerald, from the celebrated collection of the late H. P. Hope, 

 is the first beryl in the list. It is of rich colour, and nearly | inch by f inch in 

 dimensions. Another stone (No. 1284), though smaller, is a still finer speci- 

 men, faultless in cutting, shape, and colour, and almost half an inch across. 

 The aquamarines in the collection are of immense size and rich hues. One of 

 them presents almost the exact tint of the more valuable blue topaz. 



Nine specimens are catalogued as chrysolites, but at least four of these are 

 incorrectly assigned to this species. Nos. 1297 and I 3°4 are chrysoberyls, 

 while 1304 is a zircon of the variety known as jargoon. 



