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polarize light by refraction, and exhibit the black cross, with rings of 

 green and red alternately. The author observed these bells to be 

 formed of minute crystals, radiating from the apex of the bell. 



16. Mannite. — This substance the author has found, since the 

 paper was read, to give circular crystals more easily and cex'tainly 

 than any other. Those from the solution in acetic acid are the 

 finest. The black concentric circles, indicating absence of matter, 

 are peculiarly marked ; and the sectors shade off so perfectly into the 

 arms of the cross, as to give the discs the appearance of being formed 

 of four solid cones. The discs are sometimes elongated into conical 

 forms, with the black cross at the summit. A crust of opaque crys- 

 talline matter, that is, not in optical contact, often covers them, and 

 often breaks off, shewing the circular crystal below. The cones have 

 frequently two, three, or four black arches crossing them. In some 

 of the larger discs, each successive ring is formed of radiating branches, 

 radiating from the margin of the ring within. 



17. Oxalurate of Ammonia [jpure). — This salt, to which the author's 

 attention was called by Professor Gregory, gives beautiful negative 

 circular crystals, and rarely fails to yield them. With weak solutions 

 the discs are small and exactly resemble those of the lithoxanthate 

 of ammonia. Professor Gregory thinks that the two salts are iden- 

 tical, but that the lithoxanthate contains a little colouring matter. 

 With strong solutions, the salt yields discs often nearly opaque, but 

 surrounded by concentric rings of marginal radiations, of different 

 tints. In some large crystals, the central circle consists of green of 

 the second order, with a faint black cross, descending to the white of 

 the first order ; the next ring, which is separated by a narrow black 

 band from the first, exhibits the white, which rises to the yellow of 

 the second order, and again descends to the white of the first, com- 

 pleting the second ring. Three similar rings follow in succession, 

 and each of the five has a uniform tint throughout its circumference, 

 proving a uniform thickness in each band. These crystals, when a 

 number are seen in the dark field, are singularly beautiful. This salt 

 yields cones like those of mannite, and these have, in the centre of 

 the black cross, a second cross bisecting the luminous sectors. 



18. Hippunc Acid gives fine circular crystals with alcohol. In 

 these, the radial lines are often divided by black spaces as broad as 

 the luminous lines ; and the whole disc is covered with numerous 

 minute concentric circles, at equal distances from one another. In 



