Chemical Science. 157 
strongly detonating.—Baryta combines with the acid apparently 
in two proportions ; the first crystallizes in dull white grains, which 
detonate powerfully, and are difficultly soluble in water.—Strontia 
resembles baryta in its compounds.—Zinc forms a salt in small 
yellow granular crystals, very soluble, and very heavy.—Potash 
produces a salt which crystallizes in long white brilliant plates, 
having a disagreeable metallic taste, not affecting test-paper, dis- 
solving completely in eight parts of boiling water, and detonating 
powerfully when heated or struck. It contains 85:08 of acid, and 
14-92 of base.—Soda has always produced a salt in small rounded 
plates, brown and brilliant ; they are lighter and more soluble than 
the preceding, but otherwise resemble it. ‘They contain 88-66 of 
acid, and 11-34 of base.—Ammonia with fulminating silver leaves 
no residuum. Berthollet’s compound being formed at the same time 
with the other. On cooling, a large quantity of granular crystals 
are obtained, which are difficultly soluble, and have a strong me- 
tallic taste. They detonate even in the liquid when touched by a 
glass rod, but fortunately if excess of alkali be present the deto- 
nation does not extend to the neighbouring portions. 
2. On the unequal Dilatation of a Crystal in different directions, 
by heat.—On measuring the mutual inclinations of the planes of a 
crystal of carbonate of lime at different temperatures, M. Mits- 
cherlich observed that they varied sensibly with the temperature, 
the variation sometimes amounting to 8'.5 from 32° to 212. Fahr. 
When the temperature rose, the obtuse diedral angles diminished, 
or in other words the short axis of the rhomboid expanded more 
than the other diagonals, so that its form approached to that of the 
cube. M. Mitscherlich concluded, therefore, that the double re- 
fraction of the crystal would at the same time diminish; a result 
confirmed by an experiment which he afterwards made with M. 
Fresnel in the manner adopted by that philosopher in 1817, to 
render more sensible the changes in the tints of plates of sulphate 
of lime. M. Fresnel had then observed, that elevation of tempe- 
rature sensibly diminished the double refraction of sulphate of 
lime ; and according to the recent experiments of the two philoso- 
phers the same effect is produced, though in a much less degree, 
on rock crystal. This experiment, however, requires repetition. 
It appears, therefore, that generally an uniform elevation of tem- 
perature in acrystal diminishes its double refraction. M. Mits- 
cherlich thinks that heat ought always to separate the molucules of 
a crystal farthest apart in that direction in which they are most 
contiguous.—Ann. de Chim. xxv. 109. 
3. Difference of crystalline Forms of the same Substance.—M. 
Mitscherlich, who first observed the remarkable fact that a body 
may affect two different crystalline forms, has, in a memoir on this 
