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like burned born or cartilage. Tbeir figureremained unchanged and they all became of anc-pake white, retaining part of their originalglofs. r lhel"e diflolved in acids with great of-fer vefcence, and when they had not been pre-vioufly roafted, the iolution remained clear, butafter roafting, the charred gluten was left un-touched. Neither ammonia nor acetited leaddifcovered any phofphat, fo that thefe fhellsfeem to confift entirely of carbonat of lime ce-mented by a fmall portion of animal gluten.

The Nacreous Shells are moftly bivalved, andhave a fofter texture, a greater glofs on theinner fide, and often {hew that iridefcent playof colours which is fo particularly beautiful inthe mother of pearl. All the oyfter and mufcleipecies are of this kind. When put in the firethey give a fenfible odour of burned horn withfome frnoke, and exfoliate. When the roaftedihells' v/ere immerfed in nitric acid a muchgreater proportion of charcoal was left than inthe former kind. But the moft ftriking differ-ence was with the unburned fhell, for whenimmerfed in the acid a violent effervefcenceand 'tapid folution took place at firft, but dimi-nithed gradually, fo that the carbonic acid wasonly difengaged at intervals. In two days timethe whole of the calcareous part was diflolved,but a reticulated feries of membranous layerswas left, the outermoft of which was the epi-dermis or fkin of the natural {hell, fo that thewhole formed a tender mafs of membranes ftillretaining the form of the fiiell. Each mem-brane therefore has its correfponding ftratumof calcareous earth interpofed between it andthe fucceeding membrane, and hence as everyadditional ftratum exceeds in extent that whichwas previoufly formed, the fhell becomesftronger and thicker in proportion as the ani-mal grows. Hence the vaft thicknefs of fhellin the large full-grown oyfters that yield themother of pearl. The nitric folution of thefefhells {hewed no indication of any phofphat, fothat the earth is merely carbonat of lime.

A fecond clafs of fhells has been ftated to bethe Cruftaceous, fuch as form the covering ofthe lobfter, crab, &c. Pieces of this fhellwere immerfed in nitrous and acetous acids, ineach of which they diffolved with moderateeffervefcence to a confidefable degree, but in afhort time the adtion ceafed and a yellowifhwhite elaftic cartilage was left, which retainedthe original form. ' In the acetous acid thecolouring matter alfo remained and was folublein alcohol. Thefe folutions contain both phof-phat and carbonat of lime, and thus it is the

prefence of phofphat of lime, though the quan-tity is comparatively fmall, which diftinguilhesthe cruftaceous from all the teftaceous fhellswhether porcellanous or of nacre.

It may be added that pearls refemble exadllyin chemical compofition, the nacre or motherof pearl; and that egg-fhell bears the ftrongeil:affinity with the porcellanous ihells.

SIDERITE.

Is a name given by Bergman to a fuppofedpeculiar metallic fubftance which is the princi-pal caufe of the brittlenefs of certain kinds ofbar iron. It has fince been difcovered to bephofphat of iron.

SIENITE. Sienit. Wern.

This is one of the .compound primitive rocksand confifts eflentially of cryftals and grains ofhornblende imbedded in felfpar. When thefelfpar is compadl the mafs has a porphyriticcharacter and is often called Sienitic porphyry.Quartz and mica are occafionally found in fienitebut in very fmall quantity; in this ftate it isfometimes called granitelle.

Sienite is commonly in mafs, and is rarelyeither fchiftofe or ftratified. It fometimes con-tains metallic veins as at Altemberg in Saxony .

SILEX. ■ Kiefelerde. Germ.

This earth is generally found in a ftony ftate,and from its forming nearly the entire compo-fition of flint, it has acquired the name of filexor filiceous earth. It is found in equal or per-haps greater purity in rock cryftal and quartz,and in white fand, and on the whole it isprobably the moft abundant earth on the faceof the globe. Moft of the ftony combinationsof filex are remarkable for their hardnefs, andwill very readily ftrike fire with fteel.

Silex when pure is white, and perfectly voidof tafte and fmell. It is infoluble in water, andis incapable of artificial cryftallization.

It likewife contradls but a very flight unionwith water when wetted, and is perfedtly voidof plafticity, fo that after wetting it dries morefpeedily than any other earth. Its fpecific gra-vity as it exifts in quartz or flint is about 2.6.

It is perfeclly infufible by itfelf even inoxygen gas.

This earth is abfolutely infoluble in everyacid except the fluoric, unlefs in that ftate offine divifion in which it exifts at the momentof its reparation from its alkaline folution byacids. Its peculiar habitudes with the fluoricacid have been mentioned under that article.

The fixed' alkalies readily diffolve filex by theaffiftance of heat, and even in the cold by longdigeftion. If flint or fand or any other filiceous