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LI. On the Chemical Composition of the Silurian and Cambrian 

 Limestones. By David ?^aBEs, F.G.S., A.I.C.E.^ 



OUR information as to the chemical composition of the lower 

 stratified rocks, particularly of England, is so imperfect 

 that it is uncertain whether even a single analysis is recorded. 

 It seemed therefore desirable that this deficiency should be sup- 

 plied ; and the more particularly, as it is of geological import- 

 ance to know whether these rocks differed from, or were analo- 

 gous to, their higher equivalents in chemical composition. 



The analyses of limestones here brought forward are intended 

 to form part of a series of analyses of the various lower rocks ; 

 but as from uncontrollable circumstances some time is likely to 

 elapse before this investigation can be pursued further, it is con- 

 sidered best to communicate the results hitherto obtained. 



The mode of analysis adopted was as follows, subject to some 

 slight variations in particular cases : — A known weight was de- 

 composed by hydrochloric acid in a previously tared carbonic 

 acid apparatus, and the loss estimated as carbonic acid ; the in- 

 soluble matter filtered from the solution was dried at 300° F., 

 weighed, and then ignited to a red heat and reweighed ; the last 

 weight was considered as insoluble matter, and the difference of 

 the two weights as organic matter. 



The filtrate was boiled with a little nitric acid and precipitated 

 by ammonia, which threw down the sesquioxide of iron, alumina, 

 and any phosphoric acid present in the limestone ; this was col- 

 lected, ignited and weighed, and then dissolved in hydrochloric 

 acid and molybdate of ammonia with excess of nitric acid added : 

 the characteristic yellow phosphomolybdate of ammonia precipi- 

 tated was, after standing some time, filtered off, washed with 

 dilute nitric acid, dissolved off the filter by ammonia, and preci- 

 pitated by a magnesian salt as usual. 



The amount of phosphoric acid deducted from that of the 

 precipitate gave the quantity of iron and alumina present, which 

 were not separated. Some part of the iron was evidently pre- 

 sent as protoxide, which makes a slight error in the amounts 

 given in the tabulated analyses. 



The lime was precipitated as oxalate, and determined as usual 

 with the precaution of reheating with some carbonate of ammonia. 

 In one case tlij lime contained some manganese, which was sepa- 

 rated by weak acetic acid and determined. 



The magnesia was precipitated from the solution, from which 

 the oxalate of lime had been filtered, by phosphate of soda and 

 ammonia and determined as usual. 



In one case the lime was determined as sulphate instead of 



* Communicated by the Author. 



