t'» 1 I m.w yos ii MUSEUM 



perfectly pure, the impurities seldom 



fall Idle they may increase to Buch to extent as 



to prohibit (.•ailing the reek a limestone. The impurities com- 



:ilv present i alumina, iron, magnesia and organic 



iiuif sulfuric acid are also met with. 



nt either as pure quartz; combined with 

 alumina in tin- form of clay; or l< quently as an element 



h as mica, hornblende or pyroxene. Silica 

 may j Uy be 1«»< »k»<l «.n as an inert impurity displacing so 



much carbon At high temperatures, however, when 



the carbonic acid has been driven off and oxid of lime left, the 

 lilica will flux the lim<' with g Alumina ifl usually 



pn clay. With an inci of the lal I 



limi •rnicnt rock. \i present to the extent of 



on! lumina is an inert impurity like Bilica, but, when 



present in larger amounts as a constituent of clay, it facilitates 

 the expulsion of the carbonic acid gas. The reason for this is 

 that clay contains chemically combined water, which passes off 

 <»nly at a red heat or at the same time as the carbonic acid gas. 

 - provides an atmosphere of watery vapor into which the 

 carbon dioxid escapes quicker than it would if passing off into 

 gas i own kind. 



D and alkali-, if present in appreciable quantity, render 

 one more easily fusible, and may necessitate the hand- 

 picking of the burned rock to separate clinkers. Limestones 

 d contain appreciable amounts of magnesia. AVhen the 

 amount of MgO is "• • or higher, they are called magnesian lime- 

 stones, but, when it reaches 18£ or 20#, the term dolomite is more 

 [uently employed. Organic matter is rarely absent from 

 lim< ■ '1 a very small amount may impart a gray or even 



blaek color to the rock While a total of !■ or 5£ of impuriti 

 n min-h when only a few tons of stone a day are 

 h it 1" - an appreciable item when the consumption at 



- amou r 350 tons of limestone a day. 



