06 THE CANADIAN NATURALIST. [Sept. 



combiues with the equally refractory bases, forming' readily fusible 

 compounds. The simple silicates, formed by the union of silica or 

 silicic acid with one base, are not always fusible. Those of the 

 alkalies and iron oxides are, but the silicates of alumina (clay), 

 magnesia (serpentine), and lime (wollastonite),are almost or com- 

 pletely infusible. Nevertheless, the three latter combined form 

 the scoriae of most frequent occurrence in the arts, namely, those 

 of iron furnaces. In these slags the proportion of silica present 

 often mounts as high as 75 per cent., while those from puddling 

 furnaces do not contain more than 35. The former are termed 

 very acid or siliceous, and the latter very basic slags. Such 

 variations in the silica contents of these compounds are accom- 

 panied by corresponding changes in their chemical and physical 

 properties. Basic slags are more easily fused than siliceous slags, 

 although the latter do not solidify as rapidly as the former. 



The same variations in the quantity of silica w'hich occur in 

 farnace slags are also to be found in original rocks, and just as 

 furnace scoria have been ranged under diflerent chemical formulae, 

 so, likewise, it has become possible to classify original rocks in a 

 similar manner. When the student of chemistry has gradually 

 added an acid to an alkali, or other base, until the mixture 

 neither reddens litmus nor browns turmeric paper, he has formed 

 a neutral salt consisting of one atom of bas3 to one of acid, such 

 as sulphate of iron (FeO S.O3) and nitrate of potash (KO N.O..0- 

 The salts of the peroxides, although frequently possessing acid 

 properties, are, nevertheless, also regarded as neutral or normal 

 and contain, for every atom of base, three of acid, such as per- 

 sulphate of iron (re2 O3 3 SO3) or tersulphate of alumina (Ag 0^ 

 3 SO3). Similarly in mineralogy those silicates are regarded as 

 neutral which contain one atom of monoxide combined with one 

 of silica acid or silica, or one atom of sesquioxide combined with 

 three of silica. Thus the mineral leucite, which consists of one 

 atom of potash, one of alumina, and four of silicic acid, may be 

 regarded as the type of a neutral mineral. Its formula is KO. 

 AI2 O3. 4 Si. O2 and it wall be observed that its bases contain 

 four while its acid contains eight equivalents of oxygen. Neutral 

 or monosilicates, therefore, are those in which the proportion of 

 oxygen in the bases, to that in the acid, is as 1 is to 2. If we 

 search among crystalline rocks for those in which this oxygen 

 ratio exists, we shall find them to be w^ell-defined rock species 

 which are not usually considered from, a chemical point of view 



