440 SCIENCE PROGRESS. 



names Grossular, Uvarovite, Melanite, and Almandine. 

 None of them exist in nature as absolutely pure compounds, 

 but only as substances which can be regarded as mixtures 

 of these five compounds with one another and with such 

 compounds as Mg 3 Al, (Si 4 ) 3 and Mn 3 Al 2 (Si 4 ) 3 , 

 magnesia — and manganese — garnets which are more or less 

 hypothetical ; the name applied to a given specimen being 

 that of the simple compound to which it most closely 

 approximates. 



Now there are here two possibilities to be distinguished. 

 Two compounds A and B are related in form and composi- 

 tion. If they are isomorphous in the strict sense of the 

 term, they are also capable of crystallising together in 

 indefinite proportions to make an intermediate compound 

 mA + n¥> where m and n are any numbers; if they are not 

 strictly isomorphous they may crystallise together to form a 

 compound 7?iA + ?/B where ;;/ and n are simple whole 

 numbers, and this is not infrequently similar in form to 

 the components. Further, both A and B may exist as 

 minerals, or A may be a known mineral and B an artificial 

 compound, or A may be a known mineral and B a com- 

 pound whose existence is only inferred from that of the 

 intermediate compounds mA + 72 B, or, as the last extreme, 

 both A and B may be only hypothetical compounds intro- 

 duced to explain the composition of the minerals mA + nB. 



Thus, for example, in the carbonate series mentioned 

 above it is not yet known whether Dolomite is an 

 isomorphous mixture of Magnesite Mg C0 3 with Calcite 

 Ca C0 3 , or a definite compound Mg Ca (C0 3 ),, or a mixture 

 of this double salt with Maonesite and Calcite. In the 

 Scapolite group the various minerals included under that 

 name are generally acknowledged to be isomorphous 

 mixtures of two or more silicates, but different views are 

 held upon the possible constitution of these primary con- 

 stituents. 



Again, there is some uncertainty whether isomorphous 

 mixtures to form homogeneous crystals can take place in 

 all proportions and are accompanied by continuous changes 

 in the physical properties of the substance, or whether they 



