664 REPORT OF NATIONAL MUSEUM, 1897. 



auother. The specimen of emery (Oat. No. 18117, U.S.N.M.) is a mix- 

 ture in variable proportions of corundum, oxide of aluminum, with 

 magnetite, oxide of iron. The next specimen (Cat. No. 35943, U.S.N.M.), I 



known as red felsite, is a feldspar mixed with quartz and other miner- 

 als and is the result of a melting- together of the original minerals. 

 The next (Cat. No. 49085, U.S.N.M.) is an intimate mixture of zircon, 

 thorite, and limonite. 



The specimens of pyrite altering to limonite (Cat. No. 13577, U.S.N.M.), 

 of corundum to dauiourite (Cat. No. 49G11, U.S.N.M.), of almandite to 

 chlorite (Cat. No. 8121G, U.S.N.M.), of iolite to chlorophyllite (Cat. No. • 

 82456, U.S.N.M.), and of pyroxene to serpentine (Cat. No. 39327, 

 U.S.N.M.) show some of the variations resulting from metasomatosis. 

 The important instances of variable composition due to chemical sub- 

 stitution are illustrated by several series of minerals, among which 

 may be noted the amphiboles, tremolite, CaMg3(Si03)4; actinolite, 

 Ca(Mg,Fe)3(Si03)4; cummingtouite (Fe,Mg)Si03, and griinerite, FeSiOs, 

 in which the calcium and magnesium are gradually replaced by iron. 



RELATION OF WATER TO COMPOSITION. 



Water is an important constituent of many minerals, and is contained 

 in them in a variety of forms. It may exist as water of crystallization, 

 that is, simply as water; or as water of constitutiou, that is, as hydro- 

 gen and oxygen in the relative proportions to form water, though prob- 

 ably not actually existing as such. In the first instance the water is 

 readily expelled from the mineral on heating; and the proportion of 

 water so occurring is fixed, each molecule of the anhydrous substance 

 being combined with some definite number of molecules of water. In 

 the second case the mineral usually yields water only at a high tem- 

 l)erature or on continued heating, and the water so occurring probably 

 does not exist as such, but rather as its elements; the compound is 

 broken up by heat with the liberation of basic hydrogen which com- 

 bines with oxygen to form water. Finally water may combine with the 

 same anhydrous substance in several different proportions to form as 

 many dilferent compounds. These relations are illustrated by the next 

 series of twenty-four specimens. 



RELATION OF WATER TO PHYSICAL PROPERTIES. 



Water, when combined as such in definite equivalent proportions, 

 may vary in amount according to the physical conditions under which 

 the compound separates from solution. The water thus contained 

 often has a great influence on the physical properties of the substance. 

 Thus borax at ordinary temperatures separates from solution with 

 four and a half parts of water and crystallizes in oblique rhombic 

 prisms; at 80° it separates with two parts of water and crystallizes in 

 octahedrons. In some cases the water is so feebly combined that it 

 gradually separates when the comi)Ound is exposed to dry air, the sub- 



