The Study of Mineralogy. 237 
a true natural system can exclude none. To the establish- 
ment of such a system, a clearer view of the nature and — 
relations of physical and chemical phenomena than that 
generally received will materially aid us. 
§ 2. Matter is susceptible of changes of volume of two 
kinds. (1) Those produced from without, by variations of 
temperature and of pressure, which changes are constant 
and regular. Effecting no essential alteration in species, 
they may be called extrinsic or, as the result of external 
dynamic agencies, mechanical changes. (2) Those which 
have been described as due to “internal disturbances,” 
which effect specific alterations in character. These con- 
stitute chemical or wnat may be called intrinsic changes, 
and differ from the last in that, instead of being constant 
and regular, they are periodic and subordinated to definite 
and unforeseen relations of volume. Intrinsic changes of 
volume in matter connote chemical as distinguished from 
dynamical processes. In chemical union we have intrinsic 
contraction or condensation (variously designated as inter- 
penetration, compenetration, identification, integration, 
unification); and in chemical decomposition, intrinsic ex- 
pansion or division. These changes may be either homo- 
geneous, involving one species of matter, or heterogeneous, 
involving two or more species. The first includes so-called 
polymerization and depolymerization, which may be (es- 
cribed as homogeneous intrinsic union and homogeneous 
intrinsic division; constituting what we have called collec- 
tively chemital metamorphosis. Those intrinsic changes 
which involve two or more species we have included under 
the title of chemical metagenesis; the process being one of 
heterogeneous intrinsic union or of heterogeneous intrinsic 
division. In the former, intrinsic contraction involves vol- 
umes of unlike species, and in the latter, intrinsic expan- 
sion resolyes a species into two or more unlike species, 
The relations to volume of all such changes are most simple 
and evident in the case of gases and vapours; but the same 
laws of intrinsic contraction and expansion by volumes 
apply alike to gases and to the liquid and solid species 
