628 EEPOKT— 1888. 



gravity, hardness, optical characters, texture and structure, including crj-stallisation ; 

 while the chemical method regards the results of chemical analysis alone, and mixed 

 methods consider these in connection with crystallisation, and even endeavour to 

 take into account other phj'sical characters. The defects of all the methods 

 hitherto devised are obvious, and no system of classification can be complete which 

 does not assign a value and a place to all characters whatsoever. There exists in 

 the nature of things such an interdependence of these that a true natural system 

 can exclude none. To the establishment 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 pro- 

 duced 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 constitute chemical or what 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 distin- 

 guished from dynamical processes. In chemical union we have intrinsic contraction 

 or condensation (variously designated as interpenetration, compenetration, identi- 

 fication, integration, unification) ; and in chemical decomposition, intrinsic expansion 

 or division. These changes may be either homogeneous, involving one species of 

 matter, or heterogeneous, involving two or more species. The first includes so- 

 called polymerisation and depolymerisation, which may be described as homo- 

 geneous intrinsic union and homogeneous intrinsic division ; constituting what we 

 have called collectively chemical metamorphosis. Those intrinsic changes which 

 involve two or more species we have included under the title of chemical meta<jene- 

 sis ; the process being one of heterogeneous intrinsic union or of heterogeneous 

 intrinsic division. In the former, intrinsic contraction involves volumes of unlike 

 species, and in the latter, intrinsic expansion resolves 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 

 formed by their condensation. In all of these chemical changes temperature and 

 pressure play an important part, and beyond certain limits the extrinsic or dynamic 

 changes thereby produced themselves provoke chemical changes. These in their 

 turn are accompanied by thermic changes, the study of which is the object of 

 thermo-chemistry. 



§ 3. All chemically stable forms of matter may theoretically, by Qufficient 

 elevation of temperature, assume, even under the greatest pressure, a gaseous con- 

 dition ; the more or less dense polymeric vapours thus produced being subject to 

 intrinsic expansion or depolymerisation on diminution of pressure. By reduction 

 of temperature these pass, as may be seen under favourable conditions, through 

 successive polymerisations, or processes of intrinsic contraction, into liquid (or solid) 

 species; the passage from the vapour to the liquid being apparently continuous. 

 The ideal gas is wholly obedient to the dynamic inSuence of pressure, according to 

 Boyle's law, to which the ideal solid is wholly indifferent. These ideal forms are, 

 however, constant only within certain limited ranges of temperature and pressure, 

 beyond which even the so-called permanent gases become liquid or solid by intrinsic 

 changes. 



The regularity of the extrinsic variations in volume for gases and vapours, 

 within certain known limits, enables us for such bodies to determine their specific 

 gravity, for which purpose atmospheric air at 0°and 7fiOmm. is taken as unity. If 

 for this we substitute hydrogen gas (represented as IL = 2'0), the lightest body 

 known, at the same temperature and pressure, the specific weight of an equal 

 volume of any given vapour or gas, calculated for this standard temperature and 

 pressure, is its equivalent weight, or in tho language of the popular hypothesis, the 

 molecular weight of the species. Extending the same method from normal gases 



