90 THOMAS ST EERY HUNT ON A NATURAL SYSTEM IN 



examination. The eonceiition that the chemical elements enter as snch into combination, 

 and there retain their volumes is, however, I believe, inadmissible in chemical philosophy, 

 The view which I have constantly maintained, and have set forth in the present essay, is 

 that differences in density, such as we have just considered, are not dependent on 

 variations in the hypothetical units adopted for conA'euience in calculation, but belong to 

 the species as an integ-er, and correspond to a greater or less condensation of its mass ; that 

 is to say, to the identification in a constant volume of a greater or less number of chemical 

 units. The very terms of atom and molecule, which we apply to these imaginary units 

 and to the mass, are concessions to a popular terminology borrowed from physics, and are 

 not only inadequate but to a certain extent misleading when applied to chemical opera- 

 tions. I venture in this connection to reprint the words employed in 18*74,' in the discus- 

 sion of this same question : — 



" The phenomena of chemistry lie on a plane above those of physics, and, to my appre- 

 hension, the processes with which the latter science makes us acquainted can afford, at 

 best, only imperfect analogies when applied to the explanation of chemical phenomena, to 

 the elucidation of which they are wholly inadequate. In chemical change, the uniting 

 bodies come to occupy the same space at the same time, and the impenetrability of matter 

 is seen to be no longer a fact ; the A'olume of the combining masses is confoirnded, and all 

 the physical and physiological characters which are our guides in the region of physics 

 fail us, gravity alone excepted ; the diamond dissolves in oxygen gas, and the identity of 

 chlorine and of sodium are lost in that of sea-salt. To say that chemical union is, in its 

 essence, identification, as Hegel has defined it, appears to me the simplest statement con- 

 ceivable. The type of the chemical process is found in solution, from which it is possible, 

 under changed physical conditions, to regenerate the original species. Can our science 

 affirm more than this, and are we not going beyond the limits of a sound philosophy when 

 we endeavor, by hypotheses of hard particles with void spaces, of atoms and molecules, 

 with bonds and links, to explain chemical affinities ; and when we give a concrete form to 

 our mechanical conceptions of the great laws of definite and multiple proportions to which 

 the chemical process is subordinated ? Let us not confound the image with the thing 

 itself, until, in the language of Brodie, in the discussion of this very question, ' we mistake 

 the suggestions of fancy for the reality of nature, and we cease to distinguish between 

 conjecture and fact.' " 



§ 129. We here terminate for the present the discussion of the principles which, as 

 we have claimed, serve " to enlarge and simplify the plan of chemical science," and, as a 

 necessary result thereof, to form the basis of a Natural System in Mineralogy. "We have 

 endeavored to set forth in some detail the application of these principles to the Silicates, 

 and more briefly to the non-silicated oxyds, which we have included in the order of 

 Oxydâtes, as well to the order of Metallates. We have, moreover, given {§ 110, 111) an 

 outline of a system of classification which embraces all natural mineral species. It is 

 here the place to repeat the language employed in 1867, and already cited in § 13 : 

 that all chemical species really belong to the mineral kingdom, and that " in this 

 extended sense, mineralogy takes in, not only the few metals, oxyds, sulphids, silicates, 



' A Century's Progress in Theoretical Chemistry ; being an address at the grave of Priestley, July 31, 1874, 

 reprinted from the American Cliemist for August and September, 1874. See .also " The Domain of Pliysiology," I-. 

 ]•:. and I). Philos. Mag., Octolier, 1881, § IS, VJ. 



