78 T. STERRY HUNT : SUPPLEMENT TO 
“the relations of this unit-weight and unit-volume to those of the molecule to which it 
belongs are unknown.” It was not at the time of writing the above essay, apparent to the 
author, that the principles laid down elsewhere in its pages, and first enunciated more 
than thirty years earlier, necessarily lead to a simple solution of the problem of these 
unknown relations, and of that presented by the distinction between what has been 
designated “the chemical molecule” and the “molecule of the physicist.’ As already 
stated by the author in 1853, the solution of these questions is to be found in the discovery 
of ‘a definite and constant relation..... between its vapor-density and the specific 
gravity of a species in its solid state,” so that “the volume of the condensation in passing 
from a gaseous to a solid state being known, the equivalents of solids, like those of vapors, 
might be determined from their specific gravities.’ It was farther maintained that “ all 
species crystallizing in the same shape have the same equivalent volume, so that their 
equivalent weights, as in the case of vapors, are directly as their densities.” Misled by the 
notions then current, the author failed to attain the conception of the volume as a constant 
quantity, and hence wrote that “the atomic volumes of crystallized species are the com- 
parative volumes of their crystals.” The true conception of the meaning of volume was, 
however, embodied in his assertion, made at the same time, that “ the doctrine of chemical 
equivalents is that of the equivalency of volumes,” and in his declaration that “the simple 
relations of volumes which Gay Lussac pointed out in the chemical changes of gases 
apply to all liquid and solid species,” so that “the application of the atomic hypothesis to 
explain the law of definite proportions becomes wholly unnecessary.” “These views”, it 
was then said, “will be found to enlarge and simplify the plan of chemical science,” and 
they were at the same time farther characterized as “ 
basis of a sound theory of chemistry.” 
§ 18. The subject thus set forth in 1853, was farther discussed in 1867, when it was 
asserted that “the gas or vapor of a volatile body constitutes a species distinct from the 
principles which may serve as the 
same body in a liquid or solid state, the chemical formula of the latter being some multiple 
of the first ; and the liquid and solid species themselves often [probably always] constitute 
two distinct species of different equivalent weights.” ' From this it follows that freezing, 
melting and vaporization are chemical changes. The union of many volumes of vapor or 
gas, in a single volume of a liquid or a solid, is a process of chemical combination, while 
vaporization is chemical decomposition. Such decomposition is either with or without 
specific difference, and examples of these two modes are seen respectively in heterogeneous 
decomposition, and in integral volatilization, which latter is the breaking up or dissociation 
of a polymeric species into simpler forms haying the same centesimal composition. Both of 
these processes are subordinated to the same laws of pressure and temperature, and involve 
similar thermic changes in the relations of the bodies concerned. In this enlarged con- 
ception of the chemical process we find a solution of the problems above proposed, and an 
explanation of the distinction which has been drawn between “the chemical molecule” 
and “the molecule of the physicist.” That the latter has a much less simple constitution 
than the former, as calculated from the results of chemical analysis and from vapor-den- 


1 On the Theory of Chemical Changes and Equivalent Volumes, Amer. Jour. Sci., March 1853; L. E. & D. Philos. 
Mag. [4] V. 536, and in a German translation in the Chemisches Centrablatt for 1853 (p. 849) ; reprinted in Hunt’s 
Chem. and Geol. Essays, pp. 426-437 ; where also will be found (pp. 453-458) the paper of 1867, quoted above, on the 
Objects and Method of Mineralogy, from the American Journal of Science for May, 1867. 
