clarke.] INTRODUCTION. 9 
and the specific gravity are the most important. From identity of 
form, or complete isomorphism between two species, we infer similarity 
of chemical structure, and the inferences thus drawn are often of the 
highest value. On the other hand, dissimilarity of form and identity 
of composition indicate isomerism, as for example in the cases of 
andalusite and kyanite; and here again we secure evidence which bears 
directly upon the study of chemical constitution. From the specific 
gravity the so-called molecular volume of a species may be computed, 
and that datum gives suggestions as to the relative condensation of a 
molecule in comparison with others of similar empirical #omposition. 
For instance, leucite and jadeite are empirically of similar type 5 but 
the latter has by far the greater density, together with superior hard- 
ness. It is, therefore, presumably more complex than leucite, and this 
supposition must be taken into account in considering its ultimate 
formula. 
From what may be called the natural history 01 a mineral, still 
another group of data can be drawn, relating to its genesis, its con- 
stant associations, and its alterability. In this connection pseudo- 
morphs become of the utmost interest, and, properly studied, shed 
much light upon otherwise obscure problems. An alteration product 
is the record of a chemical change, and as such has weighty signifi- 
cance. The decomposition of spodumene into eucryptite and aibite, 
the transformation of topaz into mica, and many like occurrences in 
nature, are full of meaning with reference to the problem now under 
consideration. Just here, however, great caution is necessary. Min- 
eralogical literature is full of faulty records regarding alterations, and 
many diagnoses need to be revised. Pseudomorphs have been named 
by guesses, based upon their external appearance, and often a com- 
pact mica has been called steatite or serpentine. In every case an 
alteration product should be identified with extreme care, both by 
chemical and by microscopical methods; for without such precautions 
there is serious danger of error. In each instance the supposed fact 
must be scrupulously verified. 
Closely allied to the study of natural alterations is their artificial 
production in the laboratory. The transformation of leucite into anal- 
cite, and of analcite back into leucite, is a case in point, and the admi- 
rable researches of Lemberg furnish many other examples. Work of 
this character is much less difficult than was formerly supposed, and its 
analogy to the methods of organic chemistry renders its results highly 
significant. Atoms, or groups of atoms, may be split off from a mole- 
cule and replaced by others, and the information so gained bears directly 
upon the question of chemical structure. With evidence of this sort 
relations appear which could not otherwise be recognized, and these 
relations may be closely correlated with observations of natural occur- 
rences. 
Evidence of the same or similar character is also furnished by the 
