18 The Progress of Physical Discovery. [JAW* 



according to certain regular proportions ; and that the various crystals 

 of the same kind, formed upon the same nucleus, differ from each other, 

 because the laminae of each of them also decrease in different proportions. 

 This theory M. Haiiy verified by a succession of experiments, and deter- 

 mined, by analysis and trigonometrical measurement, the forms of the 

 nucleuses and elementary molecules of all known crystals. His Treatise on 

 Mineralogy, in 1801, may be said to have created a new science a science 

 worthy of the most honourable mention, not only by reason of its own 

 importance, but because it affords an example within our own memory 

 of the happy results of the experimental or Baconian method of search- 

 ing after the truth. The subject has received a fresh light from 

 M. Haiiy's Treatise on Precious Stones, in 1817, and by the researches of 

 M. Budant, reported to the French Academy in 1817 and 1818. The 

 latter has drawn attention to the mechanical combinations which take 

 place in crystallization, by the interposition of heterogeneous Substances 

 between the molecules of the real crystal, and to the extraordinary pre- 

 dominance which particular substances enjoy in such combinations, by 

 virtue of which they compel other bodies to yield to their forms and 

 laws, although those bodies compose by far the greatest portion of the 

 combination, and have peculiar forms of their own. Sulphate of iron, 

 for instance, in solution with sulphate of copper, in the proportion of 

 one part to nine of the latter, has been found to crystallize the whole 

 mass, in the form peculiar to itself, viz. an acute rhomboid, though the 

 form of crystallization of the integrant molecule of sulphate of copper is 

 an irregular oblique-angled parallelipiped. The means by which the 

 rhomboidal molecules are enabled to range themselves to form the gene- 

 ral crystal, notwithstanding the interruption of a superior number of 

 molecules of another figure, is still a mystery which presents an exten- 

 sive field for inquiry. The causes which change the forms of salts 

 from those of their primitive molecules to secondary figures, have also 

 been investigated by M. Beudant. It had been ascertained by Fourcroy 

 and Vanquelin, that the presence of uric acid gave to sea salt an octo- 

 hedral form, whilst in pure water it crystallized in tubes like its con- 

 stituent molecules; and that upon muriate of ammonia its eifect is 

 exactly the reverse. This and other instances led M. Beudant to sub- 

 mit the crystallization of salts to the influence of all circumstances 

 capable of affecting it ; and he discovered that chemical precipitates and 

 mixtures, in the same solution, vary materially the secondary forms, and 

 that they depend in some degree also upon the proportions which the 

 crystallizing principles bear to the crystallized substances. Similar 

 researches have also been applied to minerals ; but here the impossi- 

 bility of experiment necessarily limits the extent of our knowledge. It 

 was however shewn by M. Mitscherlich, in 1824, that the mutual incli- 

 nations of the surfaces of crystal of carbonate of lime varied considerably 

 with the temperature: so much so, that from 3' to 100 the difference was 

 8^'. It appears to be established that, in general, heat, distributed uni- 

 formly in a crystal, diminishes its double refraction, and, in M. Mits- 

 cherlich's opinion, that it always tends to scatter the molecules of crystal 

 the most at the point where they are condensed the closest. Further 

 observation will probably confirm this opinion ; but experience with 

 regard to crystals is at present imperfect, though its fundamental prin- 

 ciples, as a science, have, as we stated, been laid down by the Abbe 

 Haiiy. The question whether the same substance must of necessity 



