80 HISTOET OF MINEEALOGY. 



combination of chemical, crystallographical, physical, and 

 optical properties into some lofty generalisation, is probably 

 a triumph reserved for some future and distant period." 



Meantime, it is satisfactory to know, that if the theory of 

 mineralogy continues uncertain, its practical departments 

 have received considerable extension and improvement. 

 Other philosophers have also promoted, by their discoveries 

 in kincfred studies, the progress of this pursuit. The chemical 

 researches of Sir Humphry Davy have proved the metallic 

 bases of the earths and alkalis ; while other inquirers have 

 shown that acidity is not an absolute but a relative quality, 

 and that there are substances which, united to certain 

 bodies, act as acids, and with others become bases. "While 

 the invention of the reflective goniometer, by Dr. Wollaston, 

 with the addition of the mirror by Mr. Sang, has brought 

 the measurement of crystals to mathematical precision ; the 

 application of the blowpipe to the purposes of mineralogy, 

 by Andreas Swab, has formed an era in the study ; this 

 instrument, in the hands of Bergman, Grahn, and Berzelius, 

 has essentially contributed to perfect our inquiries into the 

 physical and chemical relations of the science. 



It may be expedient, before we enter further on the 

 subject, to give an outline of the general qualities of elemen- 

 tary substances. 



The chemist divides bodies into simple and compound ; 

 the simple being, those out of which nothing different from 

 themselves can be obtained ; the compound, those which con- 

 sist of two or more elements. It may be cited, as a fact 

 indicative of the progressive character of chemical inquiry, 

 that the number of simple substances, known in 1787, was 

 seventeen; in 1802, it was twenty-eight; whereas we are 

 now acquainted with fifty-five. If it be objected, that many 

 substances now considered simple may probably be compound 

 bodies, which more perfect instruments, or more powerful 

 re-agents, will enable us to discover ; it may be fairly assumed 

 that there are elementary substances yet undiscovered, which 

 further researches may bring to light. 



The elementary bodies at present known may be conve- 

 niently classed under the following heads : 



Five gases, or vapours oxygen, hydrogen, nitrogen, 

 chlorine, and fluorine. 



