Notices respccling New Books. 381 



by placing his joined glasses upon his drawings, at right angles to 

 them, and looking at them, in the same manner, saw them multi- 

 plied ; but the number of reflections could be calculated. Dr. 

 Brewster, by putting the reflectors in a tulie, and attaching 

 thereto, and at right angles to them, two discs of ghiss with ob- 

 jects interposed, form.s an optical instrument capable of produ- 

 cing an incalculable (if not an infinite) number of combinations, 

 by merely making the discs, or the whole instrument, to revolve 

 on its axis, while the eye looks through it. If the previous ap- 

 plication of any known principle to the construction of instru- 

 ments, is to be considered and held as embracing all future ap- 

 plications of the same principle, there can be no new inventions ; 

 for to obtain knowledge of a principle, not before known, is a dis- 

 covery, and not an inueniion: no person can invent a principle; 

 but he may apply a principle, when known, to a new purpose, 

 and this new application with the new means emplojed, is what 

 constitutes a new invention. T. 



LXV. Notices respecting New Books. 



A Treatise on the General Principles of Chemical Analyses. 

 Translated from the French oJ'L. J. Tmkard, Member of 

 the Institute cf France, Professor of Cneinistry, &c. &c. 

 By Arnold Merrick. Sao. 346 pages. 



In -the original this Treatise forms t1;e concluding volume of 

 Thenard's Chemistry, published in 18l(j. Some slight altera- 

 tions and many additions, consisting chiefly of extracts from the 

 other volumes, have been made by the translator, to render the 

 work more complete as a treatise. The translator states that 

 possessing, as we do, the excellent works of Daiton, Daw, 

 Henry, Murray, and Thomson, a translation of the whole of 

 Thenard's Elementary and Practical Treatise on Cheniistrv was 

 quite unnecessary; " but as we have no separate and consenient 

 work in English on chemical analyses, (the Essays of Berjimau 

 and Kirwan having been long out of print,) it has been judsed 

 that a translation of Thenard's treatise on that subject wo^ild 

 be a valuable acquisition to the practical chemist. It is hoped 

 that the present translation will be found sufficiently perspicuous 

 faithful and concise," and the translator modestly adds that " it 

 pretends to no other kind of merit." 



We consider the work before us as a valuable acquisition to 

 the practical chenust. Its nature will be better understood from 

 a summary of the contents than from an extract. 



Contents . 



