Parkes's Chemical Catechism by Brayley. 215 



ing, in comparative detail, the history of the subjects more summarily 

 treated of in the text or Catechism itself. 



In the preparation of the present edition, it is stated by the editor, 

 Mr. Brayley Jun., in his " Advertisement," that it has been his pur- 

 pose to introduce every new fact in Chemistry, possessing a degree 

 of general importance entitling it to notice in a popular elementary 

 work, vi^hich had been discovered since the date of the preceding edi- 

 tion (1826), and to adapt the Chemical Catechism to the actual con- 

 dition of the science j but at the same time sedulously to preserve that 

 character and general arrangement, which had acquired, for the pre- 

 vious editions, a popularity so extensive and so lasting. He has endea- 

 voured, therefore, he states in continuation, to treat every new sub- 

 ject in the manner in which the Author would himself have treated it -, 

 and in those cases in which the Author's original remarks were af- 

 fected by the progress of science, he has been careful rather to make 

 them valid, by giving them a correct turn, than to omit or materially 

 to alter them. Where also, in former editions of the Work, a parti- 

 cular statement has been made upon an important subject, which sub- 

 sequent discoveries have impugned, the Editor has frequently retained 

 the statement, adding the requisite corrections, instead of expunging 

 it ; in order that readers who have derived their views on the subject 

 alluded to from former editions, and who may refer to it as treated 

 in the present, may observe the correction, and be thus informed of 

 the truth as now known. The same course has been pursued, he in- 

 timates, in other instances, for the sake of preserving a notice of the 

 history of the science on the point in question. 



The principal subjects now introduced into the Chemical Catechism 

 for the first time, or the history of which, it is stated, has been so mate- 

 rially improved as to render them virtually new, appear to be the fol- 

 lowing: — the phsenomena of the Conduction and Radiation of Caloric; 

 the new earth Thorina, as now recognised by Berzelius j the Vegeto- 

 alkalies; the new mineral alkali, Lithia^ Bromine, Fluorine, and 

 Boronj the Metals of the Earths, as obtained and described by Ber- 

 zelius, Wohler, Oersted, and Bussyj and the new metal more re- 

 cently discovered, Vanadium. A Table of Chemical Equivalents has 

 also been added to the '' Chemical Tables " at the end of the volume ; 

 and the higher temperatures mentioned in the " Table of the Effects 

 of Heat," have been corrected, agreeably to the pyrometrical re- 

 searches of Mr. Daniell. A variety of other subjects are also noticed 

 for the first time in the present edition, but more briefly than the for- 

 mer, and chiefly in notes, in connexion with the history of the sub- 

 stances or principles to which they relate. Among these we observe 

 Saussure's experiments on the variations in the proportion of carbonic 

 acid contained in the air; the identity of gases and vapours as shown 

 by Mr. Faraday j the present state of our knowledge respecting the 

 proportions of oxygen and nitrogen in atmospheric air ; the true 

 nature of glass j the use of alumina in the form of clay in retaining 

 subterranean waters, and throwing them up as springs to the earth's 

 surface; the geological history of common salt ; the management of 

 Dr. Wollaston's test for potash ; the history of the supposed amalgam 



