Parkes's Chemical Catechism^ by Bray ley. 219 



by the undulations of a subtle fluid of extreme rarity and immense elasti- 

 city, in a manner exactly corresponding to that in which the undulations of 

 the air impart the sensation of soimd to the ear. This subtle fluid is con- 

 ceived to pervade not only all the spaces between the sun and the planets, 

 — so as to transmit to the latter the undulations impressed upon it by the 

 action of the former, — but also the earth's atmo!?phere itself, and under 

 various modifications, all the bodies, whether solid or fluid, of which the 

 earth, so far as we know of it, consists; as water, and rocks, and minerals of 

 all kinds. This hypothesis has very recently been supported by Professor 

 Airy of Cambridge, by experimental evidence so demonstrative of its truth, 

 that Professor B. Powell of Oxford, who is well qualified, by the attention 

 which he also has paid to the subject, to pronounce an opinion upon it, has 

 recently declared that the language of comparison between the two theories 

 or hypotheses of the nature of light has ceased to be admissible; which is 

 equivalent to a declaration that the undulatory hypothesis presents a true 

 view of the nature of light ; which thus appears to be, not a substance itself, 

 but merely an affection of a substance, which has received the name of ether 

 or the ethereal medium; as sound is not a substance itself, but merely an 

 affection of a substance, which in this case is the air." 



Professor Daniell, in his first paper on his nev^r register-pyrometer, 

 given (from the Philosophical Transactions for 1830) in the Phil. 

 Mag. and Annals, N.S. vol. x. p. 191 et seq. has shown that had the 

 degrees of Wedgwood's pyrometer been valued from the determina- 

 tion of the fusing point of iron by MM. Clement and Desormesj (a 

 determination agreeing with that made by Mr. Daniell himself with 

 his original pyrometer), the result would have better corresponded 

 with the whole series of phsenomena investigated. Instead of 130° Fahr. 

 as fixed by the inventor, or 62^5 as corrected by M. Guyton, those de- 

 grees would have been estimated at about 20° Fahr. In pursuance of 

 this estimate, Mr. Brayley, when correcting the higher temperatures 

 given in Mr. Parkes's '* Table of the Effects of Heat " (p. 606-7), 

 agreeably to Mr. Daniell's researches and to those of one or two 

 other chemists, has added the corrected temperatures by Wedgwood's 

 pyrometer also, which, as the indications of that instrument, however 

 inexact, are still often referred to, will be useful to the student in 

 correcting them. We subjoin the range of temperatures from Wedg- 

 wood's zero upwards. 



Fall. Reau. Cent. Wedg. 



." Iron red heat in day-light 1272 551 /OO 



Enamel colours ^?^rw/, or ia7*n^-iw*, on ~1 i^qo fiO'i 7'Sr fi 



porcelain J 



Bronze melts, copper ^, tin ^ 1446 629 'J'f'iii 



, copper ^, tin i 1534 668 835 



Diamond burns ? 1552 676 845 14 



" Orange heat (Prinsep) 1650 719 899 



Brass melts, copper ^, zinc -^ 1672 730 9U 



Brass melts, copper 4, zinc ^ 1690 737 921 21 



Bronze melts, copper -f-i., tin tV 1750 794 955 



Silver melts '. .' 1873 818 1023 28 



Copper melts 1996 862 1091 



* " This is a technical terra used by enaraellers, glass and porcelain paint- 

 ers, &c., to denote the fixing of the colours they employ, by means of vitri- 

 faction, on the substances painted upon.'' 



2F2 



