Mr. H. F. Talbot on obtaining Homogeneous Light. 35 



thoms. The depression at Montague main colliery, three miles 

 and a half to the westward, was, several years ago, calculated at 

 ninety fathoms (See sections in the Geological Transactions, 

 First Series, vol. iv. pp. 34 and 36.) 



[To be continued.] 



IX. On a Method of obtaining Homogeneous Light of great 



Intensity. By H. F. Talbot, Esq. M.P. F.R.S.* 

 A S it is a desideratum f in optical science to procure perfectly 

 ■**■ homogeneous light of sufficient brightness for many im- 

 portant experiments, I am glad to be able to communicate a me- 

 thod which in a satisfactory manner supplies that deficiency. 

 It is only requisite to place a lump of common salt upon 

 the wick of a spirit-lamp, and to direct a stream of oxygen gas 

 from a blowpipe upon the salt. The light emitted is quite 

 homogeneous, and of dazzling brightness. If instead of com- 

 mon salt we use the various salts of strontian, barytes, &c. 

 we obtain the well-known coloured flames, which are charac- 

 teristic of those substances, with far more brilliancy than by 

 any other method with which I am acquainted. 



X. Reviews, and Notices respecting New -Books. 



The Alphabet of Scientific Chemistry for the use of Beginners. By 

 James Rennie, M.A. Professor of Zoology in King's College, 

 London, &c. 



WE shall not enter into a minute examination of this work, but 

 offer a few observations, which may preserve the public from 

 being led to suppose, that because the author is Professor of Zoo- 

 logy in King's College, he is competent to write on the subject of 

 Chemistry. 



In page 76, in treating of nitric acid, a gas is mentioned which we 

 never heard of before ; it is called nitric gas. Now if this term has 

 any meaning, it must be nitric acid gas ; but unluckily this acid does 

 not exist in the aeriform state. Further on, it is stated that this nitric 

 gas combines with oxygen to form nitrous acid; — of course for nitric 

 gas, we must read nitric oxide gas } indeed a note serves to correct 

 this blunder of the text. 



After speaking of the nitric and nitrous acids, the author says, 

 that "with smaller proportions of oxygen, two other acids are formed, 

 the hyponitric, and the hyponitrous acids." No such acid as thehypo- 



* Communicated by the Author. 



f See the following remarks by Professor Airy, in the Philosophical 

 Magazine for January last, p. 29 : — " I have made no experiments here with 

 homogeneous light, and I know that, on account of its extreme faintness, 

 however obtained, little confidence can be placed in results," &c. &c. 



Also, at p. 104, Professor Airy says that it is well-known *' how faint is 

 light of anv reasonable degree of homogeneity." 



F2 



