THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



SEPTEMBER' 1843. 



XXI. On the Decomposition of Carbonic Acid Gas and the 

 Alkaline Carbonates, by the Light of the Sun ; and on the 

 Tithonotype. By John W. Draper, M.D., Professor of 

 Chemistry in the University of New York*. 



T^OR many years it has been known that the green parts of 

 -*- plants under the influence of the sunlight possess the power 

 of decomposing carbonic acid and setting free its oxygen. It is 

 remarkable that this, which is a fundamental fact in vegetable 

 physiology, should not have been investigated in an accurate 

 manner. The statements met with in the books are often far 

 from being correct. It is sometimes said that pure oxygen 

 gas is evolved, that the decomposition is brought about by the 

 so called "chemical rays;" these, and a multitude of other 

 such errors pass current. So far as my reading goes no one 

 has yet attempted an analysis of the phenomenon by the aid 

 of the prism, the only way in which it can be truly discussed. 



In a paper by Dr. Daubeny, inserted in the Philosophical 

 Transactions for 1836f, two facts which I shall verify in this 

 communication are fully established. These are, — 1st, the 

 constant occurrence of nitrogen gas in mixture with the oxy- 

 gen, an observation originally due to Saussure, or some earlier 

 writer; and, 2nd, that the act of decomposition is due to the 

 light of the sun. This latter result, obtained by employing 

 coloured glasses or absorbent media, has not been generally 

 received. Doubt will always hang about results obtained in 

 this way, and nothing but an analysis by the prism can be 

 satisfactory. It has happened, therefore, in books of credit 

 published since that time, that other interpretations of the 

 phenomena have been given. (Johnston's Agr. Chem., Lect. 

 5. § 7.) (Graham's Chem., p. 1013.) 



* Communicated by the Author. 



t Noticed in Phil. Mag. S. 3. vol. viii.p. 416.— Edit. 



Phil. Mag. S. 3. Vol. 23. No. 151. Sept. 1843. M 



