DR. DAUBENY'S EXPERIMENTS. 



CHAPTER XV. 



ON THE DECOMPOSITION OF CARBONIC ACID GAS AND THE ALKALINE CARBONATES BX 

 THE LIGHT OF THE SUN, AND ON THE TITHONOTYPE. 



(From the London, Edinburgh, and Dublin Philosophical Magazine for September, 1843.) 



CONTENTS : Dr. Daubeny's Experiments. Importance of the Subject. Decomposition 

 in the Prismatic Spectrum. Decomposition under Absorbent Media. Decomposition 

 is due to Light. Disturbing Causes. Analysis of Gas evolved. Decomposition of 

 Saline Bodies. Production of Nitrogen. Disappearance of Oxygen. Character of 

 Chlorophyll. 



Tithonotypes in Copper. Detithonizing Power of Gases. 



772. FOR 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 "chem- 

 ical 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. 



773. In a paper by Dr. DAUBENY, inserted in the Philosophical Transactions for 1836, 

 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 oxygen, an observation 

 originally due to SAUSSURE, or some earlier writer ; and, 2d, that the act of decompo- 

 sition is due to the LIGHT of the sun. This latter result, obtained by employing col- 

 oured 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. Johnson's Agr. Ckcm., 

 Lect. v., 7 ; Grahams Chem., p. 1013. 



774. In its connexions with modern organic chemistry and physiology, the experi- 

 ment of the decomposition of carbonic acid by leaves assumes extraordinary interest. 

 To no other single experiment can the same importance be attached. When we re- 

 member that this decomposition is the starting-point for organization out of dead mat- 

 ter, that, commencing with this action of the leaf, the series of organized atoms goes 

 forward in increasing complexity, and blood, and flesh, and cerebral matter are at its 

 terminus, it is clear that unusual importance belongs to precise views of this, the com- 

 mencing change. The beams of the sun are the authors of all organization. 



775. There is but one way by which the question can be finally settled, and that is 

 by conducting the experiment in the prismatic spectrum itself. When we consider the 



