DECOMPOSITION OF CARBONIC ACID, ETC. 
91 
drawn to the iron in order to perceive it. From six to twelve 
of these are prescribed in the course of a day. It may also 
be given mixed with bread. 
Yours, with respect, 
Alexander H. Hoff. 
ART. XXIIL-ON THE DECOMPOSITION OF CARBONIC ACID 
AND THE ALKALINE CARBONATES BY THE LIGHT OF 
THE SUN. By John W. Draper, M. D., Professor in the Uni- 
versity of New York City. 
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 investi- 
gated in an accurate manner. It is not known that any 
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. 
It is the object, of Prof. D.'s paper to prove, 1st. That the 
light of the sun is the true cause of the decomposition, the 
rays of heat and the so-called "chemical rays" not partici- 
pating therein, as Graham, Johnston, and other writers on 
vegetable chemistry suppose. 2d. That it is the yellow 
light, or most luminous ray, that is mainly concerned. 3d. 
That leaves evolve not pure oxygen gas, but a mixture of 
oxygen and nitrogen in regulated proportions. 4th. That 
there is an extensive class of salts which is decomposed 
under the same circumstances, and therefore the phenome- 
non is rather to be attributed to a digestive than to a respi- 
