92 Abstraction of Nitrogen from tke Atmosphere. 
moisture and atmospheric air. The other metals, for the 
greater part, are similarly circumstanced. 
The iron which is found in the noncontemporaneous veins 
of the primitive, and of the transition crystalline rocks, and 
all the iron of the mechanical rocks, is in the state of the red 
oxide, and of the hydrate of this oxide. Our common ores 
of iron, are the hy drates, all of which are of modern origin, 
when compared with the iron of the primitive class. Whence 
the oxygen of these ores of iron, if not from the atmos- 
phere? 
The objection to ae abstraction of oxygen by means of 
iron, &c. is, that plants, when exposed to light, have the pro- 
rty of decomposing carbonic acid, retaining the carbon, 
and setting the oxygen free. It is very true, however, that this 
process goes on only whilst they are acted upon by the direct 
ray of the sun; for, in the night, it is equally certain, that 
carbonic acid is extricated, and not oxygen- 
not the relationship between the quantity of carbonic acid 
given out in the night, with the oxygen liberated in the day, 
uncertain as we are whether the oxygen required to produce 
the carbonic acid of the night is greater or less than was 
furnished by the day, I have thouphs it better, in the present 
state of our knowledge, merely to state the facts, leaving the 
application till experiment shall have given us more certain 
ta. 
OF GYPSUM. 
No argument can be drawn from gypsum, which has a di- 
rect relationship with the subject matter in question ; but in- 
directly it exhibits important features, which seem to me to - 
be worthy of consideration. 
To the accurate observations of Mr. Eros we are in- 
debted for the important fact, that as yet no gypsum has been 
_ found in the primitive class of rocks. All the localities cited 
e and water are alone found in the primitive class; for ne 
‘sulphuric acid, combined or uncombined, is known to exist 
contemporaneously with the minerals of that age. 
4ypsum, as a rock, is found in the transition class, proba- 
bly anterior to the a deposition of salt, as in the Alps; 
