106 
the yellow or tithonic rays of the sunbeam have alone the 
power of fixing carbon. Heat itself acts a subordinate part, 
and immediately after sunset the leaves of plants no longer 
decompose the carbonic acid of the air, and they sink into a 
passive condition. The gaseous bodies brought from the 
ground by the roots percolate the delicate tissues, and 
escape into the atmosphere. Hence plants exhale carbonic 
acid at night and oxygen in the day time ; and Dr. Draper, 
speaking of the carboniferous era, justly concludes: — " If 
plants have once grown in these latitudes with excessive 
luxuriance, and in short spaces of time have withdrawn large 
quantities of carbon from the air, this is a result which is 
connected not so much with internal or external temperature, 
as with variations in the brilliancy of light."* I have, there- 
fore, no doubt that a register for these four months, of the 
temperature and quantity of light emitted, neglecting the 
cold nights, would at once afford an index of the growth and 
production of our wheat and other cereals. But as long as 
meteorologists confine their observations to the extremes of 
day and night, these are perfectly valueless in this agricul- 
tural inquiry. 
In the year 1846, when Mr. Lawes produced from the 
exhausted field the weight of 63J lbs. per bushel, the mean 
temperature of May, June, July, and August, was 63° ; and 
he concludes thus, — " If I could depend upon a constant 
climate in England, similar to that of 1846, I could produce 
annually 40 or 50 bushels of wheat upon an acre, with the 
same facility that I now produce 33 or 34 ; but as it is, that 
is, as the seasons are, were I to supply the proportion and 
quantity of mineral organic manures necessary to produce 50 
bushels, in a wet and cold summer it would unduly develop 
the circulating condition of the plant, its vascular structure 
would be increased to an injurious extent, and the crop 
* Chemistry of Plants, p. 96. 
