752 Proceedings of the Royal Society 
at the Royal Institution in 1864, and published in the “ Medical 
Times and G-azette,” contains the first estimate of the chemical effi- 
ciency of sunlight. “ Now, we have seen already, that by the life 
of plants great stores of energy are collected in the form of com- 
bustible matter, and that they are collected under the influence of 
solar light. I have shown you in the last lecture that some parts 
of solar light — the so called chemical rays, the blue and the 
violet which produce chemical action — are completely absorbed 
and taken away by the green leaves of plants ; and we must sup- 
pose that these chemical rays afford that amount of energy which 
is necessary to decompose again the carbonic acid and water into 
its elements, to separate the oxygen, to give it back to the atmo- 
sphere, and to collect the carbon and hydrogen of the water and 
carbonic acid in the body of the plant itself. It is not yet possible 
to show that there exists an accurate equivalent proportion between 
the power or energy of the solar rays which are absorbed by the 
green leaves of plants, and the energy which is stored up in the 
form of chemical force in the interior of the plants. We are not 
yet able to make so accurate a measurement of both these stores 
of energy, as to be able to show that there is an equivalent pro- 
portion. We can only show that the amount of energy which the 
rays of the sun bring to the rank is completely sufficient to produce 
such an effect as this chemical effect going on in the plant. I 
will give you some figures in reference to this. It is found in a 
piece of cultivated land producing corn or trees, one may reckon 
per year and per square foot of land 0-036 lb. of carbon to be pro- 
duced by vegetation. This is the amount of carbon, which during 
one year, on the surface of a square foot in our latitude, can be 
produced under the influence of solar rays. This quantity, when 
used as fuel and burnt to produce carbonic acid, gives so much 
heat that 291 lbs. of water could be heated 1° C. Now we know 
the whole quantity of solar light which comes down to one square 
foot of terrestrial surface during one second, or one minute, or one 
year. The whole amount which comes down during a year to one 
square foot is sufficient to raise, the temperature of 430,000 lbs. of 
water 1° C. The amount of heat which can be produced by fuel 
growing upon one square foot during one year is, as you see from 
these figures, a very small fraction of the whole amount of solar 
