of Edinburgh, Session 1871-72. 753 
heat which can be produced by the solar rays. It is only the 
1477th part of the whole energy of solar light. It is impossible 
to determine the quantity of solar heat so accurately that we could 
detect the loss of so small a fraction as is absorbed by plants and 
converted into other forms of energy. Therefore, at present, we 
can only show that the amount of solar heat is sufficient to pro- 
duce the effects of vegetable life, but we cannot yet prove that this 
is a complete equivalent ratio.” This estimate is, strictly speaking, 
the mean agricultural efficiency of a given area of land, cultivated as 
forest, and considering that active growth only takes place during five 
months in the year, we may safely adopt g^o-th of the total energy 
of sunlight as a fair value of the conserved power, on a given area 
of the earth’s surface in this latitude during the course of the 
summer. As chlorophyll in one or other of its forms is the sub- 
stance through which light becomes absorbed, and chemical 
decomposition ensues, it would he interesting to acquire some idea 
of the storage of power, effected by a given area of leaf surface 
during the course of a day, and to compare this with the total 
available energy. Here we are dealing with strictly measurable 
quantities, provided we could determine the equation of chemical 
transformation. 
Boussingault’s recent observations on the amount of carbonic acid 
decomposed by a given area of green leaf seem to me to afford 
interesting data for a new determination of the efficiency of sun- 
light. In his experiments made between the months of January 
and October under the most favourable circumstances in atmo- 
spheres rich in C0 2 one square decimetre of leaf has decomposed in 
one hour, as a mean 5'28 cc of C0. 2 , and in darkness evolves in the 
same period of time 033 cc of C0 2 . In other words, one square metre 
of green surface will decompose in twelve hours of the day, 6336 
cc of C0 2 , and produce in twelve hours of the night 396 cc of C0 2 . 
This quantity of carbonic acid decomposed does not represent 
the whole work of sunlight for the time, as water is simultaneously 
attacked in order to supply the hydrogen of the carbo-hydrates. 
Boussingault, in summing up the general results of his laborious 
researches on vegetable physiology, says, “ Si l’on envisage la vie 
vegetale dans son ensemble, on est convaincu que la feuille est la 
premiere etape des glucoses que, plus ou moins modifies, on trouve 
