THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. [November 2, 1872. 
314 
added to the nitric solution produced a voluminous 
brownish black precipitate of bismutlious sulphide. 
The moist mass applied to a bright surface of iron or 
zinc gave a copious black deposit of metallic bismuth. 
From tliis it is evident that the residue consisted 
mainly of bismuth albuminate, probably associated 
with hydrochloric acid. This result explains that 
the shrinkage in volume and weight is due to an 
exterior layer of bismuth albuminate, which, con¬ 
tracting, eliminates the absorbed water of the co¬ 
agulated albumen, and that the non-action of the 
pepsin is owing to the immediate seizure of any 
exposed albuminous surface by the bismutlious chlo¬ 
ride, as long as there is any of this salt in the 
solution. The writer finds that if the syrup of pepsin 
and bismuth is diluted with water, and the resulting 
liquid saturated with sodium chloride, the pepsin is 
completely precipitated. Hence, to assay this pre¬ 
paration by means of coagulated albumen, the 
bismuth must first be removed as sulphide. 
From this accumulation of evidence the writer is 
convinced that the usual pepsin assay, when applied 
to solutions containing bismuth, is utterly unreliable. 
THE CHEMICAL EFFICIENCY OF SUNLIGHT.* 
BY JAMES DEWAR. 
Of all the processes proposed to measure varying 
luminous intensity by means of chemical effects, no” 
one has yet been expressed in strictly dynamical 
measure. This is owing to the very small amount 
of energy to be measured necessitating very pecu¬ 
liar processes for its recognition. The chemical 
actions generally induced by light are of the “trigger” 
or “relay” description; that is, bear no necessary rela¬ 
tion to the power evolved by the transformation. 
There is one natural action of light, however, of a very 
different kind, continuously at work in the decompo¬ 
sition of carbonic acid by plants, necessitating a large 
absorption of energy, and thus enabling us to ascertain 
the proportion of the radiant power retained, through 
the chemical syntheses effected. 
So far as I am aware, the following passage, extracted 
from Helmholtz’s Lectures “On the Conservation of 
Energy,” delivered at the Royal Institution in 1864, and 
published in the ‘Medical Times and Gazette,’ contains 
the first estimate of the chemical efficiency of sunlight. 
“ Now, we have seen already, that by the life of plants 
great stores of enegy 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 car¬ 
bonic acid and water into theii elements, to separate 
the oxygen, to give it back to the atmosphere, and to 
collect the carbon and hydrogen of the water and car¬ 
bonic 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 che¬ 
mical force in the interior of the plants. A Ye 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 proportion. We can only show that the 
amount of energy which the rays of the son bring to 
* Eead before the Royal Society of Edinburgh, May 6 
18/2, and printed in the ‘ Philosophical Magazine.’ 
the earth 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 produced 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 quan¬ 
tity, when used as fuel and burnt to produce carbonic 
acid, gives so much heat that 291 lb. 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 lb. 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 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 produce 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 fa 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 chlorophyl in one or 
other of its forms is the substance through which light 
becomes absorbed and chemical decomposition ensues, it 
would be 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. 
Boussing-ault’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 deter¬ 
mination of the efficiency of sunlight. By experiments 
made between the month of January and October under 
the most favourable circumstances in atmospheres rich 
in C0 2 , one square decimetre of leaf was found to de¬ 
compose in one hour, as a mean, 5*28 cub. centims. of 
C0 2 , and in darkness to evolve during the same period 
of time 0-33 cub. centims. of C0 2 . In other words, one 
square metre of green surface will decompose in twelve 
hours of the day 63'36 cub. centims. of CO.,, and produce 
in twelve hours of the night 3-96 cub. centims. of Co. 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 carbonhydrates. Boussmgault, in 
* The rate at which the leaf functions is dependent on 
the luminous intensity. The relative amounts, therefore, 
of carbonic acid decomposed through the action of the dif¬ 
ferent coloured rays are proportional to their luminous 
power; and the curve of assimilation is found to follow the 
curve of Fraunhofer. This proves that the judgment we 
torm of equal luminous impressions is in reality due to 
equal mechanical effects associated with the different coloured 
r fys. Professor Draper, of New York, in his recent paper 
“ Gn the Distribution of Heat in the Spectrum,” by dividing 
the spectrum into two portions of equal luminous intensity” 
obtained identical thermal effects by absorption. This does 
not prove that each ray has the same total energy, but only 
that in all probability those at equal distances on either side 
of the mean wave-length in the normal light-spectrum of 
the sun are identical. 
