164 On the Measurement of the Chemical Action of Light. 
bonic acid, is only one of the numerous plans which the employ- 
ment of peroxalate of iron suggests ; for instance, we might use 
in the determination the weight of certain metals which the solu- 
tion after exposure will precipitate. Thus a portion which has 
been made and kept in the dark, may be mixed with chloride of 
gold without any action ensuing ; but if it has been illuminated, 
the amount of metallic gold precipitated is in proportion to the 
incident light. On this principle I commenced an attempt to 
determine the hourly and diurnal illumination of a given locality. 
At the bottom of a hollow metal tube, arranged as a polar axis, 
was placed a bulb containing a standard solution of the iron salt, 
and at the close of the proposed periods the weight of gold it 
could reduce was ascertamed. There is something fascinating 
in determining the quantity of light which the sun yields us by 
the quantity of gold it can produce. Upon the whole, however, 
I would recommend to those who are disposed to renew these 
attempts, to select a method depending on the volume of car- 
bonic acid, for it is always easier to make an observation than 
an experiment. 
Among the important results which may be expected from 
these new modes of photometry, and which will doubtless be 
furnished at an early period, are the hourly, diurnal, and annual 
quantities of the sunlight. These are not only important in a 
meteorological point of view, but also as respects physical geo- 
graphy, and the great interests of agriculture. The sum of 
vegetable organization is in all climates and localities a function 
of the light distributed thereto, Even so far as heat is con- 
cerned, the indications of the thermometer are of little use. It 
is not the intensity, but the absolute quantity which should be 
measured. ‘To each plant, from the moment of its germination 
to the moment of its maximum development, and the completion 
of its physiological functions, a definite quantity of heat and also 
of light must be measured out. As respects the heat in such 
inquiries, it is not the thermometer but the calorimeter which 
should be used; and as to the light, the photometers here recom- 
mended determine its quantity, but not its brilliancy, and there- 
fore answer the indications required. And since it is the light 
of the sun, and not the temperature of a locality, which is the 
effective condition of vegetable growth, we see how important, 
even in agriculture itself, these proposed determinations really are, 
I hope that these remarks may draw attention to the problem 
of the chemical action of light. To those who are disposed to 
devote themselves to such inquiries, I recommend as a photo- 
metric means a mixture of chlorine and hydrogen where great 
sensitiveness is required, and in other cases the peroxalate of iron. 
University, New York, 
July 29, 1857. 
