162 Prof. Draper on the Measurement of 
plies the prior action of light. The time has now probably come 
when the wants both of chemistry and physiology will require 
the conditions of that action to be determined. The field of 
organic analysis has been pretty completely reaped ; there is not 
now much to be done except by the gleaners. 
Even among those who have devoted themselves to experi- 
ments in optical chemistry, the tendency has been to the im- 
provement of the art of photography, rather than to the exami- 
nation of facts which are at its scientific basis. A great amount 
of information, destined ere long to be advantageously used, has, 
however, in that way been indirectly obtained. 
It is quite evident that in the contemplated inquiry the first 
thing to be done is to invent somé means for measuring with 
exactness the chemical force of light. More than twenty years 
ago I commenced making attempts with that view. These were, 
first by the comparison of stains made on paper covered with 
chloride or bromide of silver. Subsequently (December 1843) 
I described in this Journal, under the name of Tithonometer, an 
instrument which is well adapted to such inquiries. _ It consists 
of an arrangement by which there may be obtained from hydro- 
chloric acid, decomposed by a voltaic battery, a mixture of equal 
volumes of chlorine and hydrogen. This mixture will remain 
without any change in the dark; but on exposure to the rays of 
a lamp, the two gases unite in proportion to the quantity of the 
incident light. So great is its sensitiveness, that an electric 
spark, which lasts, it 1s said, less than the millionth of a second, 
affects it powerfully even at a distance, and sometimes occasions 
an explosion which destroys the tithonometer. 
By the aid of this instrument may be illustrated the change 
which I discovered that the sun’s rays occasion in the properties 
of chlorine, and likewise the prelimimary absorption of light 
which is necessary before chemical actions ensue. It is this 
period of preliminary absorption, in the case of the iodide of 
silver, which is of such interest in the art of photography—the 
period during which invisible impressions are made on the da- 
guerreotype plate and collodion film; capable of development im 
the one case by vapour of mercury, and in the other by pyro- 
gallic acid or protosulphate of iron. 
The tithonometer is the instrument of which Professor Bunsen 
and Dr. Roscoe, in an improved form, have made such excellent 
use. In its original construction I can still recommend it to 
those who are disposed to engage in these inquiries, as possess- 
ing extraordinary sensitiveness; and if suitable corrections for 
variations of temperature and pressure be applied, of sufficient 
exactness. 
To such I would in addition suggest another means for mea- 
