the Chemical Action of Light. 163 
suring the chemical action of light. It will be found well adapted 
where extreme sensitiveness is not desired. It is an aqueous 
solution of peroxalate of iron. This substance, which is of a 
golden-yellow colour, may be kept, as I found, for more than 
three years (probably for any length of time) without exhibiting 
any change, if in total darkness; but on exposure to a lamp or 
the daylight, it undergoes decomposition, carbonic acid; gas 
escaping, and the lemon-yellow protoxalate of iron precipitating. 
If set in the sunshine, it actually hisses through the escape of 
the gas. The ray which chiefly affects it is the indigo, the same 
which affects the tithonometer, and the silver compounds used 
in photography. This ray, to produce its effect, undergoes ab- 
sorption, as may easily be proved by causing a sunbeam to pass 
through two parallel strata of peroxalate, when it will be found 
that the light which has gone through the first portion is inope- 
rative on the second. 
Other properties which the solution of peroxalate of iron pre- 
sents strongly recommend it as a photometric agent to the 
chemist. Unlike solution of chlorine, it may be very conveni- 
ently confined in glass tubes by mercury. In its use there are 
two points which must be attended to :—1st, the lemon-yellow 
protoxalate must not be permitted to incrust the side of the glass 
exposed to the light, and thereby injure its transparency; 2nd, 
the solution of peroxalate must be kept nearly at a constant tem- 
perature, for its colour changes with the heat. At the freezing 
of water it is of an emerald-green ; at the boiling, of a brownish- 
yellow. With these variations of tint its absorptive action on 
light varies, and therefore its liability to be changed. 
It may be remarked that the peroxalate of iron is an excellent 
photographic agent. A piece of tissue-paper made yellow by 
being dipped in a neutral solution of it, when dried in the dark 
is very sensitive. Its invisible impressions may be developed by 
a weak solution of nitrate of silver, two grains dissolved in an 
ounce of water answering very well. 
In the application of peroxalate of iron to photometry, several 
different methods may be followed. The course I have most 
commonly taken has been to determine the quantity of carbonic 
acid produced—sometimes by volume, sometimes by weight. It 
is of course understood, that before any carbonic acid can be 
disengaged, the solution must become saturated therewith ; and 
that before we can correctly measure the quantity of light by the 
quantity of acid produced, this dissolved portion must be ascer- 
tained. In one of my photometers the expulsion of the dissolved 
gas is accomplished by exposure to a small bath of boiling water, 
in another by a stream of hydrogen. Both processes yield satis- 
factory results. 
But this method, by the determination of the produced car- 
M2 
