Dr. Woods on a New Actinometer. 45 
P.S. Since the above was written I have made some experi- 
ments with the actinometer, and it has answered my expectations. 
I believe it is a reliable register of the amount of action of light. 
The most important precaution to be taken in its use is to guard 
against change of temperature, or to have a previous knowledge 
of the extent to which the change will affect the instrument. 
Increase of temperature seems to act on it in three different 
ways. It expands the liquid and confined air, causing the liquid 
to rise in proportion to the relative bulk of the vessel and tube, 
as ina thermometer. It expels the carbonic acid from the satu- 
rated solution in addition to that produced by the light: the 
amount of carbonic acid a fluid can dissolve depends on the tem- 
perature ; the higher the latter, the less gas the fluid can con- 
tain; so that if during a lengthened observation the tempera- 
ture increases, carbonic acid is expelled independent of actinic 
action. And thirdly, the higher the stationary temperature is, 
the greater seems to be the power of the light. In this, as in 
all chemical processes, heat increases the action. For instance, 
during a day’s exposure, when the thermometer was at 36° F., 
the fluid of the actinometer rose about 3 inches, whereas in 
the same period of time, when the thermometer was at 60° F., it 
rose about three times as high. Whether this increase was alto- 
gether due to the light acting more energetically on the warmer 
fluid, or partly to the higher temperature expelling some carbonic 
acid, I cannot at present decide. 
I have used a solution of peroxalate of iron, 35 grains to the 
ounce ; and as it may facilitate matters for others who may wish 
to try the instrument, I will describe in detail how I obtained 
the solution. 
I dissolved in 6 oz. of water 1043 grs. of protosulphate of 
iron ; I added 180 grs. of sulphuric acid of 1°84 spec. grav., and 
boiled; while boiling I threw in 140 grs. of nitric acid of 1°42 
spec. grav. This caused an effervescence of nitrous acid, for which 
the operator should be prepared by having the vessel of sufficient 
capacity and under a flue. The protosalt was thus converted into 
the persalt. Red prussiate of potash should now produce no 
blue colour. I then precipitated the peroxide of iron with am- 
monia and washed with warm water. I had thus 300 ers. of 
anhydrous peroxide of iron; I diffused this in 20 oz. of water, 
and added 720 grs. of crystallized oxalic acid—an excess of 
about 10 grs. of acid. This dissolved the iron and gave me a 
solution of 712 grs. of the peroxalate—about 35 grs. to the ounce. 
The bottle of the actinometer holds about 2 ounces, and I used 
this quantity of fluid without renewing it for some weeks. How 
long it may retain its power I cannot say. 
Parsonstown, December 23, 1859. 
