Dr. Woods on a New Actinometer. 43 
The marks indicate the successive heights of the fluid at the 
different hours mentioned. The cock D was turned two or three 
times to allow the fluid to descend, as otherwise it would have 
overrun the tube. 
The thermometer is not registered in these trials, as the tem- 
perature did not vary more than one or two degrees. The acti- 
nometer was at a closed window ; the weather was dull and rainy, 
except occasionally, as at 12 o’clock December 7th, and 2 o’clock 
December 8th. In the sunshine, in the open air, the rise of the 
fluid was about three times as great as the largest space in the 
same period of time. The bore of the tube I used was Ith of an 
inch in diameter, and the solution in the bottle exposed a surface 
of about three square inches. I mention these particulars, as 
on them depends the sensibility of the instrument. The larger 
the surface of fluid exposed, of course the greater will be the 
action of the light; and the smaller the bore of the tube the 
greater will be the rise for a given evolution of carbonic acid. I 
tried a thermometer tube of about jth of an inch in diameter, 
and the fluid rose rapidly, perhaps a couple of inches in a minute, 
but in jerks and irregularly ; and I cannot yet say how far the 
bore may be diminished with utility. 
When the fluid is first exposed it shows no evolution of car- 
bonic acid, although the action of the light produces it. The 
gas is dissolved, to a certain extent, in the fluid, and until the 
latter is saturated no rise in the tube occurs. The point of 
saturation is reached after a greater or less time, according to 
the hight, generally in about fifteen minutes. This is only a 
small inconvenience ; and I got rid even of this loss of time by 
having a slight excess of oxalic acid in the fluid, and by adding 
a couple of grains of carbonate of potash, so that saturation was 
at once accomplished. But there is a more serious annoyance ; 
the fluid having been saturated with the carbonic acid, gives it 
out again even in the dark, so that until it all nearly disappears 
the fluid continues to rise ; the removal, therefore, of the actino- 
meter from the light does not immediately stop the rising. This 
would not affect the indications if the height of the fluid in the 
tube was marked immediately before removing it ; but there is a 
danger that the carbonic acid of saturation may be partially 
escaping into the space above the fluid, even during the conti- 
nuance of an observation, so that we may have two sources of 
evolution, one the action of the actinism at the moment, and the 
other the saturated state of the fluid. For instance, if a strong 
light fell on the instrument, the gas would be generated quickly ; 
if the light then diminished, the carbonic acid which would then 
be given out, might be due both to the action of the diminished 
light and to the saturated state of the fluid. I cannot say 
