Dr. Woods on a New Actinometer. 4) 
the solution, and the fluid is raised correspondingly in the tube, 
and read off on the scale C. 
This is the principle of the actinometer ; and for taking an 
occasional observation the above answers pretty well; but there 
are other circumstances to be taken into account in the construc- 
tion of a more perfect instrument. For instance, if the tempera- 
ture varies, the indications are interfered with, and that to an 
extent the greater the more sensitive the apparatus is. For it is 
obvious that the sensibility of the actinometer may be carried 
to any extent by making the tube proportion- Fig. 2. 
ally small in the bore; but then for the same 
reason any change of temperature will corre- 
spondingly affect the rising of the fluid. There- 
fore for accurate measurements it will be neces- 
sary to have a thermometer dipping into the 
solution, and a preliminary experiment made in 
order to find to what extent the change of 
temperature affects the instrument. It will 
also be convenient to have a second tube pass- 
ing through the cork, but not into the fluid, 
closed of course by a cork or stopcock when 
the instrument is in use. This tube is for 
the purpose of allowing the carbonic acid to 
escape when desirable, or for filling the vessel 
or emptying it when the solution is exhausted 
of peroxalate. The apparatus I would there- 
fore recommend, and which I have tried (only, 
however, for a few days) with apparent success 
in determining the actinic action of light, may 
be seen in fig 2. It shows half the real size of 
the instrument I used. 
A is the phial, R the tube dipping under 
the surface of the solution. C a thermometer, 
also dipping into the solution, whose graduated 
scale serves too as a scale for reading off the 
height to which the fluid ascends in the tube. 
D is the smaller glass tube passing through the 
cork, but not into the solution. When the 
fluid is raised nearly to the top of the tube 
B by the pressure of the carbonic acid, it may 
be drawn down again and set to any mark 
by opening partially the tube D until suffi- 
cient fixed air escapes to allow it to descend. SS 
I give in the following page a representation of the heights 
to which the fluid rose during different parts of two days, 
