40 Dr. Woods on a New Actinometer. 
acid which had been given off, was to be measured: but herein 
lay the difficulty ; it was required to find the amount of gold 
precipitated from its chloride by the proto-salt in the solution of 
peroxalate which had been exposed to the light, or to mea- 
sure by weight or volume the carbonic acid which had been 
evolved during the same exposure. Now, as Mr. H. C. Draper 
says in a paper published in the Photographic Journal for last 
September, “even an enthusiast would soon tire of daily following 
out these details.” Indeed the labour would be too great, even 
if the results were rigidly exact. The latter gentleman, Mr. 
H.C. Draper, suggests the weighing of the apparatus in which 
the peroxalate is exposed, both before and after the exposure, 
taking the precaution to expel the dissolved carbonic acid by 
means of a stream of hydrogen, and thus to find the amount of 
fixed air generated by the loss of weight the apparatus sustains. 
This idea is ingenious ; but the process would be very little if at 
all less troublesome than the others. 
An easy and expeditious plan for the measurement of the 
actinic effect of light is a great desideratum. It would relieve 
the art of photography of half its failures, and would be of still 
greater advantage to its science. In order to give a helping 
hand towards its attainment, I have endeavoured to render the 
use of the peroxalate of iron as a photometric agent, both ma- 
nageable and simple. 
If after exposure any process is required to define the quantity 
of change effected by the light, especially any process involving 
knowledge of chemistry or nicety of manipulation, uo doubt it 
will be neglected, except perhaps by the few “en- Fig. 1. 
thusiasts,” whose results would therefore be of limited 
value. For this reason I have endeavoured to find a 
method of measuring the photometric changes at 
once, and by the eye only, in the following manner :— 
having nearly filled a phial with a solution of peroxa- 
late of iron, I passed through its cork a glass tube | 
into the bottle, both tube and cork fitting air-tight in 
their places. This tube, open at both ends, dipped by 
one of them under the surface of the solution, so 
that when the bottle was exposed to light, any car- 
bonie acid evolved should collect over the fluid, 
pressing it into the tube ; and a scale applied to the 
latter would show at once the amount of action going 
on. A reference to fig. 1 will explain the construc- 
tion of the Actinometer. A is a low-sized square 
phial, capable of holding about two ounces. Bis the 
tube passing through the cork into the solution of === 
peroxalate of iron, The carbonic acid collects in the space over 
ise} 
MULOUN neh — 
meena = —— 
PELE Lp a) 
