Determination of Organic Matter in Potable Water. 9 
a periodic variation of the electric forces within the insulating 
fluid between the electrodes. 
The displacement in the direction of a diminution of the 
refraction-index corresponds to a raising of the temperature 
of the insulating fluid from o, 0001 to =, 1 C. ; it increases 
with the difference of electric potentials of the electrodes and 
the viscosity of the fluid, and may be owiug to a heating of 
the fluid by friction, since the fluid-particles are moved to and 
fro and stirred round by the electric forces between the elec- 
trodes. 
The increase of the refraction-index occurs, in some fluids, 
only at the commencement of the electric action ; in others it 
alternates with a diminution at short intervals during the 
entire continuance of the electric action. 
II. Upon a Photo-chemical Method for the Determination of 
Organic Matter in Potable Water. By Dr. Albert 
E. Leeds*. 
WITHOUT attempting to revive at this moment the dis- 
cussion as to the sources of error inherent in the pre- 
sent methods employed for the estimation of organic matter 
in potable waters, or as to the relative value of the inferences 
drawn from the results arrived at by the various analytical 
methods, I shall proceed to state the outlines of the new pro- 
cess, and the experiments thus far made to test its accuracy 
and applicability. The process is founded on the fact of the 
ready decomposability of certain salts of silver when exposed, 
in the presence of organic matter and in the state of solution, 
to the action of light. As a preliminary step it was needful 
to determine whether these salts were likewise reduced, even 
when organic matter was not present ; and to this end the fol- 
lowing experiments were tried : — 
1: To 100 cubic centim. of ammonia-free distilled water, 
5 cubic centim. of a decinormal solution of nitrate of silver 
was added, and the liquid exposed to the sunlight for 48 hours 
in a well-stoppered comparison-tube. 
2. The same, but after addition of ammonia just sufficient 
to redissolve the precipitate first formed. 
3. Same as 1, but the silver first precipitated as chloride, 
and then redissolved by ammonia. 
4. Same as 1, but the silver precipitated as cyanide, and 
then redissolved in cyanide of potassium. 
5. Same as 1, but the precipitate first formed with sodium 
hyposulphite just redissolved in excess of reagent. 
* Communicated by the Author. 
