118 MESSES. A. V. HAECOUET AND W. ESSON ON THE LAWS OE CONNEXION 
this liberation of iodine begins may be most accurately observed by the help of a little 
starch previously added to the liquid. 
In all the experiments whose results are here recorded the same apparatus and 
methods were employed. The apparatus consisted of a glass cylinder about 12 inches 
high and 3 broad, round which, within 2 % inches of the top, a fine line was etched: into 
the cylinder, through a bung closing its mouth, passed a thermometer and an inverted 
funnel-tube ; the latter, which occupied the axis of the cylinder and reached nearly to 
the bottom, was connected with an apparatus for generating carbonic acid ; a third hole 
in the bung, which served to give access to the contents of the cylinder, was ordinarily 
closed with a small cork. The method of performing an experiment was as follows. A 
quantity of water, purified from organic matter by redistillation Off potassic permanga- 
nate, was boiled for some time to expel dissolved oxygen, and then allowed to cool in 
an atmosphere of carbonic acid. When cold it was poured into the cylinder which had 
previously been filled with carbonic acid, and a current of this gas, ascending in large 
bubbles from the inverted funnel, was kept passing through the liquid until the close of 
the experiment. These bubbles of gas, whose diameter is nearly half that of the cylinder, 
serve the purpose of stirring the fluid constantly and to any required degree, without 
causing loss or exposure to the air, and without danger to the thermometer. Measured 
quantities of the standard solutions were then introduced according to the particulaf 
experiment which was to be made ; for example, 50 cub. centims. of hydric sulphate 
and 10 cub. centims. of potassic iodide, together with hi all cases a few cub. centims. of 
starch. Next, the liquid having been brought to the proper temperature, the cylinder 
was placed on a level stand, and so much more water added as would make the upper 
surface of the fluid exactly coincide with the line etched upon the vessel. In every 
experiment the same quantity of the remaining ingredient was taken, namely, 10 cub. 
centims. of a dilute solution of hydric peroxide*; thus the total volume was in every 
experiment the same. Two operations, however, had still to be performed before starting 
the reaction by the addition of peroxide. First, it was necessary to make sure in each 
case that the fluid contained no trace of any oxidizing or reducing substance. To this 
end the colour of the fluid was brought to the faintest possible blue by the addition, 
* The solution employed in most of these sets of experiments was prepared hy dissolving a weighed quantity 
of pure sodic peroxide in water, and adding twice the quantity of hydric sulphate required to neutralize it. The 
alkaline solution of sodic peroxide, and the solution obtained by neutralizing this with hydric sulphate, decom- 
pose slowly but perceptibly from day to day ; the addition of a second proportion of acid renders the solution 
almost absolutely stable. In some sets of experiments a pure solution of hydric peroxide was employed, which 
was obtained by distilling the acidified solution of sodic peroxide. The first portions of the distillate consist of 
water containing but little peroxide ; as the acid liquid becomes more concentrated and the temperature rises, 
hydric peroxide comes over in considerable quantities, but finally decomposition sets in, and the liquid in the 
retort effervesces with escaping oxygen. About A of the peroxide may thus be collected in a simple distillation. 
The proportion is not much increased by distilling under a diminished atmospheric pressure. It is probable 
that by supplying water continuously so as to keep the fluid in the retort at that degree of concentration at 
which the peroxide begins to come over in quantity, nearly the whole might be distilled. 
