DR. ANDREWS ON THE CONSTITUTION AND PROPERTIES OP OZONE. 
5 
course of a few minutes. With caustic potassa in the same vessel, the increase in 
weight, for the same volume of oxygen gas, was considerably greater than with the 
solution of iodide of potassium, and at the end of the experiment it was found that 
carbonate of potassa had been formed. Now as a small quantity of free potassa is 
always produced during the action of ozone on a neutral solution of iodide of potas- 
sium, it appeared not improbable that this would seize upon a portion of the car- 
bonic acid just referred to, and thus the augmentation in the weight of the apparatus 
would depend upon two distinct causes,— the ozone reaction, and the absorption of 
carbonic acid. To prevent the occurrence of the latter, it was only necessary to 
acidulate the solution of iodide of potassium, so as to prevent the formation of free 
potassa, or to boil for some time the liquid subjected to electrolysis. The acidu- 
lation of the solution alone was found to be sufficient to prevent the carbonic acid 
from being absorbed, for when this precaution was attended to, the results were the 
same, whether the electrolyte was boiled immediately before the commencement of 
the experiment or not. With this modification, the irregularities previously observed 
in different trials disappeared, and the simple and interesting result was obtained, 
that the increase in weight of the apparatus was exactly equal to the amount of oxy- 
gen deduced by calculation from the iodine set free. 
I will now describe the chief precautions which I adopted to avoid, as far as pos- 
sible, all sources of error in the following experiments, the delicacy of which will at 
once be apparent, if we consider that not more than 40 milligrammes of ozone are 
contained, under the most favourable circumstances, in 10 litres of electrolytic oxy- 
gen ; and that it was necessary to have the arrangements so perfect, that this large 
quantity of gas (supposed to be free from ozone) should traverse the apparatus with- 
out producing any appreciable change in the united weight of the vessels D and E. 
The solution of iodide of potassium employed in all the experiments had the same 
composition, although the quantity of ozone obtained in some cases was three times 
greater than in others. It consisted of 2 grins, of iodide of potassium dissolved in 
22^ grms. of a weak solution of hydrochloric acid, containing 2 per cent, of pure 
acid. As it is difficult to procure iodide of potassium entirely free from iodate of 
potassa, I always prepared, at the commencement of each experiment, two similar 
solutions, of which one was introduced into D, and the other preserved in a ground 
stoppered vial, till the experiment was finished. The amount of free iodine in both 
was determined at the same time, and the difference taken to represent the exact 
quantity of iodine due to the ozone reaction. The correction for the iodate of 
potassa in the original solution, when reduced, rarely represented more than O’OOl 
grm. oxygen, but quantities of this magnitude must not be neglected in these experi- 
ments. 
Previous to weighing the vessels D and E, one litre of atmospheric air, deprived 
of carbonic acid and carefully desiccated, was passed through the apparatus. The 
object of this precaution was to bring every part of the apparatus into the same state 
