TRANSACTIONS OF THE SECTIONS, 23 
for delicate tests; indeed, chlorine and also lime in combination with sulphuric 
acid have each the power of staining the preparation of starch and iodide of potas- 
sium. Ordinary iodide of potassium is often impure, and the material itself (gene- 
rally writing-paper) is far from being chemically pure. There has also been a 
want of uniformity in the proportions of starch and iodide of potassium employed 
by different observers; in fact, the following are the formule :— 
Formula of Schénbein, 10 parts of starch to 1 of iodide of potassium. 
Formula of Moffat, 23 parts of starch to 1 of iodide of potassium. 
Formula of Lowe, 5 parts of starch (wheat) to 1 of iodide of potassium. 
I determined upon manufacturing the starch myself without the aid of the usual 
chemicals, simply steeping the solutions in distilled water, which was repeatedly 
changed until pare starch alone remained, Starch was made from wheat, rice, 
sago, arrowroot, potato, arum, snowdrop, crocus, narcissus, tulip, and hyacinth, 
which were as pure as possible and as white as snow, To Mr. Squire, of Oxford 
Street, was entrusted the manufacture of pure iodide of potassium, part prepared 
with water, and part crystallized several times from alcohol, The materials used 
were calico, specially prepared by Mr. Joseph Sidebotham, of the Strine Works, 
and a chemically pure photographic paper as well as a very porous paper. There 
was great difficulty in getting a chemically pure paper, as nearly all were worthless 
for these experiments, At the recommendation of Dr. R, D, Thomson, 15 grains 
of prepared chalk were added to each ounce of air-dried starch to prevent sour- 
ness, This precaution is requisite for uniformity of effect, as the intensity of action 
depends upon the amount of water contained in the starch. Thus,— 
ests made with air-dried starch became coloured with five minutes’ exposure, 
Tests made with starch dried by fire-heat for one minute coloured with seyen 
minutes’ exposure, 
Tests made with starch dried by fire-heat for three minutes coloured with nine 
minutes’ exposure, 
Tests made with starch dried by fire-heat for ten minutes coloured with thirteen 
minutes’ exposure. 
Tests made with starch dried by fire-heat for thirty minutes coloured with twenty 
minutes’ exposure. 
Tests made with air-dried starch, with chalk added, coloured with twenty minutes’ 
exposure, 
aire to far succeeded, I next tried a mixture of 10 parts of starch to 1 of iodide 
of potassium as a dry-powder test; ten minutes’ exposure in the open air showed 
that the powder tests were a success, being more sensitive than the test-slips. 
My next determination was with regard to a proper formula, ¢. e. what strength 
would colour quickest. Powders of different strengths were prepared, varying in 
the proportion of iodide of potassium and wheat-starch, beginning with equal por- 
tions of each, and extending as far as 30 parts of starch to 1 of iodide of potassium. 
It was found that 1 part of iodide of potassium to 5 of wheat-starch was invariably 
the darkest, the degree of density diminishing in either direction when other strengths 
were used; thus, 1 to 43 or 1 to 53 were neither so dark. Other starches require a 
different formula. 
The next series of experiments were with the view of ascertaining the effect of 
various acids, and other chemical substances, on the ozone powders when placed 
under the same bell-glass; and the result was that hydrochloric acid, nitric acid, 
nitrous acid, chloride of lime, phosphorus, iodine, carbonate of iron, or limestone on 
which an acid had been poured, each coloured the tests rapidly, whilst sulphuric 
acid, glacial acetic acid, carbonate of lime, carbonate of iron, and ammonia produced 
no effect on the powders. It was remarked that the powder tests had the adyan- 
tage of being more sensitive, and that they also retained their maximum colour, not 
afterwards fading, as is the case with the tests of Schénbein or Moffat; and there 
is yet a more important advantage to be mentioned, for by their aid we are enabled 
to say what colours the tests, and whether it is really ozone. In the experiments it 
was found that a different colour was imparted to the powder, and that the colour 
penetrated deeper according to what coloured it; so that differences of effect took 
place by which the different materials used might be recognized, which could not be 
seen by the use of test-slips. Thus,— 
