Ozone and Ozone Tests. 163 



it is by no means easy to procure a proper chemically-pure 

 paper ; that which was used by Dr. Moffat (ordinary writing 

 paper) is very impure. 



Requiring pure starch, and uncertain what starch would 

 be the best, I set about manufacturing it myself, without 

 the usual aid of chemicals. The substances used were wheat, 

 rice, sago, potato, arrowroot, crocus, snowdrop, tulip, narcissus, 

 arum, etc.; these were reduced to powder, steeped in distilled 

 water, which was constantly changed, until the pure starch 

 alone remained, the result being perfectly satisfactory ; a starch 

 was produced from all the above substances, which, for white- 

 ness and purity, could not be surpassed. Chemically-pure 

 iodide of potassium was procured, through Mr. James Grlaisher, 

 from Mr. Squire, of Oxford Street, made expressly for these 

 experiments ; one portion prepared with water, and another 

 crystallized several times from alcohol. 



At the recommendation of Dr. R. D. Thomson, 15 grains of 

 prepared chalk were added to every ounce of air-dried starch, 

 to prevent sourness. This, unfortunately, has the effect of 

 diminishing the sensitiveness of the tests, yet appears requisite 

 for uniformity of effect, as the intensity of action depends much 

 upon the amount of water contained in the starch. The follow- 

 ing experiment will make this apparent : — A test made with 

 air- dried starch showed the presence of ozone with 5 minutes' 

 exposure. Further drying by fire-heat for one minute retarded 

 it to 7 minutes ; 10 minutes' dryiug by fire-heat retarded it to 

 13 minutes ; 30 minutes' drying by fire-heat caused the presence 

 of ozone not to be seen, with less than 20 minutes' exposure ; 

 whilst the same powders merely air-dried, yet having the addi- 

 tion of 15 grains of chalk to each ounce of starch, also occupied 

 a 20 minutes' exposure before ozone could be detected. 



With regard to the material used for tests, either cliemi- 

 cally-pure calico or chemically -pure paper answered well; but 

 ordinary writing-paper, or any other impure material, coloured 

 in course of time. Some papers would be stained and useless 

 in twelve hours. 



We have said that the method of ascertaining the amount 

 of ozone was by means of test-slips ; but, in order to ascertain 

 if something more sensitive might not be substituted, I tried 

 as a first experiment a mixture of 10 parts of starch to 1 of 

 iodide of potassium, carefully mixed together as a dry powder 

 test; a small portion of this mixture was placed in a pill-box in 

 the open air, and in the short space of ten minutes' exposure 

 it was shown that dry powder tests were an undoubted success, 

 colouring well, and much more rapidly than the test-slips. 



The next determination was with regard to a proper for- 

 mula — Schonbein using one of 10 parts starch to 1 of iodide of 

 vol. v. — no. ii r. N 



