On Ozone. 343 



ART. XXVI. — On Ozone. By Charles Smallwood, M.D., 

 LL.D., Professor of Meteorology in the University of McGrill 

 College, Montreal. 



[Continued from page 169.] 



The method now almost universally adopted for ascertaining 

 the presence of ozone in the atmosphere is from its action on the 

 iodide of potassium and starch. A portion of the iodine is set 

 free by the action of the oxygen and combines with the starch, 

 giving rise to the fine blue colour so distinctive of the presence of 

 ozone. The test papers are prepared by boiling one drachm of 

 pure starch in one ounce of distilled water, and when cold, by 

 adding ten grains of the iodide of potassium. This solution is to 

 be carefully and evenly spread upon good glazed paper by means 

 of a soft brush, or a sponge may be substituted. I have found 

 that " glazed " or "sized" paper is preferable to bifulous paper. 

 u Cream-laid letter-paper" is that used here — the solution is more 

 evenly spread over the surface. It is then to be quickly dried, 

 cut into pieces of about 4 inches long by 1 inch broad, and kept 

 in a dry place, free from light and air, until required. Schonbein's 

 ozoneometer consists of 750 slips of such paper, which is sufficient 

 for a year. I have found that strips of fine calico, being previously 

 well washed, and then dipped into the solution, answer equally 

 well. I have been in the habit of using them here thus prepared, 

 when long lengths were required, as in the apparatus where time 

 becomes an element of the observation. These strips of paper 

 are exposed to light, free from sun or rain, and are removed at 

 6 a.m. and 10 p.m., daily. These hours are adopted so as to cor- 

 respond with the other instrumental observations. The date and 

 hour is inscribed upon them, and the variable amount of ozone 

 indicated is estimated by comparison with a scale of tints. The 

 zero (°) of this scale, or ozoneometer as it is called, is perfectly 

 white, increasing gradually, until a very deep blue or really Hack 

 shade is obtained, which is denominated 10 ; the intermediate 

 shades are easily estimated. The mean of these two daily obser- 

 vations forms the daily mean. The deep shades in the ozone- 

 ometer, although kept from light and air, are subject to changes 

 by gradually becoming lighter; but a scale of artificial tints may 

 be used, which will be permanent. Dr. Moffatt of England, who 

 has devoted many years to observation on ozone, encloses his 

 slips of paper in a small box without a bottom, so as to keep it 

 shaded from light as well as rain. Observations were carried on 



