520 [April 16, 



necessary before we can declare whether ozone is present in a certain 

 fixed amount, or whether it changes from hour to hour. These correc- 

 tions have yet to be found out ; those for the height of the barometer 

 and the force and direction of the wind will be considerable. 



PART II. 



The ozone tests heretofore used have appeared to me to be un- 

 satisfactory, and, on close examination, I found them to be faulty in 

 many respects. The paper used had a glaze upon it, which pre- 

 vented the solution from penetrating it ; substances, moreover, ha d 

 been used in its manufacture which acted injuriously on the tests. 

 Again, the starch of commerce was found to be impure ; it is manu- 

 factured with lime, sulphuric acid, and chlorine, substances fatal to 

 these tests. The iodide of potassium was also impure ; and there 

 has been a want of uniformity in the proportions of starch and iodide 

 of potassium employed by different observers. 



Having found out that the starch of commerce was impure, I pro- 

 cured a jar of wheat-starch in the wet state before any chemicals 

 had been used. This was steeped in distilled water, which was 

 changed every two days until quite sweet to the taste, and, although 

 by a long process, a chemically pure starch was thus obtained. 



Sir John Herschel suggested trying other vegetable starches ; I 

 therefore made starch from rice, potato, sago, and wheat. 



I obtained chemically pure iodide of potassium from Mr. Squire 

 of Oxford Street, who forwarded me two samples made expressly for 

 these experiments, the one prepared with water, the other crystal- 

 lized several times from alcohol. 



On the recommendation of Dr. R. D. Thomson, 15 grains of pre- 

 pared chalk have been added to each ounce of air- dried starch to 

 prevent it from becoming sour from any moisture that might be con- 

 tained in it ; subsequent observations have proved that this is abso- 

 lutely requisite for uniformity of effect, as the intensity of action 

 depends upon the amount of water contained in the starch, which is 

 apparent from the following experiment : 



Tests made with air-dried starch 



a. Without further drying became coloured in 5 minutes. 



(3. After further drying by fire -heat for 1 minute became coloured 

 in 7 minutes. 



