226 OZONE. 



we commence investigations concerning it we start oft* 

 imder many disadvantages. Observers are contradictory 

 in their statements, antl it is a world of theory with only 

 now and then an isolated fact. 



The amount of ozone in the atmosphere is seldom if 

 ever correctly estimated ; like measuring the velocity of 

 tlic wind wc can approximate towards correctness; but 

 to say that we can accurately determine the exact quan- 

 tity would be at least a very doubtful statement. Yet 

 tliis determination must be the foundation of all accurate 

 knowled<re in relation to its eftccts. Various methods 

 have been employed to ascertain the presence of ozone in 

 tlie atmosphere, among which are iodide of potassium 

 with starch, and also with various other compeunds. 

 Gum guaiacmn. thallium, etc., all of which I have exper- 

 imented with, but 1 shall s})cak in detail of one only. 

 The iodide uf potassium and starch is perhajis the easiest 

 to manage, and with pure iodide free from iodate with 

 ivhich it is frequently combined, it may be prepared by 

 dissolving one part of the iodide in two hundred parts of 

 distilled water, to which solution ten parts of finely pow- 

 dered starch may be added, and all gently heated until it 

 is thickend like starch used by the laundress. In this so- 

 Uition while warm dip strips of bleached cotton cloth 

 which have been thorovighly cleansed and previously 

 dried, squeeze out the excess of starch and hang before a 

 hot fire to dry. Cloth thus prepared should be white and 

 quite stift'with the starch. Next cut in strips one inch 

 wide and three inches long, and put up in ground stop- 

 pered bottles. In this way it may be kept any length of 

 time desired. When wanted a strip can be removed and 

 exposed in a sheltered situation, away from sunlight and 

 manifest causes of impurit}' in the atmosphere. 



A wooden box open at the top and bottom two feet 



