384 THE CANADIAN NATURALIST. [Jime 



once, and then in a minute quantity, and cholera was at its height 

 during that period. On the 11th of September, a southerly breeze 

 set in, with indications of Ozone, and from day to day the number 

 of cases diminished. In a paper, read by me in Montreal, before 

 the American ^Association for the Advancement of Science at their 

 meeting in 1857, I stated that moisture in the atmosphere 

 was necessary for the development of Ozone ; this opinion has 

 been opposed by the only American observer, Captain Pope, during 

 some journeys that he made across the great plains in 1856 and 

 1857. He says : — " Ozone increases in quality, rapidly and regular- 

 ly, in receding from the low lands which border the Gulf of Mexico, 

 and is greatest on the table lands of the interior": he goes on further 

 to state that on the low lands animal and vegetable decomposition 

 is very rapid, and on the table lands very slow and with little 

 escape of offensive gases — therefore, on account of the moisture in 

 i the low lands, there should be more Ozone developed than in the 

 table lands. But another cause must, with all deference, be 

 brought to bear on the observations of Captain Pope, and it is a 

 very important one : for as already shown, there is a considerable 

 amount of fever and malaria in these wet, low lands, hence the 

 deduction that Ozone has been partially destroyed by the malaria, 

 consequently a less amount was indicated by the ozonometer on 

 the low lands than on the higher tablelands. These reasons will 

 account for Captain Pope's observations, without in the least dis- 

 paraging the theory, that moisture is necessary for the deve- 

 lopment of Ozone. The fact, that a humid state of the atmosphere 

 better developes Ozone, is confirmed by the observation of Dr. 

 Moffatt, Mr. Lowe, and other Europeans, who have paid attention 

 to the subject. I shall read a short extract from my 1857 paper, 

 showing the amount of precipitation as a test for determining its 

 presence in the atmosphere, and the amount of Ozone corres- 

 ponding to the days of precipitation ; and showing, also, the 

 diminished quantity of Ozone during the months of July, August, 

 and September, 1854, which were the months of the greatest 

 mortality during that visitation of cholera in this neighbour- 

 hood. During the visitation of cholera, in most places there 

 were high readings of the barometer. In 1854, here, the mean 

 reading for the month of July was 29.961 ; for August, 29.910 ; 

 and for September, 30.201 inches — the lowest reading during the 

 period was 29.619. The thermometer also ranged high — the 

 mean temperature for July being 76.2, and for August 68.31 ; the 



