CHEMICAL SCIENCE. 347 



tenths ; the extreme blue with which the paper was tinged when the ozone 

 was plentiful, being called ten, diminishing to zero, as the shade became less 

 strong. The presence of nitric acid would also mark this last paper. It 

 had been said that slips of this paper exposed at a high altitude had exhibited 

 a deeper shade than those exposed simultaneously at a lower one. His ob- 

 servations did not corroborate this idea, though he had exposed slips at a 

 height of eighty feet, and others at a height of only four feet, from the ground, 

 lie suggested, for the sake of uniformity and comparison, that five feet 

 should be the standard altitude in future observations. The presence of 

 ozone was usually accompanied by a low reading of the barometer, which 

 continues during the continuance of the presence of the ozone ; and was 

 usually also accompanied by ruin or snow. He had traced ozone in the at- 

 mosphere with the thermometer at 20 below, and 80 above, zero ; but in 

 general it was in large quantities in the air during falls of rain and snow. The 

 cyclometer was a sure indication of ozone, and a moist atmosphere seemed 

 necessary for its generation. He was, from this circumstance, led to compare 

 the presence of ozone with the precipitation of rain or snow, and he had these 

 results. During the last seven years there were 918 days with rain or snow, 

 and 816 days when ozone was indicated. In 1S50 there were 150 days with 

 rain or snow, and 1 10 with ozone ; in 1351, 123 days with rain or snow, and 

 135 days of ozone ; in 1852, 136 days with rain or snow, and 152 days with 

 ozone ; in 1853, 135 days with rain or snow, and 114 days of ozone ; in 1854, 

 the year of cholera, 133 days of rain or snow, with only 73 days of ozone ; 

 in 1855, 140 days of rain or snow, and 100 of ozone ; in 1856, 144 days of 

 rain or snow, and 144 of ozone. The days marked were in all cases those 

 in which the ozone exceeded 5. The small amount of ozone present during 

 the year 1354, favored the idea of a deficiency of that principle in the atmos- 

 phere during the prevalence of cholera, and the deficiency occurred during 

 almost every month of the year, though the days of rain or snow were not 

 below the average number. Here seemed to be a confirmation of the opinion 

 that there was a deficiency of ozone during the prevalence of this epidemic. 

 Southerly and easterly winds, from which rain and snow usually came, were 

 generally accompanied by indications of the presence of ozone, while north- 

 erly or westerly winds seldom led to its development. During its presence 

 there was no special condition of the atmosphere appreciable by instruments, 

 except the existence of moisture. Schonbein thought it depended on the 

 electrical condition of the atmosphere ; but, from 6,000 observations, he had 

 not been able to establish that fact. Nor was its presence always simulta- 

 neous with the Aurora Borealis, as had been supposed. The fact that a 

 moist atmosphere was necessary for its development, might account for its 

 being developed in greater quantities near the sea than elsewhere. 



ON THE CORROSION OF FRESH- WATER SHELLS. 



At a meeting of the Boston Society of Natural History, in 1856, Dr. 

 TVcinland made the following remarks upon the Corrosion of the Shells of 

 Fresh-water Clams : 



