THE METEOROLOGY AND CLIMATE OF PLYMOUTH. 437 
water on the earth in its natural state is chemically impure, and 
contains salts of various kinds dissolved in it; hence the vapour 
arising from evaporation over the surface of the earth must be 
always charged positively. The question then arises, How do we 
find clouds sometimes positively and at other times negatively 
charged? The former is already accounted for, and we never find 
clouds negatively charged except during the prevalence of dense 
mists and fogs. The explanation is then obvious. The vesicles of 
water composing the fog are good conductors (so good that only 
those who have attempted to work an electrical machine in a damp 
atmosphere really know), and take the negative electricity from the 
ground ; when the fog lifts it forms a cloud negatively charged. 
Thirdly, the chemical changes occurring in that complex product 
called life always produce positive electricity ; and when we consider 
the millions of growing organisms, both animal and vegetable, who 
can doubt that these must prove a fertile source of atmospherical 
electricity? The last source I shall mention is combustion, involv- 
ing two of the former sources ; viz., friction of the unburnt carbon 
and heated air as they ascend against the surrounding atmosphere, 
and the chemical changes involved in the formation of vapour of 
water from the hydrogen, and also of different gases from the 
hydro-carbons of the fuel. Hence, perhaps, the frequency and 
severity of the thunderstorms in large towns and in the Black 
Country. Thunder clouds are recognized by their peculiarly dense 
and black appearance; they have been observed at all heights, 
varying from 25,700 feet to not higher than 89 feet above the 
earth’s surface. “The discharge of the electric fluid called lightning 
(which, by the way, has been recently proved to occupy only the 
one-millionth part of a second) is generally accompanied by a pecu- 
liar smell, which was formerly described as sulphurous, but now is 
known to be produced by ozone. At Stutgard it was found that 
rain and snow were sometimes positively and at other times nega- 
tively charged. Out of 150 observations of the former the rain- 
drops were found to contain positive electricity in 71 cases, and 
negative in 79; whilst out of 30 observations on snow, 24 cases 
were positive, and only 6 negative, the process of crystallization no 
doubt conducing to the increased proportion of positive electricity 
in snow. Volta further showed that when the sky was free from 
clouds the air was always positively charged and the ground nega- 
tively, and that no atmospheric electricity could be discovered in 
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