'276 I'HILOSOPHICAL TRANSACTIONS. [ANNO 1J82. 



periments of Dr. Black and Wilke, and which has been afterwards much eluci- 

 dated bv Dr. Crawford, who followed the experiments of Dr. Irwin. By follow- 

 ing this analogy, it seems that, as the vapours on their condensing lose part of 

 their latent heat, on account of their capacity being diminished, so they part 

 with some electric fluid. Hence originates the positive electricity, which is al- 

 ways more or less predominant in the atmosphere, when the sky is clear, viz. at 

 that height where the vapours begin to be condensed. Accordingly, the atmos- 

 pherical electricity is stronger in fogs, in which case the vapours are more con- 

 densed, so as to be almost reduced into drops, and is still stronger when thick 

 fogs become clouds. 



Hitherto we have accounted for the positive atmospherical electricity; but it is 

 easy to account for clouds negatively electrified; lor when a cloud, positively 

 electrified, has been once formed, its sphere of action is extended a great way 

 round, so that if another cloud comes within that sphere, its electric fluid, agree- 

 ably to the well known laws of electric atmospheres, must retire to the parts of 

 it which are the most remote from the first cloud; and from thence the electric 

 fluid may be communicated to other clouds, or vapours, or terrestrial promi- 

 nences. Thus a cloud may be electrified negatively, which cloud, after the 

 same manner, may occasion a positive electricity in another cloud, &c. This 

 explains, not only the negative electricity, which is often obtained from the at- 

 mosphere in cloudy weather; and the frequent changes from positive to negative 

 electricity, and contrarywise in stormy weather; but also the waving motion often 

 observed in the clouds, and the hanging down of them, so as nearly to touch 

 the earth. After the fore-mentioned discoveries we need no longer wonder at 

 the appearance of lightnings in the eruptions of volcanos, as was particularly 

 observed in the late dreadful eruption of Mount Vesuvius. The few experiments 

 I have made show, that the quantity of smoke, but much more the rapidity 

 with which it is produced, tends to increase the electricity which arises from 

 combustion, &c. How great must then be the quantity of electricity that is 

 produced in such eruptions ! 



