138 
JOURNAL OF THE PLYMOUTH INSTITUTION. 
consisted of pale white phosphorescent streams of light of a pyra- 
midal form, which appeared to arise from the northern portions of 
the horizon, and extend with their pale luminous tips towards the 
zenith, and not unfrequently beyond it. Their motion was some- 
what tremulous, and they gave the impression rather of rapid 
growth and development of the light in the peculiar form which it 
took than of the actual shooting of the light itself. In both cases 
the light of the stars was usually seen through it. Its elevation 
was so considerable as to be above all clouds, and when crimson 
was the prevailing colour, the clouds through which it was visible, 
and which partly intercepted the light, assumed the colour of 
violet, deeper in proportion to the density of the cloud and paler 
at the edges. 
As we approached higher latitudes the phenomena increased in 
brilliancy, until, when near the poles, it shone out with all its 
splendour, not in the horizon, but in the zenith, diffusing its light 
on all sides, and partly compensating for the absence of the sun. 
The seasons of the year when it was most frequently visible were 
about the periods of the equinox, less during the winter, and still 
less during the summer. 
Various causes had been assigned to these phenomena, but as 
scientific investigation had advanced they had been unquestionably 
proved to be electrical in their character, and to have been deve- 
loped by atmospheric agencies, and it was the object of the present 
lecture to endeavour to show how these conditions were brought 
about- To this end it was necessary first to examine the different 
phenomena exhibited by electricity under various conditions, no- 
ticing the points of similarity between our artificial modes of deve- 
loping it, and those observed during its production in the great 
laboratory of nature. 
Electricity then was excited by various processes, evaporation 
and condensation being, however, the only two which were con- 
cerned in the subject of the evening. Every particle of moisture 
evaporated from the surface of the earth carried up a certain por- 
tion of electricity, which may be termed its normal quantity. The 
influence of heat modified the manifestations of this quantity. 
Increase of temperature increased the bulk of the vapour, and a 
development of negative electricity was the consequence. Con- 
densation, on the other hand, diminished its volume, and the 
normal quantity thus appeared in excess, or positive. 
