238 PHYSICS. 
animals exist, this being especially the case in warm countries during 
summer and the beginning of autumn, just after sunset. Reliable observations 
by scientific men on this phenomenon are very few in number; owing to the 
rarity of its occurrence. The distinguished astronomer, Bessel, while 
travelling in a boat to Bremen on the 2d of Dec. 1807, saw, during the very 
dark and rainy night, several hundreds of these lights over a piece of 
ploughed moorland. They were mostly of a bluish color, without much 
motion. In many parts of Spain and Italy, especially about Bologna, and 
on some dry hills near Nizza, great flames are sometimes seen to ascend 
from the earth to a height of twelve feet. They vanish suddenly, again 
become enkindled, and appear to be extinguished neither by wind nor rain, 
changing their places with considerable rapidity. There is no doubt that 
many of the accounts we have of the ignis fatuus, especially those of ancient 
writers, are highly tinctured with superstition and fear, to which we may 
also ascribe the idea that on approaching the light it always recedes and 
leads its follower astray. : 
According to Volta, the real ignis fatuus consists of carburetted hydrogen 
gas inflamed by electricity. Gehler supposed it to be due to a phosphorescent 
matter generated by putrefaction. Parrot considered it as a marsh gas 
(probably a mixture of phosphuretted hydrogen and other gases) inflamed by 
the atmosphere. Berzelius asserts, however, that this theory is untenable, since 
the gases mix very rapidly, and the peculiar unpleasant smell of the gas just 
mentioned would be readily perceptible where such meteors occur. Muncke 
believed that in most cases hydrogen containing phosphorus in solution, if 
not phosphuretted hydrogen itself, must be considered as the cause of the 
phenomenon, but that some accounts and appearances are attributable to 
the phosphorescence of decaying animal or vegetable matter. In other cases, 
especially in warm countries, it is exceedingly probable that luminous 
insects, like the firefly, the glow-worm, &c., have given rise to the appear- 
ance in question. 
A phenomenon of much more frequent occurrence is presented by the 
shooting stars. These luminous bodies, like stars, seem to glide rapidly 
across the heavens. As the subject has already been referred to under the 
head of astronomy, it will here be necessary to make only a few supple- 
mentary remarks. 
The elevation of the shooting stars, that is, at the beginning and end of 
their being visible, is very various, ranging, according to the observations and 
measurements of Bensenberg and Brandes, from four to thirty-five miles. A 
series of observations made in 1823, on one hundred shooting stars, gave 
4 of 1—3 miles, 15 of 3—6 miles, 22 of 6—10 miles, 35 of 10—15 miles, 
13 of 15—--20 miles, 3 of about 30 miles, 1 of 45—46 miles, 1 of about 60 
miles, and 1 of over 100 miles. Olbers, however, considered all determi- 
nations of over thirty miles as highly problematical. Among thirty-six of 
the hundred just referred to, their observers estimated that twenty-six went 
downwards, nine upwards (the angle of inclination from 6°—68° to the 
horizon), and one horizontally. The orbits of most had a westerly direc- 
tion, opposite to the motion of our earth in space. The average velocity 
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