Page:Popular Science Monthly Volume 2.djvu/223

 the northwest. "We know by various indications that the air is highly electrified. The wind blows too freshly to place our water-dropper out-of-doors; accordingly, it is placed upon a table in the laboratory, and we notice the indication of the instrument. The air is indeed highly charged—the spot of light is thrown to a greater distance upon the scale than we have often noticed. Let us record the reading; it is as much as the positive pole of twelve Daniell cells gives. Now, lighting our Bunsen gas-burner, and placing it near our water-dropper, let us observe the spot of light; it does not change its position materially at first.

Now, after the lapse of a few minutes, it falls to a lower position upon the scale, now it goes down to zero. Now it mounts again, but in a contrary direction to its first indication, showing a slight negative charge in the air, which before was strongly charged positively. Can this be due to the presence of the flame? And, if it is due to the flame, are not great fires capable of influencing the electrical state of the atmosphere, to a greater or less extent? Such are the questions which force themselves upon us. We must, in the first place, examine our flame. Soldering a platinum wire to the copper connecting wire, and dispensing with the water-dropper, we examine the outer and inner cone of the flame. At all points in the luminous portion of the flame an indication of negative electricity is obtained; at all points in the immediate neighborhood of the flame, but exterior to it, there are signs of a positive charge in the heated air. The inner cone of partly-consumed gas is neutral, or slightly negative.

The flame, therefore, is negative, and it tends by its presence to reduce the positive charge of electricity which generally characterizes the air of fine weather to zero, or to change it to a feebly negative-charge. We learn from various writers upon meteorology that the normal electricity of the atmosphere is positive. Herschel in his work on Meteorology says, p. 125: "Out of 10,500 observations made at the Kew Observatory in 1845–'47, 10,176 showed positive, and only 364 negative electricity—the latter being almost always accompanied with heavy rain." Sir William Thomson, in the proceedings of the Literary and Philosophical Society of Manchester, March, 1862, relates some experiments which tend to show that clearing-up weather is preceded, and in many cases foretold, by a change in the atmosphere from a negative to a positive charge of electricity.

We can, therefore, conclude with some probability of truth that great fires, by changing the electrical state of the atmosphere, have an influence upon the production of rain. The state of our knowledge, however, in regard to the part that electricity plays in atmospheric changes, is very meagre. The question of the truth of the popular belief that great fires are followed by rain still remains unanswered; and we can only hope that we have thrown a little more light upon it by our research.