Page:Popular Science Monthly Volume 11.djvu/302

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288
THE POPULAR SCIENCE MONTHLY.

in life, put up with being left behind, although we had the first start, and I have no choice left but to resign myself.

The presence of carbonic acid in the soil and its periodical motion are for the present a bare fact. Other places, with different soils, must be examined under varying circumstances, and for longer periods, before an explanation can be attempted.

The first question which naturally meets us is that about the origin of this gas. It cannot spring from the humus of the surface, because at Munich and Dresden its quantity is smallest in the immediate neighborhood of the surface, where the humus lies, and increases in proportion to the distance thence. As the amount of carbonic acid in the ground-air generally increases the nearer this is to the ground-water, we should be at first sight inclined to assume that it evaporates from it. Is it not a fact that the ground-water which feeds wells and sources contains this gas? And is it not well known that many a well's shaft contains so much carbonic acid as to extinguish a burning candle at the distance of a few feet only from its opening? This assumption, however, is not justified for several reasons, according to the researches and experiments made at Munich: 1. There are two months in the year when the amount contained in the upper stratum, which is at the greatest distance from the ground-water, is larger than in the lower. 2. I have examined simultaneously, at given places, the amount of the gas both in the groundwater and in the ground-air, and have investigated whether, according to the laws of diffusion and absorption, either had a surplus of the gas, and was accordingly in a condition to receive or yield some of it. In every case the amount of carbonic acid in the ground-air was larger by fifty per cent, than in the ground-water, so it is clear that it is the water which receives its carbonic acid from the air, and not vice versa.

Hereby the question about the origin of the gas is certainly not yet answered, and would have been left equally unsettled if we had to ask, Whence comes all the carbonic acid which is found in the groundwater? All this water is precipitated from the atmosphere, from rain or snow. In entering the soil as meteoric water its amount of carbonic acid is exceedingly small. By help of Bunsen's analytical tables it is easy to calculate, from the quantity of carbonic acid in the atmosphere, and the absorbing power of water for this gas, that one pint of rain-water at the average temperature and barometrical pressure can only contain a very small fraction of a grain of carbonic acid, and this has been proved further by analytical experience. But the analysis of the pump-water in Munich which was poorest in carbonic acid showed that it contained on an average 112 to 1910 grain of the free gas. The ground-water at the places of examination stands about sixteen feet from the surface. It is therefore evident that the meteoric water, which is the sole source of the ground-water, must more than centuple