some becoming old and indisposed to erupt unless angered by throwing stones down the throat.
It is evident, however, that Bunsen's theory of geyser-eruption is independent of his theory of geyser-formation. A tube or fissure of any kind, and formed in any way, if long enough, would give rise to the same phenomena. The Yellowstone geysers have mounds or chimney-like cones, but it is by no means certain that the whole length of their eruptive tubes has been built up by siliceous deposit. Bunsen's theory of eruption none the less, however, applies to these also. The more chimney-like form of the craters in the case of the Yellowstone geysers is probably due to the greater abundance of silica in solution.
| THE HYGIENIC INFLUENCE OF PLANTS. |
By MAX VON PETTENKOFER.
THE animal kingdom is, as we know, dependent on the vegetable kingdom, which must have existed on the earth before men and animals could live upon it. We may, therefore, rightly call plants children of the earth. But, in so doing we use the language of metaphor, as when we speak of "Mother Earth." The earth does not directly bring forth either plants or animals. Every plant is the child of a mother-plant, descends from one of its own kind like ourselves; but plants derive their nourishment directly from earth, air, and water, and, although generated by plants, are nourished directly by the inorganic breasts of Nature, and imply no other organic life but their own. Had plants a voice, they would more correctly speak of "Mother Earth" than ourselves.
Plants live directly on the lifeless products of earth, and we live directly on the products of plants or on animals which live on them; our existence implies other organic life, and our nourishment is not derived so directly from the earth as that of plants. Since the vegetable world comes between us, we should rather call earth our grandmother than our mother. At all events it is an affectionate relationship.
We have a natural feeling of close affinity with the vegetable world, which expresses itself not only in our love of foliage and flowers, but in our fondness for metaphors derived from the vegetable world and its processes. If we were to reckon up how many metaphors in every-day life and in poetry are derived from the vegetable world, and how many from other spheres of Nature, we should find a great excess of the former.
Our material relations to plants are also very numerous. The question we are now concerned with is not what food or what medi-
