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Rh its original amount of carbonic acid before it reaches the wells. Thus much is certain, that the source of the carbonic acid must be sought for in the soil, and for this reason the more natural supposition is, that the soil yields the gas and gives it to the water and to the air simultaneously, but naturally with greater facility and in greater quantity to the air than to the water. The sources of the carbonic acid in the soil have now to undergo a stricter investigation; the probability is, they owe their origin to organic processes in the soil.

Allow me, now, a few more concluding and valedictory words.

In the introduction to my lectures I thought it incumbent upon me to give you my views about popular lectures in general. Those views necessarily excluded the possibility of disposing, in a few hours, of any one subject of hygiene in such a manner as to impart to my audience a thorough theoretical and practical knowledge. My hesitation in selection lasted some time. I might have collected and described the last works and tendencies in the field of hygiene, pointing out what had practically succeeded, and what ought to be aimed at further—and there is a series of interesting points and facts, forming a most grateful subject for lecturing; or I might have attempted to give you a survey, a kind of bird's-eye view of the whole domain of hygienic science. There is a charm, in the contemplation of a grand and beautiful distant landscape, in marking, first, the more interesting points; then to let the eye wander round them till it comes to the next striking point, and to enjoy to the utmost the sight of the rich view.

I might, perhaps, have succeeded in satisfying your expectations up to a certain point, but I thought it preferable to direct and to concentrate your attention mainly upon one single object which is known to every one, and which seems to be so thoroughly examined that many believe that there is very little to say about it—the air, in its hygienic relations to man's clothing, to his dwelling-place, and to the soil on which he builds.

It is such a natural error to imagine that we cannot but understand everything with which we are in continual intercourse; but, if we take the trouble of looking a little more closely into everything of which we make daily use, we shall soon make the humiliating discovery that we are acting preëminently according to instinct and tradition, and much less by personal understanding. Each period has its own task, to contribute and to create something by which civilization gains materially or ideally. But if at any one period we examine into the daily life of its generation, we shall find a great deal more that is inherited than self-acquired. This fact ought to make us modest and zealous, but also just and thankful toward our forefathers, who did not possess or know many things which we possess and know now.

As animals make use of Nature and her laws in a multifarious and surprisingly appropriate manner, so does man also. Each carrier on the road makes use of the laws of motion and friction, and of those