GROUND-AIR IN ITS HYGIENIC RELATIONS. 289 



its original jimount of carbonic acid before it reaches the wells. Thus 

 mucli 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 ; tlie 

 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 hesita- 

 tion in selection lasted some time. I might have collected and de- 

 scribed 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 con- 

 centrate your attention mainly upon one single object which is known 

 to every one, and which seems to be so thorouglily 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 tlie humiliating dis- 

 covery that we are acting preeminently according to instinct and tra- 

 dition, 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 mod- 

 est 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 



VOL. XI. 19 



