286 THE POPULAR SCIENCE MONTHLY. 



the frozen soil did not allow the gas to escape straight upward, but 

 drove it into the house. I have told you already why I take the 

 frozen soil to be not more air-tight than when not frozen. In such 

 cases the penetration of gas into the houses is facilitated by the cur- 

 rent in the ground-air caused by the house. The house, being warmer 

 inside than the external air, acts like a heated cliimney on its sur- 

 roundings, and chiefly on the ground upon which it stands, and the 

 air therein, which we will call the ground-air. The warm air in the 

 chimney is pressed into and up the chimney by the cold air surround- 

 ing the same. The chimney cannot act without heat, and the heat is 

 only the means of disturbing tlie equilibrium of the columns of air 

 inside and outside the chimney. The warm air inside is lighter than 

 the cold air outside ; and this being so, the former must float upward 

 thi'ough the chimney, just like oil in water. It continues to do so as 

 long as fresh cold air comes into its neighborhood from outside. As 

 soon as we interruj^t this arrival, the draught into tlie chimney is at 

 an end. Any other way of looking at the action of chimneys leads to 

 erroneous views, which have many times stopped the progress of the 

 art of heating and ventilating. 



Thus our lieated houses ventilate themselves not only through the 

 walls but also through the ground on which the house stands. If 

 there is any gas or other smelling substance in the surrounding 

 ground-air, they will enter the current of this ventilation. I have 

 witnessed a case in Munich, where not the least smell of gas could be 

 detected in the street, but a great quantity of gas found its way into 

 the c;i'ound-floor room of a house where no gas Avas laid on. In 

 another case the gas always penetrated into the best heated room 

 and produced an illness of its inmates, which was taken for typhoid 

 fever. 



The movement of gas through the ground into the house may give 

 us warning tliat the ground-air is in continual intercourse with our 

 houses, and may become the introducer of many kinds of lodgers. 

 These lodgers may either be found out, or cause injury at once, like 

 gas ; or they may, without betraying their presence in any way, be- 

 come enemies, or associate themselves with other injurious elements, 

 and increase their activity. The evil resulting therefrom continues 

 till the store of these creatures of the ground-air is consumed. Our 

 senses may remain unaAvare of noxious things, which we take in, in 

 one shape or another, through air, water, or food. 



"We took rather a short-sighted view all the while, when we be- 

 lieved that the nuisances of our neighbors could only jooison the water 

 in our pumps ; they can also poison the ground-air for us, and I see 

 more danger in this, as air is more universally present, and more 

 movable, than water. T should feel quite satisfied if, by my lectures, 

 you were convinced of this important fact, if of none other. 



England has given proof how the public health can be improved, 



