GROUND-AIR IN ITS HYGIENIC RELATIONS. 285 



surface of the column it arrives at than the outer air on the surface 

 of the other columu. If tliere were no liquid in the manometer, the 

 moved air would finally flow out of the manometer, and, as you see, 

 now that I have emptied the manometer, nearly blow out this candle. 



In this way I believe that I liave convinced your senses that the 

 air can move through porous soils. 



If the air in the ground can be set in motion by the pressure of 

 air or wind against its surface, there can be no doubt that the same 

 can be effected by differences of temperature, and by diffusion, and 

 generally by all causes which can produce movement of gases. As 

 long as the air in the ground is of a different temperature or compo- 

 sition from the free atmosphere, there must be exchange and motion. 

 I will only, in order to leave no doubt on your mind, direct your 

 attention to several well-known facts, which can only be explained 

 by the change of the ground-air. 



All Christian nations bury their dead in the earth, to give back to 

 dust what came from dust. There are burial-grounds in which a 

 corpse decays completely in six to seven years, and others in which it 

 takes twenty-five to thirty years. The regulations about a second 

 occupation of a ground depend on this difference, and therefore towns 

 with an equal population may be obliged to have burial-grounds of 

 very different sizes. There are other circumstances of some influence 

 on the process of decay, but the principal one is the amount of, and 

 the facility for, the change of the air in the soil. Rubble and sandy 

 soils do the work much quicker than marl and clay soils. Striking 

 experiences in this respect have been made on the French battle- 

 fields, chiefly near Sedan, where a Belgian chemist, Louis Creteur, 

 had to disinfect the large dead-pits. The bodies were buried in 

 chalk, quarry, rubble, sand, argillite, slate, marl, or clay soils, and 

 the sad work lasted from the beginning of March to the end of June. 

 In rubble the decay had taken place fully, but in clay the bodies were 

 surprisingly well kept, even after a very long time, and even the 

 features could be identified. 



As the processes of putrefaction and decay are intimately con- 

 nected with the activity of certain lower organisms, which prey upon 

 the dead, it is sufficiently clear that these organisms must thrive dif- 

 ferently in different kinds of soil. A lively change of air and water 

 in the ground appears to be of great influence in this respect ; the 

 more air in the ground the richer the underground life. 



Remarkable testimony as to the permeability of the ground, and 

 of the foundations of our houses, has been given by gas emanations 

 into houses which had no gas laid on. I know cases where persons 

 were poisoned and killed by gas which had to travel for twenty feet 

 under the street, and then through the foundations, cellar-vaults, and 

 flooring of the ground-floor rooms. As these kinds of accidents hap- 

 pened only in winter, they have been brought forward as a proof that 



