100 THE HOME, FARM AND BUSINESS CYCLOPAEDIA. 



to the air in certain districts by stagnant water and decayed vegetable 

 matter. It is now generally acknowledged that this noxious quality is in 

 reality a subtle poison, which acts on the human system through the 

 medium of the lungs, producing fevers and other epidemics. 



Putrid matter of all kinds is another conspicuous source of noxious 

 effluvia. The filth collected in ill-regulated towns ill-managed drains 

 collections of decaying animal substances, placed too near or within private 

 dwellings are notable for their effects in vitiating the atmosphere, and 

 generating disease in those exposed to 'them. In this case also, it is a 

 poison diffused abroad through the air which acts so injuriously on the 

 human frame. 



The human subject tends to vitiate the atmosphere for itself, by the 

 effect which it produces on the air which it breathes. Our breath, when 

 we draw it in, consists of the ingredients formerly mentioned ; but it is 

 in a very different state when we part with it. On passing into our lungs 

 the oxygen, forming the lesser ingredient, enters into combination with 

 the carbon of the venous blood (or blood which has already performed its 

 round through the body) ; in this process about two-fifths of the oxygen 

 is abstracted and sent into the blood, only the remaining three-fifths being 

 expired, along with the nitrogen nearly as it was before. In place of the 

 oxygen consumed, there is expired an equal volume of carbonic acid gas, 

 s-uch gas being a result of the process of combination just alluded to. Now, 

 carbonic acid gas, in a larger proportion than that in which it is found in 

 the atmosphere, is noxious. The volume of it expired by the lungs, if free 

 to mingle with the air at large, will do no harm; but, if breathed out into 

 a close room, it will render the air unfit for being again breathed. Sup- 

 pose an individual to be shut up in an air-tight box: each breath he emits 

 throws a certain quantity of carbonic acid gas into the air filling the box ; 

 the air is thus vitiated, and every successive inspiration is composed of 

 worse and worse materials, till at length the oxygen is so much exhausted 

 that it is insufficient for the support of life. He would then be sensible of 

 a great difficulty in breathing, and in a little time longer he would die. 



Most rooms in which human beings live are not strictly close. The 

 chimney and the chinks of the doors and windows generally allow of a 

 communication to a certain extent with the outer air, so that it rarely 

 happens that great immediate inconvenience is experienced in ordinary 

 apartments from want of fresh air. But it is at the same time quite cer- 

 tain that, in all ordinary apartments where human beings are assembled, 

 the air unavoidably becomes considerably vitiated, for in such a situation 

 there cannot be a sufficiently ready copious supply of oxygen to make up 



