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or clouds, and consequently no rain. This suspended matter consists of 

 small particles of organic and inorganic substances, produced by the 

 breaking down and decay of various natural objects, of various kinds of 

 dust produced in our manufactures, and last, but not least, of small living 

 bodies knowia as micro-organisms derived from putrefying animal and 

 vegetable matters. In addition to these, there are present in foul air 

 vai-ying quantities of gases of various kinds, derived from manufactories 

 and putrefaction, and also some very deadly organic substances found in 

 the air respired by animals. It has been shown by Mr. Aitken that thei-e 

 are as many particles in one cubic inch of air taken from a room in which 

 the gas is burning as there are inhabitants in Great Britain, and in three 

 inches of gas from a Bunsen flame as many as there are inhabitants 

 in the whole world. The method by means of which the number of 

 micro-oi-ganisms in the atmosphere may be determined is extremely 

 simple, though requiring great care. A certain quantity of air is allowed 

 to filter slowly through a flask containing a layer of sterilised nutritive 

 gelatine. The micro-organisms settle upon this, and immediately begin to 

 develop into small colonies, which are visible to the naked eye. These are 

 then counted, and from the numbers thus obtained the number of 

 micro-organisms present in a given amount of air can be easily calculated. 

 The number of micro-organisms present in the air as compared with the 

 dust particles is extremely small, and they vary in different localities, 

 being least in pure country air, such as moorland air, and greatest in 

 densely populated districts and in very close, stufiy- rooms. The organic 

 matter found in the breath of animals is perhaps one of the most injurious 

 of the air impurities which we have to consider. It has been shown 

 recently that this is of such a nature as to act, when in sufficient 

 quantities, as a deadly poison. Recent experiments have shown that 

 animals which have been made to breathe the same air over and over 

 again have ultimately died, being poisoned by the organic matters 

 respired. What these poisons are, where and how they are formed, are 

 facts not yet completely known, but it is extremely important for us to 

 realise that they are of so injurious a natui-e. These are the substances 

 which give the offensive, sometimes fetid, smell so often found in shut-up' 

 living and sleeping rooms, and which cause the headache and depression 

 so constantly experienced in what is commonly called a stufiy atmosphere. 

 Worse than this, it is supposed that many of our terrible zymotic diseases 

 are in great measure assisted in their ravages by such an atmosphere, and 

 many authorities on the subject show that the disease generally known as 

 consumption, so common in England, is in large part at least caused by 

 the constant breathing of such a tainted atmosphere. 



Professor Carnelle3''s experiments were here alluded to, in which he 

 has shown that over-crowding is accompanied by foul air — as shown by 

 the presence of micro-organism and organic matter — increased death-rate 

 and reduction of mean age of death. It was further pointed out that in 

 respect of foul air, well-ventilated sewers were considerably better than 



