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districts, where the results of the conglomeration of people do not 
exist, must be of typical purity ; but we find this is not a correct 
representation of the air surrounding many farmsteads. On the 
other hand, it is almost invariably contaminated with gaseous and 
solid bodies in small quantity resulting from the decay of organic 
substances. 
I have recently been engaged on some experiments in connec- 
tion with this subject. The object has been to determine the 
relative tendency of the air in different situations to induce 
fermentation. 
I have exposed certain chemical solutions capable of under- 
going fermentation near to several farmsteads, and have found 
them to ferment more rapidly there than in the open country ; 
thus proving that the air of those places contains bodies which act 
as ferments. Iam hoping, therefore, to continue these experiments 
further, and to obtain comparable results, so that the process may 
be a new and more effectual method of examining the air as a 
guide in sanitary matters. 
In the evidence given before a Committee of the House of 
Commons on the Scheme for the Supply of Water to Manchester 
from Thirlmere, it was shown that in the Vale of St. John’s, near 
Keswick, a very high rate of mortality—in proportion to the number 
of inhabitants—exists there from Consumption ; and, although I 
believe no special reason was suggested for this, yet it is probable 
that the emanations from undrained and boggy lands in the 
immediate neighbourhood may be the cause, for minute organisms 
no doubt originate from these conditions, and pollute the sur- 
rounding air. 
And now, as to the third, and what we will consider the last, 
source of air pollution,—that due to Manufactures and to Combus- 
tion. We have here at least two distinct contaminations: one in 
the form of black smoke, which we see and often remark upon; 
the other which we do not see, and which is therefore frequently 
never contemplated. 
Black smoke acts injuriously in two ways; one is by charging 
the air with small solid particles of carbon, which are then breathed 
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