4 7 
ON THE AIR AND WATER OF TOWNS, 17 
these cases to assign other reasons for the feelings experienced, and to 
attribute much to the change of scene and occupation; and I have read it 
not long ago asserted, that the air in the streets of London and of the tops 
of distant hills probably differed only in the temperature. 
_ Priestley found that after shutting up a mouse in air a considerable time 
it seemed to become weak and to be slowly dying, but if he put a fresh 
“mouse into the same air at this period it instantly died. We can bear the 
gradual deterioration with ease, but we often find ourselves surprised at the 
state of the air in which we find our friends sitting, perfectly unconscious of 
any want of attention to their sanitary state. 
_ The air has often been called a general receptacle for all impurity ; 
Nature has made it a universal purifier by giving it so large an amount of 
free oxygen. It is oxygen which purifies, and bodies which are impure 
have a tendency to volatilize, after which they become pure. 
No doubt the air of a town contains a portion of all exhalations which 
arise inatown. These are such as come from living bodies in the first 
instance ; exhalations which can never be got rid of, but which it is probable 
are not at all dangerous, unless accumulated. There are also exhalations 
from the refuse matter of animals, and from combustion of fuel. These are 
the chief points. Various manufactures give out various effluvia, and no 
man that has walked through a large town with attention can have failed to 
perceive that no street is entirely free from effluvia, and that every one 
seems to have a peculiarity of its own. 
The smell is a delicate guide to this, and although custom causes us to 
forget that odour to which we are much exposed, a frequent change gives 
‘us still more acuteness, and both houses and streets may fairly he com- 
plained of when the inhabitants are little aware of it. 
_ That animals constantly give out a quantity of solid organic matter from 
the lungs may readily be proved by breathing through a tube into a bottle, 
when the liquid or condensed breath will be collected at the bottom of the 
bottle ; or by breathing through a tube into water, when a solution of the 
same substance will be found in the water. This would scarcely require 
ao if we considered that breath so frequently has an organic smell ; per- 
_ haps rather it always has an organic smell, and when it is bad the smell is 
_ ofte offensive, containing decomposing organic matter. 
_ Ifthis condensed breath be put on a piece of platinum, or on a piece of 
white porcelain and burnt, the charcoal which remains and the smell of 
_ organic matter will be conclusive. If it be allowed to stand for a few days 
(about a week is enough), it will then show itself more decidedly by be- 
coming the abode of small animals. These are rather to be styled animal- 
cules, and very small ones certainly, unless a considerable quantity of liquid 
be obtained : they may be seen with a good microscope. Animalcules are 
now generally believed to come from the atmosphere and to deposit them- 
selves on convenient feeding-places ; that is, they only appear where there 
is food or materials for their growth, and they prove of course the existence 
of that continuation of elements necessary for organic life. At the same 
time their presence is a proof of decomposing matter, as their production is 
one of the various ways in which organized structure may be broken up. 
‘Such a liquid must of course be an injurious substance, giving out constantly 
vapours of an unwholesome kind. 
__ I mentioned some time ago that I had got a quantity of organic matter 
“er rok windows of a crowded room, and I have since frequently repeated 
8. c 
