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because you require only to wash with pure water once or 
twice, and you will obtain a washing which contains no 
ammonia. It is only superficial. 
This ammonia on the surface is partly the result of the 
decomposition of organic matter continually taking place and 
adhering to everything in dwellings. The presence of organic 
matter is easily accounted for, but it is less easily detected 
than ammonia. It is probable that the chief cause of the 
presence of ammonia on surfaces in houses and near 
habitations is the direct decomposition of organic matter 
on the spot. If so, its presence, being more readily 
observed than organic matter itself, may be taken as a test 
and the amount will be a measure of impurity. A room 
that has a smell indicating recent residence will, in a certain 
time, have its objects covered with organic matter, and 
this will be indicated by ammonia on the surface of objects. 
After some preliminary trials, seeing this remarkable con- 
stancy of comparative results, and the beautiful gradations 
of amount, it occurred to me that the same substance must 
be found on all objects around us whether in a town or not; 
I therefore went a mile from the outskirts of Manchester 
and examined the objects on the way. Stones that not 
twenty hours before had been washed by rain, showed 
ammonia. It is true that the rain of Manchester contains it 
also, but considering that only a thin layer would be 
evaporated from these stones, it was remarkable that they 
indicated the existence of any. The surface of wood 
was examined — palings, railings, branches of trees, grass 
(not very green at the time), all showed ammonia in no 
very small quantities. It seemed as if the whole visible 
surface around had ammonia. I went into the house and 
examined the surfaces in rooms empty and inhabited, tables, 
chairs, ornaments, plates, glasses, and drawing-room orna- 
ments. A (parian) porcelain statuette, under a glass, showed 
some ammonia, a candlestick of the same material (but un- 
