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POPULAR SCIENCE REVIEW. 
enable the observer to perceive the different lines of research 
he has adopted: — 1. The aeroscope affords a very convenient 
method for obtaining specimens really representing the nature 
of the true atmospheric dust. 2. Specimens of dust washed 
from exposed surfaces cannot be regarded as fair indices of the 
constituents of atmospheric dust, since they are liable to con- 
tain bodies which may have reached the surface otherwise than 
by means of the air, as well as others which are the result of 
local development. 3. Specimens collected by gravitation also 
fail to indicate the nature and amount of organic cells con- 
tained in the atmosphere, as the heavier amorphous and 
inorganic constituents of the dust are deposited in relative 
excess due to the method of collection. 4. Dew also fails to 
afford a good means of investigating the subject, as it is impos- 
sible to secure that all the bodies really present in a specimen 
of it should be collected into a sufficiently small space ; and, 
moreover, because it is liable to accidental contaminations, and 
also affords a medium in which rapid growth and development 
are likely to take place. 5. Distinct infusorial animalcules, 
their germs or ova are almost entirely absent from atmospheric 
dust and even from many specimens of dust collected from 
exposed surfaces. 6. The cercomonads and amoebae appearing 
in certain specimens of pure rain-water appear to be zoospores 
developed from the mycelial filaments arising from common 
atmospheric spores. 7. Distinct bacteria can hardly ever be 
detected among the constituents of atmospheric dust, but fine 
molecules of uncertain nature are almost always present in 
abundance ; they frequently appear in specimens of rain-water 
collected with all precautions to secure purity, and appear in 
many cases to arise from the mycelium developed from atmo- 
spheric spores. 8. Distinct bacteria are frequently to be found 
amongst the particles deposited from the moist air of sewers, 
though almost entirely absent as constituents of common 
atmospheric dust. 9. The addition of dry dust (which has been 
exposed to tropical heat) to putrescible fluids is followed by a 
rapid development of fungi and bacteria, although recognisable 
specimens of the latter are very rarely to be found in it while 
dry. 10. Spores and other vegetable cells are constantly pre- 
sent in atmospheric dust, and usually occur in considerable 
numbers ; the majority of them are living and capable of growth 
and development : the amount of them present in the air ap- 
pears to be independent of conditions of velocity and direction 
of wind ; and their numbers are not diminished by moisture. 
11. No connection can be traced between the numbers of 
bacteria, spores, &c., present in the air and the occurrence of 
diarrhoea, dysentery, cholera, ague or dengue ; nor between the 
presence or abundance of any special form or forms of cells, 
