ON THE MOLECULAR ORIGIN OF INFUSORIA. 
59 
the atmosphere. Then the forms he assumes to be organic, are 
not necessarily so. They are exceedingly frequent among 
mineral substances, and siliceous rounded forms are common, 
which of course resist sulphuric acid. 
Numerous investigations have been made, both before and 
since M. Pasteur wrote, to determine the nature of dust floating 
in the atmosphere — of that dust, for example, which a ray of 
sunlight reveals to us, when admitted into a chamber. It con- 
sists, for the most part, of different kinds of starch corpuscles ; 
the debris of clothing, especially filaments of cotton, silk, and 
wool ; the results of different kinds of combustion, wTiether of 
coal or of wood ; various mineral bodies, globular or ovoid, 
amorphous or crystalline ; and minute fragments of insects and 
vegetables ; very rarely small seeds and microscopic animal- 
cules. 
These constituents vary to such an extent in different locali- 
ties, as to enable the observer, in some cases, to determine 
whence the dust was collected. Starch corpuscles abound in 
the neighbourhood of flour^mills and bakeries ; fragments of 
clothing where there have been crowded assemblies of persons, 
cotton and wool being predominant if the persons belong to the 
poorer classes, and silk if the upper classes have been present ; 
the products of combustion predominate in smoky localities ; 
mineral particles on the roads and highways ; seeds, fragments 
of vegetables and insects, in market places, gardens, &c., &c. 
But although these constituents of the air vary in different 
places, infusoria, produced in all of them, are identically the 
same.* 
This has been tested in various ways. The dust has been 
ransacked to discover organic germs — collected and carefully 
examined with the microscope, near the soil, and on the sum- 
mits of the highest buildings, not only in frequented, but in 
desert places ; in crowded assemblies, as well as in empty 
Grothic cathedrals and ancient vaults — in the ancient palace of 
Karnack, on the banks of th^ Nile ; in the tomb of Ehamses II. 
at the extremity of the Desert ; as well as in the central cham- 
bers of the great pyramid of Grhizeh. The chief element of the 
dust collected in these places has been found to be starch cor- 
puscles.f Large quantities of air have been drawn through 
tubes by aspirators, and collected on cotton, in distilled water, or 
projected on glass. The feathery snow, which, falling through 
the atmosphere, may be well supposed to collect its contents, 
has been melted, and the precipitate carefully collected. The 
emanations of marshy places, such as those of the Maremma in 
* Poucket’s Nouvelles Experiences/’’ p. 73, et seq. 
t Pouchet’s Heterogenie,” p. 446. 
