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316 THE NORTHERN MICROSCOPIST. 

THE DUST FROM BOILER FLUES UNDER THE 
MICROSCOPE. 
UR attention has recently been called to the minute particles 
() of dust found in the flues leading from boiler and other fur- 
naces, and there are so many points of interest connected with this 
apparently insignificant matter that a few words on the subject may 
not be out of place. 
So long ago as 1868 Mr. J. B. Dancer published his observations 
in this direction in the form of a pamphlet, and not long since he 
kindly sent us a slide of these particles from which the phenomena 
he mentions might easily be observed. 
When coal undergoes combustion there is always a certain 
amount of ash which does not pass away in the gaseous form. A 
good deal of this is carried by the draught through the flue into 
the chimney, but a certain proportion of it, including most of the 
heavier particles, falls and is left behind on the bottom and sides 
of the flues. 
If this be examined under a power of 40 or so diameters it will 
be seen to consist largely of ferruginous matter with crystalline 
substances and a vast number of minute spheres. ‘These latter are 
best separated for inspection by washing the dust with water in 
such a manner as to let the lighter particles float away. If the 
remainder, consisting of the desired spheres and other heavier 
particles, be dried and sprinkled upon an inclined glass plate, a 
gentle tap will set the balls rolling and they can thus be easily 
collected. Some are transparent, some opaque white, and others 
variegated like polished agates. ‘The majority, however, appear 
like a rusty cannon ball or are of a brilliant black colour. Almost 
all have an aperture in them similar to that of a bomb-shell, and 
appear to be hollow, as though a gas had been imprisoned, and 
undergoing expansion from the heat, had forced an opening in the 
side of the cell and escaped. 
Mr. Dancer gives his opinion as to the probable constitution of 
some of these balls. He thinks that the transparent ones consist 
of silicates of soda and potash, which may also be present, com- 
bined with lime and alumina, in the opaque white. Most of the 
others contain iron in different combinations, as ferrous oxide, 
black magnetic oxide, and sulphide. Some of the black balls, he 
conjectures, are of metallic iron coated externally with a silicate. 
There seems a greater amount of iron present than can be 
accounted for by the quantity always present in coal-ash, and 
perhaps this represents the tear and wear of the ironwork about 


