DEFINITE FIGUEES BY THE DEPOSITION OF DUST. 
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■consists of particles strung together and having a distinctly fibrous structure (fig. 49). 
It is in this form that the dust exists when forming pictures. It has already been 
stated that magnesia dust, if allowed to deposit on mercury, forms the ordinary 
cross; on the contrary, if it be allowed to deposit on water at about 1 / , oi on a 
mixture ot water with a little alcolrol or glycerine, then the deposit which forms on 
the surface breaks up, as the dust sinks, into a figure having a cellular form (fig. 49 a). 
As before stated, other powders than magnesia act in the same way. For instance, 
a figure, corresponding exactly with those described as jiroduced by the action ot a 
pin, and magnesia, is also produced witli fine fungus spores, dust from ashes, or 
ammonium chloride. 
It is interesting to note that if the warmed glass be rubbed with a piece of flannel, 
and then exposed to the dust, in place of a fine even deposit a very strongly fibrous 
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