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namely, the mill chimney, and soon found that many tons of the 
mysterious spheres collected annually in the furnace flues, and 
that the smaller ones went up the chimney literally in millions. 
They were particles of the siliceous ash of coal, fused in the 
furnace gases and whirled in the spheroidal state by the draught 
into the cooler atmosphere. While the inference from this incident, 
concerning air-carried impurities in a mill making fine paper, is 
perfectly plain, the occurrence of this spherical glass dust in the 
atmosphere of manufacturing centres suggests the usefulness of 
enquiries in other directions. For example, what is the physio- 
logical effect of such dust on living organisms ? What would 
take place on the leaves of trees sprinkled with such dust and 
exposed to sunlight, each little sphere acting as a “ burning glass ” ? 
What is their effect when breathed into the human lung ? 
All paper-mills are troubled at some time with slimy growths 
in their water pipes. These give rise to dirty specks in the paper. 
They may be divided into two kinds, brown slime from fresh- 
water pipes, and grey or black slime from white-water pipes 
(conveying water containing cellulose in suspension). The paper- 
maker calls them “ slime,” and they are most difficult to get rid of. 
He doesn’t know much about them except that they give trouble 
in making clean paper, and that they come and go mysteriously. 
I have found that both kinds of slime are growths of higher 
bacteria. The brown slime from fresh-water consists of iron 
bacteria, and the grey slime from white-water of bacteria that 
grow during the fermentation and decomposition of cellulose. 
Both are anaerobic and grow rapidly in a warm situation, especially 
in a slightly acid medium. 
While much time is wasted in attempting to clean pipes 
in the paper-mill by mechanical means, much better results 
may be attained by studying the conditions that produce the 
growths. 
All paper contains a small percentage of grit or sand. This is 
u subject that has not received the attention due to it, and it is 
one that can only be dealt with properly by the microscopist. 
The sources of this grit are numerous, and can only be traced up 
with the aid of a mineralogical microscope. A large proportion of 
the grit in printing-papers may be traced to the china-clay used 
in loading, and when excessive causes undue wear and tear of 
process-blocks. I have found a small percentage of grit in a 
foreign paper specially made for wiping the front lens of oil- 
immersion objectives. I do not advise the use of paper of any 
kind for cleaning photographic or other lenses. 
And so on ; in every department of a manufacturing process 
such as paper-making interesting applications of the microscope 
may be found, and the more detailed the study the wider the 
field. 
