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Transactions of the Society. 
rubber, a small free-hand section, which need not be thin, of the 
patch is made and placed on a microscope slide and covered with 
a few drops of xylol. The rubber swells and becomes semi-trans- 
parent. After a few minutes a fairly thick cover slip is placed on 
the rubber and a gentle pressure applied to the cover so that a 
thinner film of rubber is formed. Examination shows the bubbles 
of various shapes and sizes, as illustrated in fig. 3. On gradually 
increasing the pressure on the cover-slip the air bubbles disappear, 
leaving a very thin transparent film of rubber. 
Another defect which may appear in smoked sheet is what is 
known as “ rust.” When an apparently perfect sheet is pulled 
strongly, and afterwards allowed to return to its original shape, it 
is sometimes found that a thin film of dust or powder has made 
its appearance on the surface of the sheet. This defect is often 
called “ stretching rusty.” 
The rust originates from the serum which remains on the sheet 
after the coagulum is taken out of the tank or pan. The proteids, 
etc., in the serum are decomposed by bacteria, but fungi, such as 
yeasts, etc., also may play an important part in this, the result 
being a film is formed composed of bacteria, fungus spores and 
proteid matter. Fig. 4 shows rust scraped from a slightly rusty 
sheet, while fig. 5 shows the same under a higher magnification. 
The rust was stained with cotton -blue before photographing. 
Moisture in sheet can be readily distinguished from very small 
air bubbles when a small piece of the rubber is allowed to swell 
up in xylol ; the water can be seen forming a very fine emulsion 
with the xylol. Fig. 6 illustrates this. 
Ckepe. 
Dirt in either sheet or crepe can be shown by treatment with 
xylol and the application of a slight pressure, but in this case the 
dirt does not disappear as do air bubbles. The nature and origin 
of the dirt can usually be determined by this examination. Fig. 7 
shows dirt in pale crepe. Fig. 8 illustrates the same with a 
higher power. 
Very often the small particles are found to be due to small 
pieces of bark or cortex of the tree which have fallen into the 
EXPLANATION OF PLATE XII. 
Fig. 1. — The appearance of bubbles in smoked sheet when examined by trans- 
mitted light, x 2/3. 
Fig. 2. — The appearance of bubbles by reflected light, x 1/1. 
Fig. 3. — Bubbles in sheet, x 200. 
Fig. 4. — “ Rust ” film from surface of smoked sheet. X 46. 
Fig. 5. — Rust film, x 200. 
Fig. 6. — Moisture in dry smoked sheet, x 200. 
