SECTIONAL TRANSACTIONS.—A. 429 
in the absence of tension. The discovery that deaminated fibres are in- 
capable of taking a ‘set’ permanent to steam, gives an opportunity of 
interpreting the molecular mechanism of the setting process. ‘The problem 
is interesting because its solution involves the combined use of chemical 
and X-ray methods, and because of the many analogies between the action 
of steam on strained fibres and the heat coagulation of egg albumin. 
Mr. J. THEwWLIs.—Fibre structure in teeth. 
X-ray analysis reveals that tooth enamel, like several other growing 
structures (e.g. asbestos, cellulose), consists of fibres. "Tooth enamel con- 
sists mainly of calcium, oxygen and phosphorus, with the possible addition 
of chlorine or fluorine, and the atoms of these elements are arranged in the 
same way as in the mineral apatite. The crystals of apatite, and hence of 
enamel, are hexagonal, and the arrangement of the enamel fibres is such that 
the hexagonal axes of the individual crystallites all tend to be parallel— 
i.e. the hexagonal axis is the fibre-axis. 
In human enamel there are two sets of fibres, one with the fibre-axis 
inclined at about 20° to the normal to the surface of the tooth, and on the 
same side as the tip; the other with the fibre-axis inclined at about 10° 
to the normal to the surface, and on the opposite side to the tip. In dog’s 
enamel the fibre-axis is at right angles to the surface of the tooth. 
Variations in degree of fibreing are found in_ histologically normal 
enamel, and three kinds of enamel can be distinguished. In human teeth 
it is found that one kind is associated with clinically immune teeth, and the 
other two with clinically susceptible teeth. 
Mr. Oar BLocu.—Principles and applications of infra-red photography 
(12.30). 
A photograph of the audience was taken at the beginning of the 
demonstration, the hall being illuminated with infra-red light. 
The renascence of interest in infra-red photography is due to the syn- 
thesis of a new tricarbocyanine dyestuff which renders possible the produc- 
tion of emulsions of considerably higher speed to infra-red radiation and 
greater cleanliness in processing than existed previously. 
Much interest has been created by the long-distance photographs in 
which there has been a considerable amount of haze penetration owing to 
the lessened scatter of infra-red light by moisture particles constituting 
haze or mist. But there are many other applications of the process to the 
arts and sciences, and examples of these are shown and discussed. They 
include astronomy, medical work, photo-micrography, investigation of 
dyed materials, spectroscopy, the detection of obliterated, over-written or 
erased writings. Some of these depend upon the power of infra-red 
radiation to penetrate beneath the surface of tissues, etc., which are opaque, 
or nearly so, to visible radiation. In short, since most photography is record 
work depending upon differentiation, wherever differences exist between the 
infra-red photograph and the ordinary photograph, the process is one which 
may have a useful application. 
AFTERNOON. 
Excursion to Daventry Broadcasting Station. 
