|
9{ | | | | -38.2 | | |
\b......|4.9620| 2.3310 | 2.4180 | | -19.4 | -63.4 |
| | | | | | |
/a.|3.4900| 0.3925 | 2.7920 | | -104.2 | -76.0 |
Ghatti{ | | | | -140.8 | | |
\b.|3.2450| 0.4605 | 2.8385 | | -106.0 | -72.4 |
| | | | | | |
/a.|2.2550| 0.2900 | 1.8078 | | -106.04 | +68.0 |
Ghatti{ | | | | -147.05 | | |
\b.|2.6635| 0.2845 | 2.3360 | | -102.04 | -66.2 |
----------+------+--------+--------+-----------+------------+-----------+

The hygrometric nature of a gum or dextrin is a point of considerable
importance when the material is to be used for adhesive purposes. The
apparatus which we finally adopted after many trials for testing this
property consists simply of a tinplate box about 1 ft. square, with two
holes of 2 in. diameter bored in opposite sides. Through these holes is
passed a piece of wide glass tubing 18 in. long. This is fitted with
India rubber corks at each end, one single and the other double bored.
Through the double bored cork goes a glass tube to a Woulffe's bottle
containing warm water. A thermometer is passed into the interior of the
tube by the second hole. The other stopper is connected by glass tubing
to a pump, and thus draws warm air laden with moisture through the tube.
Papers gummed with the gums or dextrins, etc., to be tested are placed
in the tube and the warm moist air passed over them for varying periods,
and their proneness to become sticky noted from time to time. By this
means the gums can be classified in the order in which they succumbed to
the combined influences of heat and moisture. We find that in resisting
such influences any natural gum is better than a dextrin or a gum
substitute containing dextrin or gelatin. The Ghattis are especially
good in withstanding climatic changes.

Dextrins containing much starch are less hygroscopic than those which
are nearly free from it, as the same conditions which promote the
complete conversion of the starch into dextrin also favor the production
of sugars, and it is to these sugars probably that commercial dextrin
owes its hygroscopic nature. We have been in part