Thereupon the stop-cock is again turned, the water is held at the point N, and the pressure is indicated by the height of the liquid in the gauge. The degree of pressure is the measure of the consistency of the glue-jelly.
The initial contact between the jelly and the rubber diaphragm is always the same, all jellies therefore having the same initial pressure. When the diaphragm is forced down into the jelly, the pressure required depends entirely upon the resistance that the jelly offers. The slightest difference in the consistencies of the various jellies will alter the pressure required, the differences being accurately recorded by the gauge upon the scale.
Advantages of this apparatus are that the relative value of the jelly compared with the standard is expressed in concrete figures; the method of operation is simple; and the instrument is so sensitive that it will record a change in reading between two samples of glue in which a difference of ¹⁄₁₀₀th ounce of dry glue is used. Repeated tests may be made on the same jelly, as the surface is not broken. With a little practice a single glue may be tested in twenty seconds, or less.
THE FINGER TEST
One of the most satisfactory methods of determining jelly-strength—and the one perhaps in most general use, is the finger test.
In this test the various glue-jellies are arranged before the tester, who presses each with the tip of the finger, comparing it with the standard as to resistance power. While this may seem to expose the final decision too greatly to the personal equation, as represented by the personality of the glue-tester, it is nevertheless true that an expert develops the most extraordinary precision, arriving at conclusions that are corroborated by the results of other tests and by the results in actual practice. The work of the glue-tester is analogous to that of the coffee-taster and the tea-taster, or experts in other lines, who through a highly developed and keenly discriminating sense of taste, or touch, or smell, determine with extreme nicety the physical characteristics of the substances that they are accustomed to test.
SOME SIMPLE, PRACTICAL TESTS FOR GLUE USERS
To perform the tests described requires a degree of experience and an equipment beyond the reach of the ordinary buyer and user of glue.
Certain tests may however be made that are of great value in determining important facts about the glue it is intended to buy and use.
These tests could not be used as a basis for grading glue scientifically, but they are exceedingly valuable in determining its purity and its adaptability to the work in hand.