INDUSTRIAL A N D ENGINEERING CHEMISTRY
April, 1923
that time was 39 per cent. In the 89 per cent humidity the same glue molded in 14 days. The moisture content a t the time of molding was 33.1 per cent. Before molding, the ground glue always matted together in the form of a loose cake. There was no sign of matting in the 75 per cent humidity. At the beginning of molding a sharp rise in the weight of the glue was noted. The same rise in weight was also noticed, but to a lesser extent, in the glue strips. As soon as mold appeared on the strips the test was stopped and their strength determined. The strength data, therefore, may possibly be in error, owing to a slight mold growth, though it is not probable that the error so introduced is greater than the errors in testing. The observation of the writers in the casting of glue sheets in the laboratory indicates that the mold spores take from 2 to 3 days to produce visible mold growth on the surface of the glue. The moisture content of the glue in the various humidities 3 days before visible mold growth appeared might, therefore, be the point above which mold growth may occur and below which there is not sufficient moisture content to permit growth. The moisture content of the glue in the dishes 3 days before molding appeared is as follows: 100 per cent humidity-33 7 per cent 89 per cent humidity-32.3 per cent St3 per cent humidity-32.8
per cent
This seems to indicate that approximately 32 per cent moisture is required for germination of mold spores. Putting a safe limit a t 30 per cent moisture gives us, from the curve
375
in Fig. 4, a relative humidity of 85 as the highest humidity at which hide or veneer glue can be held a t a temperature of 80" F. without danger of mold growth. From the data shown under (B) of this report it is apparent that the strength of both glues a t 85 per cent has been materially lessened below that required for breaking the wood. From this it seems that if glue specimens were subjected to work a t this humidity, mold growth would have practically no effect, because the glue would not be sufficiently strong to hold the glued joint. On the other hand, it might be a factor if the glued joints were stored under high humidities and not subjected to strains.
CONCLUSIONS 1-The hygroscopicity of a veneer and a standard hide glue is the same a t the same temperatures, as far as the investigation goes. It seems probable, therefore, that all hide glues have the same hygroscopicity a t the same temperature. 2-The hygroscopicity of both the veneer and the standard hide glue is greater a t 70" than a t 80" F. 3-The tensile strength of glue may vary from a t least 20,000 Ibs. per sq. in. to practically zero, depending upon the moisture content of the glue and the grade of glue. At 30 per cent moisture the two glues tested had a tensile strength of 1700 and 700 Ibs..per sq. in. 4-It seems very likely that mold cannot maintain itself below a moisture content of 30 per cent, a t which moisture content the glue has already failed in strength.
New Form of Melting-Point Apparatus' By H.A. Bell MALLINCKRODT CHEMICAL WORKS, ST. LOUIS,M O .
This apparatus, which is a modification of the Thiele apparatus, is made of Pyrex glass and is about 12 in. in height. It consists of a sulfuric acid bath, containing the thermometer and the melting-point tube with stirrer, the heat being controlled by a water rheostat. The purpose of the double bath is to prevent any sudden changes of temperature that might cause the sample to melt before the thermometer could register the true temperature; also the direction of circulation is opposile that of a Thiele apparatus. This is to promote more uniform heating of the bath. The middle leg of the apparatus should be somewhat larger than the outside legs, so as not to restrict the flow of the acid. The middle leg is made of 3/8-in. tubing and the outer legs of 11/32-in. tubing. The middle leg is wound with 50 ft. of No. 34 B & S gage nichrome wire, the ends of which are brought out and attached to brass rings on the outside legs. The rheostat is composed of a 2- x 20-in. glass tube, having openings at top and bottom to permit the removal or adjustment of the electrodes. A small tube is sealed in the side of the top for the escape of the products of electrolysis. The electrodes should be capable of adjustment of from 1/2 to 16 in. of separation, and are insulated from accidental contact with the operator's hand by inclosing the connecting rod of the adjustable electrode with a small glass tube. The apparatus is connected to an electric-light plug, or other source of power furnishing 110 volts, by means of a plug, A. One lead from this plug is soldered to the end of the connect1 Received
February 23, 1923.
ing rod of the adjustable electrode B. This rod is incased snugly in a glass tube and capped with 8 hard-rubber handle, C. The temperature and rate of temperature rise are obtained by varying the distance between the electrodes, D, or by changing the concentration of the salt solution E. The second lead from the light plug is connected to a snap switch, F, and the second lead from the snap switch is connected to the upper terminal of the heating unit G. The lower terminal to G is connected to the lower electrode of the rheostat H. The current is turned on at F. G heats up the sulfuric acid con, tained in its core. The acid becomes lighter , than the acid in the two outside legs and rises into the body of the apparatus. The circulating acid strikes the base of the inside bath and spreads out conelike, thereby heating up the inside bath from all directions a t the same time. The apparatus is easy to operate and gives accurate and consistent results.
