The Ghattis apparently are all lævo-rotatory, and give much less alcoholic precipitates than the gum arabic. The precipitation moreover was in the opposite direction, that is, the most lævo-rotatory gum was thrown down by the alcohol. The appended table shows the nature of the precipitates and the respective amounts from two Ghattis and two gum arabics. It will be observed that the angle of rotation in three of the cases is decidedly less both for precipitate and filtrate than for the original solution:
SPECIFIC ROTATORY POWERS OF GUMS.
| Gum used. | Weight Gum Waken. Grms. | Weight Alcohol Precipitate. | Weight Gum Filtrate. | αJ Original Gum. | αJ Alcohol Precipitate. | αJ Filtrate. | |
| 3 | a b | 5 5 | 2.7940 3.5805 | 1.9415 0.8910 | + 66.2 | + 58.4 + 57.4 | + 53.7 - 52.5 |
| 9 | a b | 5 4.9620 | 2.3315 2.3310 | 2.3736 2.4180 | - 38.2 | - 20.8 - 19.4 | - 67.5 - 63.4 |
| Ghatti: | a b | 3.4900 3.2450 | 0.3925 0.4605 | 2.7920 2.8385 | -140.8 | -104.2 -106.0 | - 76.0 - 72.4 |
| Ghatti | a b | 2.2550 2.6635 | 0.2900 0.2845 | 1.8078 2.3360 | -147.05 | -106.04 -102.04 | + 68.0 - 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 able to confirm these results by a series of tests of the same gums in India, but have not yet obtained information as to their behavior in the early part of the year.
The fermentation of natural gum solutions is accompanied by a decrease in the viscosity of the liquid and the separation of a portion of the gum in lumps. Apparently those gums which contain most sugar, as indicated by their reduction of Fehling's solution, are the most susceptible to this change. Oxalic acid is formed by the fermentation, which by combination with the lime present renders the fermenting liquid turbid, and also some volatile acid, probably acetic.
We have made some experiments with a gum which readily fermented--in a week--as to the respective value of various antiseptics in retarding the fermentation. Portions of the gum solutions were mixed with small quantities of menthol, thymol, salol, and saccharin in alkaline solution, also with boric acid, sodium phosphate, and potash alum in aqueous solution. Within a week a growth appeared in a portion to which no antiseptic had been added; the others remained clear. After over five months the solutions were again examined, when the following results were observed:
| Antiseptics. | Solution after Five Months. |
| Menthol in KOH | Some growth at bottom, upper layer clear. |
| Thymol in KOH | Growth at top, gum white and opaque. |
| Salol in KOH | Growth at top, gum black and opaque |
| Saccharin in KOH | White growth at top. |
| Boric acid | Remained clear; did not smell. |
| Sodium phosphate | Slight growth at top. |
| Potash alum | Slight growth at top. |
The solution to which no antiseptic had been added was of course quite putrid, and gave the reactions for acetic acid.
In the earlier part of this paper we have given a short account of the chief characteristics of the more important gum substitutes. The following additional notes may be of interest.