The yield of furfuraldehyde by the breaking down of arabin and metarabin was thought possibly to be of some value in differentiating the natural gums from one another, but we have not succeeded in obtaining results of much value. 0.2 gramme of a gum were heated with 100 c.c. of 15 per cent. sulphuric acid for about 2½ hours in an Erlenmeyer flask with a reflux condenser. After this period of time, further treating did not increase the amount of furfuraldehyde produced. The acid liquid, which was generally yellow in color, was then cooled and neutralized with strong caustic soda. The neutral or very faintly alkaline solution was then distilled almost to dryness, when practically the whole of the furfuraldehyde comes over. The color produced by the gum distillate with aniline acetate can now be compared with that obtained from some standard substance treated similarly. The body we have taken as a standard is the distillate from the same weight of cane sugar. The tint obtained with the standard was then compared with that yielded by the gum distillate from which the respective ratios of furfuraldehyde are obtained. The following table shows some of these results:
| Substance. | Comparative Yield of Furfuraldehyde. | Amount of Glucose Produced. |
| Cane sugar | 1.00 | .. |
| Starch | 0.50 | .. |
| Gum arabic | 1.33 | 34.72 |
| Gum arabic | 1.20 | 43.65 |
| Ghatti, 1 | 1.00 | 26.78 |
| Ghatti, 2 | 1.33 | 22.86 |
| Metarabin | 1.75 | .. |
The amount of reducing sugar calculated as glucose is also appended. This was estimated in the residue left in the flask after distillation by Fehling's solution in the usual way. The yields of furfuraldehyde would appear to have no definite relation to the other chemical data about a gum, such as the potash and baryta absorptions or the sugar produced on inversion.
The action of gum solutions upon polarized light is interesting, especially in view of the fact that arabin is itself strongly lævo-rotatory αD = -99°, while certain gums are distinctly dextro-rotatory. Hence it is evident that some other body besides arabin is present in the gum. We have determined the rotatory power of a number of gum solutions, the results of which are subjoined. On first commencing the experiments we experienced great difficulty from the nature of the solutions. Most of them are distinctly yellow in color and almost opaque to light, even in dilute solutions such as 5 percent. We found it necessary first to bleach the gums by a special process; 5 grammes of gum are dissolved in about 40 c.c. of lukewarm water, then a drop of potassium permanganate is added, and the solution is heated on a water bath with constant stirring until the permanganate is decomposed and the solution becomes brown. A drop of sodium hydrogen sulphate is now added to destroy excess of permanganate. At the same time the solution becomes perfectly colorless.
It can now be cooled down and made up to 100 c.c., yielding a 5 per cent. solution of which the rotatory power can be taken with ease. Using a 20 mm. tube and white light the above numbers were obtained.
| Gum or Dextrin. | Solution used. Per Cent. | αD |
| Aden, 1 | 5 | - 33.8 |
| Cape, 2 | 5 | + 28.6 |
| Indian, 3 | 5 | + 66.2 |
| Eastern, 4 | 5 | - 26.0 |
| Eastern, 5 | 5 | - 30.6 |
| Senegal, 6 | 5 | - 17.6 |
| Senegal, 7 | 5 | - 18.4 |
| Senegal, 8 | 2½ | - 19.6 |
| Senegal, 9 | 5 | - 38.2 |
| Senegal, 10 | 5 | - 25.8 |
| Amrad | 2½ | + 57.6 |
| Australian, 1 | 5 | - 28.2 |
| Australian, 2 | 5 | - 26.4 |
| Brazilian, 1 | 2½ | - 36.8 |
| Brazilian, 2 | 2½ | + 21.0 |
| Dextrin, 1 | 5 | +148.0 |
| Dextrin, 2 | 5 | +133.2 |
| Ghatti, 1 | 5 | - 39.2 |
| Ghatti, 2 | 5 | - 80.4 |
These numbers do not show any marked connection between the viscosity, etc., of a gum and its specific rotatory power.
When gum arabic solution is treated with alcohol the gum is precipitated entirely if a large excess of spirit be used. With a view to seeing if the precipitate yielded by the partial precipitation of a gum solution was identical in properties to the original gum, we examined several such precipitates from various gums to ascertain their rotatory power. We found in each case that the specific rotatory power of the alcohol precipitate redissolved in water was not the same as that of the original gum. In other words these gums contained at least two bodies of different rotatory powers, of which one is more soluble in alcohol than the other. O'Sullivan obtained similar results with pure arabin. The experiments were conducted in the following manner:
(a.) Five grammes of a dextro-rotatory gum (No. 3 in table) were dissolved in 20 c.c. of water. To the solution was added 90 c.c. of 95 per cent. alcohol. The white precipitate which formed was thrown on to a tared filter and washed with 30 c.c. more alcohol. The total filtrate therefore was 140 c.c. The precipitate was dried and weighed = 2.794 grammes or 55.88 per cent. of the total gum. The precipitate was then redissolved in water, bleached as before and diluted to a 5 per cent. solution. This was then examined in the polarimeter. Readings gave the value αD = +58.4°. The previous rotatory power of the gum was +66°. Now the alcohol was driven off from the filtrate, which, allowing for the 11.95 per cent. of water in the gum, should contain 32.17 per cent. of gum. The alcohol-free liquid was then diluted to a known volume (for 5 per cent, solution), and αJ found to be + 57.7°. This experiment was then repeated again, using 5 grammes of No. 3, when 3.5805 grammes of precipitate were obtained, using the same volumes of alcohol and water. The precipitate gave αJ = +57.4°; the filtrate treated as before, only the percentage of gum dissolved being directly determined instead of being calculated by difference, gave αJ = + 52.5°.
(b.) Another gum (No. 9) with αJ = -38.2° and containing 13.86 per cent, of moisture, gave 2.3315 grms. of precipitate when similarly treated. The precipitate gave when redissolved in water αJ = -20.8°. The filtrate containing 39.5 per cent, real gum gave αJ = -67.5°, so that the least lævo-rotatory gum. was precipitated by the alcohol.