[Continued from SUPPLEMENT, No. 830, page 13110.]

GUM ARABIC AND ITS MODERN SUBSTITUTES.[¹]

By Dr. S. RIDEAL and W.E. YOULE.

Subjoined is a table giving the absolute viscosity of various gums. A comparison of the uncorrected viscosities with the corrected shows the great importance of Slotte's correction for dextrins and inferior gum arabics; in other words, for solutions of low viscosity, while it will be observed to have little influence upon the uncorrected η obtained for the Ghatti gums and the best samples of gum arabic.

TABLE OF ABSOLUTE VISCOSITIES OF 10 PER CENT. GUM AND DEXTRIN SOLUTIONS.

Sample.	η Uncorrected.	η Corrected.	Z Water = 100.
Gum arabic	0.1876	0.1856	1,233
Cape gum	0.1575	0.1555	1,029
Indian gum	0.0540	0.0470	311
Eastern gum	0.0689	0.0639	417
Gum arabic	0.0550	0.0480	317
Senegal	0.0494	0.0410	271
Senegal	0.0468	0.0380	251
Senegal	0.0627	0.0557	364
Gum arabic	0.0511	0.0430	285
Water	0.0149	0.0124	100
Ghatti	0.2903	0.2880	2,322
Ghatti, 5 per cent	0.0903	0.0828	688
Ghatti, 5 per cent	0.1391	0.1350	1,089
Ghatti, 5 per cent	0.1795	0.1760	1,420
Ghatti, 5 per cent	0.1527	0.1485	1,198
Ghatti, 5 per cent	0.1139	0.1083	873
Ghatti, 5 per cent	0.1419	0.1369	1,104
Dextrin	0.0398	0.0255	169
Dextrin	0.0341	0.0196	129
Dextrin	0.0455	0.0380	306
Gum substitute	0.0318	0.0224	180
Gum substitute	0.0318	0.0224	180
Amrad	0.0793	0.0708	570
Australian	0.0378	0.0283	228
Australian	0.0365	0.0268	216
Brazilian	0.0668	0.0627	506
Brazilian	0.0516	0.0445	359
Ghatti	0.3636	0.3621	2,920

In the column for η corrected the differences due to the use of different instruments are of course eliminated. The absolute viscosity of water at 15° C. determined in four different instruments is shown below. Poiseuille's value for water being 0.0122.

Instrument.	1.	2.	3.	4.
η corrtd. of water.	0.0109	0.01185	0.0124	0.0120
K₁ value.	0.000000898	0.000000863	0.000000932	0.00000052
K₂ value.	0.235	0.2175	0.226	0.0204

The above values for various gums and dextrins were obtained at a constant temperature of 15° C. and are compared with water at that temperature. It is of the utmost importance that the temperature of the water surrounding the bulbs should be adjusted for each series of experiments to the temperature at which the absolute viscosity of the water was determined. As far as we have ascertained, in gum solutions there is a steady diminution in viscosity with increase of temperature until a certain temperature is reached, beyond which increase of heat does not markedly influence the viscosity, and it is possible that above this "critical point," as we may term it, the gum solutions once more begin to increase in viscosity. The temperature at which the viscosity becomes stationary varies somewhat with different gums, but broadly speaking it lies between 60° C. and 90° C., no gums showing any marked decrease in viscosity between 80° C. and 90° C.

The experiments we have made in this direction were conducted as follows. The 300 c.c. bottle containing the gum was placed in a capacious beaker full of hot water, and the viscosity instrument was also surrounded with water at the same temperature. Thermometers were suspended both in the beaker and the outer jar. The viscosity at the highest temperature obtained, about 90° C., was then taken and repeated for every fall of 4° C. till the water reached the temperature of the air.

The values so obtained gradually diminished with the increase of temperature. From the η values obtained the Z values were calculated, using water at 15° C. as a standard. From the Z values thus obtained taken as the ordinate, and the temperature of each experiment as the abscissa, curves were plotted out embodying the results, examples of which are given below. The curves yielded by three gums 2, 7, and 8 changed between 90° C and 100° C., while gum sample 4 has a curve bending between 60° C. and 70° C. Experimentally this increase of viscosity of the latter gum above 60° C. was confirmed, but the critical point of the other solutions tried approaches too nearly to the boiling point of water for experiments to be conducted with accuracy, as the temperature of the bulbs diminishes sensibly while the experiment is being made.

If viscosity values have been determined it is possible to calculate the remaining or intermediate values for Z at any particular temperature from the general equation--Zt = A + Bt + Ct²

As an example of the mode of calculation we may quote the following. A gum gave the following values for Z at the temperature stated:

Gum. 50° C. Z_50° = 228

Gum. 30° C. Z_30° = 339

Gum. 20° C. Z_20° = 412

from which the constants--

A = 592.99 B = -10.2153 C = 0.0583

can be obtained, and thus the value of Z_t° for any required temperature. The numbers calculated for gums all point to a diminution in viscosity up to a certain point, and then a gradual increase. A comparison of some of the figures actually obtained in some of these experiments, compared with the calculated figures for the same temperature, shows their general agreement.

Curves showing viscosity change with temperature for three typical gums. A--Arabic VII. B--Senegal VIII. C--Ghatti 15.

EFFECT OF TEMPERATURE UPON VISCOSITY--GUM VII.

Temperature. ºC.	η	Z found.	Z calculated.
50	0.0283	228	228.00
45	0.0305	246	246.55
42	0.0352	284	266.75
38	0.0368	297	289.00
34	0.0410	330	313.06
30	0.0419	339	339.00
26	0.0445	359	367.80
22	0.0492	398	396.47
20	0.0511	412	412.00
18	0.0531	428	428.00

EFFECT OF TEMPERATURE UPON VISCOSITY.--GUM VIII.

Temperature. ºC.	η	Z found.	Z calculated.
50	0.0430	347	347
46	0.0475	383	371.14
42	0.0502	405	397.09
38	0.0510	411	424.73
34	0.0575	463	454.06
30	0.0602	485	485
26	0.0637	513	517.82
22	0.0667	538	552.25
20	0.0707	570	570
18	0.0755	609	583.07

The constants for the first gum are those given in the preceding column, while for the latter they were--

A = 771.9: B = -11.15: C = 0.053

As will be observed, the effect of heat appears to be the same upon the two typical gum arabics quoted above, an increase of temperature from 18° C. to 50° C. decreasing the viscosity by nearly one half in both cases, and the same seems to be true of most gum arabics. Roughly also the same holds good for Ghattis, as the following numbers show:

Gum.	Z at 18° C.	Z at 50° C.
Gum arabic.	1016	579
Gum arabic.	428	228
Gum arabic.	609	347
Gum arabic.	581	258
Ghatti.	572	306
Ghatti.	782	418

The following table shows the effect of heat upon the viscosity of a typical Ghatti:

GHATTI GUM NO. 15.--VISCOSITY.

Temperature. °C.	η	Z.
50	0.0517	418
46	0.0581	468
42	0.0628	506
38	0.0726	585
34	0.0788	635
30	0.0857	691
26	0.0889	717
22	0.0919	741
20	0.0946	763
18	0.0964	777

There is therefore no essential difference in the behavior of a Ghatti and a gum arabic on heating. Some interesting results, however, were obtained by heating gums, both Ghattis and arabics, at a fixed temperature for the same time, cooling, and then after making the solutions up to the original volume taking their viscosities at the ordinary temperature. The effect of heating for two hours to 60° C., 80° C., or 100° C. was a small permanent alteration in viscosity of the solution, and it would therefore seem desirable that gum solutions should be made up cold to get the maximum results. The following numbers illustrate this change, viz.:

		After heating to
Gum Arabic 10 Per Cent.	Without heat.	60°C.	80°C.	100°C.
Z at 18°C	570	468	470	517
Z at 30°C	485	400	422	439
Z at 50°C	347	287	258	301
Ghatti gum No. 15, 5 per cent. Z at 18°C.	1,104	780	660	758

The variation of viscosity with strength of solution was also studied with one or two typical gums. A 10 per cent. is invariably more than twice as viscous as a 5 per cent. solution. The following curve was obtained from one of the Ghattis. Similar results were shown by other gums.

Variation of Viscosity, with Dilution. Ghatti No. 888.

It would seem, therefore, that strong solutions, say of 50 per cent. strength, would be more alike in viscosity than solutions of 5 per cent. strength of the same gums. In other words, the viscosity of a gum solution should be taken as nearly as possible to the strength it is used at, to obtain an exact quantitative idea of its gumming value.

The observation of this fact was one of the circumstances which decided us to use 5 per cent. solutions for the determination of Ghatti gum viscosities, the ratio between the 5 per cent. and 10 per cent. solutions of gum arabics being roughly the same as that between the respective weights required for gumming solutions of equal value.

From observation of the general nature of the solutions of Ghatti gums, and from the fact that when allowed to stand portions of the apparently insoluble matter passed into solution, the hypothesis suggested itself that metarabin was soluble in arabin, although insoluble in cold water. If this hypothesis were correct, it would explain the apparent anomaly of Ghattis giving solutions of higher viscosity than gum arabics, although they leave insoluble matter behind. The increase in viscosity would be due to the thickening of the arabic acid by the metarabin. Moreover, the solutions yielded by various Ghattis leaving insoluble matter behind would be all of the same kind, viz., a saturated solution of metarabin in arabin more or less diluted by water. Still further, if the insoluble residue of a Ghatti be the residual metarabin over and above that required to saturate the arabin, then it will be possible to dissolve this by the addition of more arabin in the form of ordinary gum arabic. In order to see if this were the case the following experiments were performed. Equal parts of a Ghatti and of a gum arabic were ground up together and dissolved in water. The resulting solution was clear. It was diluted until of 10 per cent. strength, and its viscosity then taken:

	Contains 50 per Cent. Ghatti.
A. Pressure 200 mm	η	Z
Temperature 15° C	0.2517	2,030

The viscosity of this solution therefore was considerably greater than the mean viscosity of the 10 per cent. solutions of the Ghatti and the gum arabic, viz., (0.288 + 0.0636)/2 = 0.1758 for the calculated η. Hence it is evident that the increase in viscosity is due to the solution of the metarabin.

Next a solution was made from a mixture of 70 per cent. Ghatti and 30 per cent. gum arabic. This was also clear and gave a considerably higher viscosity than the previous solution.

	Contains 70 per Cent. Ghatti.
B. Pressure 200 mm	η	Z.
Temperature 15° C	0.3177	2,562

It will be obvious that the increase of viscosity over the previous solution in this case must be due to the smaller amount of the thin gum arabic which is present, i.e., in the first case there is more gum arabic than is required to dissolve the whole of the insoluble metarabin. Further experiments showed that this is also true of the second mixture, as the viscosities of the following mixtures illustrate:

Strength of Solution.	η	Z.
C. 80 per cent. Ghatti.	0.3642	2,937
D. 75 per cent. Ghatti.	0.33095	2,669
E. 77.5 per cent. Ghatti.	0.4860	3,819

This last solution E we called for convenience the "maximum viscosity" solution, as we believe it to be a 10 per cent. solution containing arabin very nearly saturated with metarabin. As will be observed, its viscosity differs widely from those of solutions C and D, between which it lies in percentage of Ghatti. The first named solution C contains too little of gum arabic to dissolve the whole of the metarabin. Consequently there is a residue left undissolved, which of course diminishes its viscosity. The second solution D is too low in viscosity, as it still contains too much of the weak gum arabic, and as will be seen further on, a very slight change in the proportions increases or decreases the viscosity enormously.

We next tried a series of similar experiments with a Ghatti containing far less insoluble residue and which consequently would require less gum arabic to produce a perfect solution. Mixtures were made in the following proportions, viz.:

----	13.3 per Cent. Ghatti.
F. Pressure 200 mm	η	Z.
Temperature 15° C	0.0976	787

----	86.6 per Cent. Ghatti.
G. Pressure 200 mm	η	Z.
Temperature 15° C	0.4336	3,497

This latter solution is approaching fairly closely to our "maximum viscosity" with the previous Ghatti, and probably a very slight decrease in the amount of gum arabic would bring about the required increase in viscosity.

When these experiments were first commenced we were still under the impression, which several months' experience of working with gums had produced, namely, that the Ghattis were quite distinct in their properties to ordinary gum arabics. But the new hypothesis, and the experiments undertaken to confirm it, showed clearly that if the viscosity of a gum solution depends on the ratio of metarabin to arabin, then there is no absolute line of demarkation between a Ghatti and a gum arabic. In other words, there is a constant gradation between gum arabic and Ghattis, down to such gums as cherry gum, consisting wholly of metarabin and quite insoluble in water. Therefore those gum arabics which are low in viscosity consist of nearly pure arabin, while as the viscosity increases so does the amount of metarabin, until we come to Ghattis which contain more metarabin than their arabin can hold in solution, when their viscosity goes down again.

From these observations it would follow, that by taking a gum of less viscosity than the gum arabic previously used to dissolve the Ghatti, less of it would be required to do the same work. We confirmed this suggestion experimentally by taking another gum arabic of viscosity 0.0557 at 15° C. A mixture containing 93.3 per cent. of this Ghatti and 6.7 per cent. of our thinnest gum arabic gave a clear solution which had the highest viscocity we have yet obtained for a 10 per cent. solution.

H. Pressure 200 mm.
Temperature 15° C.

η
0.5525

Z.
4,456

This gum arabic may be regarded as nearly pure arabin (as calcium and potassium, etc., salt). By diluting the new "maximum viscosity" solution, therefore, with the 10 per cent. solution of the gum arabic in fixed proportions we obtain a series of viscosities which are shown in the following curve.

Curve Showing Influence of Ghatti upon Viscosity.

Besides obtaining this curve for change in viscosity from maximum amount of metarabin to no metarabin at all, we also traced the decrease in viscosity of the "maximum" solution by dilution with water. The following numbers were thus obtained, and plotted out into a curve.

Having obtained this curve, we are now in a position to follow up the hypothesis by calculating the surplus amount of insoluble matter in a Ghatti. For, let it be conceded that the solution of any Ghatti leaving an insoluble residue is a mixture of arabin and metarabin in the same ratio as our "maximum" solution, only more diluted with water, then from the found viscosity we obtain a point on the curve for dilution, which gives the percentage of dissolved matter.

Now to show the use of this: The Z value for a 10 per cent. solution of the second Ghatti at 15° C. is 2,940. This corresponds on the curve to 8.4 dissolved matter. 10-8.4 = 1.6 grammes in 10 grammes, which is insoluble.

CHANGE OF VISCOSITY WITH DILUTION--"MAXIMUM" SOLUTION. 15° C. TEMPERATURE.

Percentage.	η	Z.
10	0.55250	4,456
9	0.42850	3,456
8	0.35120	2,832
7	0.27660	2,230
6	0.22290	1,797
5	0.16810	1,355
4	0.11842	955
3	0.08020	647
2	0.06190	499
1	0.03610	291

Curve of Variation in Viscosity on Dilution of the "Maximum" Solution.

We have already shown that a "maximum" viscosity solution of this gum is formed when 6.7 per cent, of thin gum arabic is added to it, and therefore 6.7 parts of a thin gum arabic are required to bring 16 parts of metarabin into solution. A convenient rule, therefore, in order to obtain complete solution of a Ghatti gum is to add half the weight in thin gum of the insoluble metarabin found from the viscosity determination. But the portion of the gum which dissolved is made up in a similar manner (being a diluted "maximum" solution).

Therefore the 84 per cent. of soluble matter contains 58 parts of metarabin, and the total metarabin in this gum is 58 + 16 = 74 per cent, on the dry gum.

With these solutions of high viscosity some other work was done which may be of interest. The temperature curves of the mixtures marked E, G, and F were obtained between 60° C. and 15° C. The two former curves showed a direction practically parallel to that at the 10 per cent. solutions, and as they were approaching to the "maximum" solution, this is what one would expect. Mr. S. Skinner, of Cambridge, was also good enough to determine the electrical resistances of these solutions and the Ghattis and gum arabics employed in their preparation. The electrical resistance of these gum solutions steadily diminishes as the temperature increases, and the curve is similar to those obtained for rate of change with temperature. Although the curves run in, roughly, the same direction, there does not appear to be any exact ratio between the viscosities of two gums say at 15° C. and their electrical resistances at the same temperature; hence it would not seem possible to substitute a determination of the electrical resistance for the viscosity determination. The results appear to be greatly influenced by the amount of mineral matter present, gums with the greatest ash giving lower resistances.

Experiments were conducted with two Ghattis and two gum arabics, besides the mixtures marked E, F, and H. Comparison of the electrical resistances with the viscosities at 15° C. shows the absence of any fixed ratio between them.

Gum or Mixture.	°C.	Ohms Resistance.	Z Viscosity at 15° C.
Ghatti, 1	10	5,667	1,490
Ghatti, 2	15	2,220	2,940
Arabic 1	15	1,350	605
Arabic 2	10	2,021	449
Mixture F	15	1,930	787
Mixture E	11.3	2,058	3,919

While performing these experiments, an attempt was made to obtain an "ash-free" gum, in order to compare its viscosity with that of the same gum in its natural state. A gum low in ash was dissolved in water, and the solution poured on to a dialyzer, and sufficient hydrochloric acid added to convert the salts into chlorides. When the dialyzed gum solution ceased to contain any trace of chlorides, it was made up to a 10 per cent. solution, and its viscosity determined under 100 mm. pressure, giving the following results at 15° C.:

--------	η	Z
Natural gum	0.05570	449
"Ash-free" gum	0.05431	438

Thus showing that the viscosity of pure arabin is almost identical with that of its salts in gum.

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 α_D = -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.	α_D
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 α_D = +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 α_J 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 α_J = +57.4°; the filtrate treated as before, only the percentage of gum dissolved being directly determined instead of being calculated by difference, gave α_J = + 52.5°.

(b.) Another gum (No. 9) with α_J = -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 α_J = -20.8°. The filtrate containing 39.5 per cent, real gum gave α_J = -67.5°, so that the least lævo-rotatory gum. was precipitated by the alcohol.

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.	α_J Original Gum.	α_J Alcohol Precipitate.	α_J 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.

The ashes of most gum substitutes, consisting chiefly of dextrin, are characterized by the high percentage of chlorides they contain, due no doubt to the use of hydrochloric acid in their preparation. The soluble constituents of the ash consist of neutral alkaline salts, but as a rule no alkaline carbonates, and it is thus possible to demonstrate the absence of any natural gum in such a compound. We have seldom noticed the presence of any sulphates in such ashes, but when sulphurous or sulphuric acids have been used in the starch conversion it will be found in small quantities.

We have already pointed out that the potash absorption value of a gum is low and that dextrins give high numbers, but the latter vary very considerably, and as the starch and sugar present also influence the potash absorption value, it does not give information of much service. The following table shows the kind of results obtained:

Sample.	KOH absorbed.	Starch. Per Cent.	Real Gum. Per Cent.
Dextrin, 1	25.40	1.99	..
Dextrin, 2	19.70	13.13	..
Dextrin, 3	7.57	24.72	..
Artificial gum, 1	19.70	10.98	9.00
Artificial gum, 2	13.70	8.05	23.50
Starch	9.43	100.00	None

The baryta absorptions seem to be chiefly due to the quantity of starch present in the composition:

Sample.	Starch. Per Cent.	BaO absorbed. Per Cent.
Dextrin, 1	1.99	1.75
Dextrin, 2	13.13	3.53
Dextrin, 3	24.72	5.64
Starch	100.00	23.61

The viscosity of a dextrin or artificial gum is determined in exactly the same way as a natural gum, using 10 per cent. solutions. It would probably be an improvement to use 10 per cent. solutions for many of the dextrins, as they are when low in starch extremely thin.

The hygroscopic nature of dextrins renders them unsuitable for foreign work, but when the quantity of starch is appreciable, better results are obtainable. A large percentage of unaltered starch is usually accompanied with a small percentage of sugar, and no doubt this is the explanation of this fact. An admixture containing natural gum of course behaved better than when no such gum is present. Bodies like "arabol" made up with water and containing gelatin are very hygroscopic when dry, although as sold they lose water on exposure to the air. Gum substitutes consisting entirely of some form of gelatin with water, like fish glue, are also somewhat hygroscopic when dried. The behavior of these artificial gums and dextrins on exposure to a warm moist atmosphere can be determined in the same apparatus as described for gums.

The process we have adopted for estimating the glucose starch and dextrin in commercial gum substitutes is based on C. Hanofsky's method for the assay of brewers' dextrins (this Journal, 8, 561). A weighed quantity of the dextrin is dissolved in cold water, filtered from any insoluble starch, and then the glucose determined directly in the clear filtrate by Fehling's solution. The real dextrin is determined by inverting a portion of the filtered liquid with HCl, and then determining its reducing power. The starch is estimated by inverting a portion of the solid dextrin, and determining the glucose formed by Fehling. After deducting the amounts due to the original glucose and the inverted dextrin present, the residue is calculated as starch. A determination of the acidity of the solution is also made with decinormal soda, and results returned in number of c. c. alkali required to neutralize 100 grammes of the dextrin. Results we have obtained using this method are embodied in the following table:

ANALYSIS OF GUM SUBSTITUTES

No.	Glucose.	Dextrin.	Starch.	Moisture.	Gum, &c.	Ash.	Acidity. c.c.
1	8.92	81.57	1.99	10.12	None	0.207	57.3
2	7.19	71.46	13.13	10.40	None	0.120	44.8
3	1.29	69.42	24.72	4.17	1.12	0.280	5.22
4	8.40	60.98	10.98	10.09	9.02	0.530	20.0
5	10.60	44.98	8.05	12.20	23.57	0.600	52.0
6	14.80	11.57	36.46	34.87	1.89	0.580	8.0
7	8.00	29.61	26.78	33.98	0.88	0.750	88.0
8	2.29	52.38	37.65	None	7.335	0.315	9.6

In those cases in which the substitute is made by admixture with gelatin or liquid glue the quantity of other organic matter obtained can be checked by a Kjeldahl determination of the total nitrogen. If a natural gum is added, it will be partially converted into sugar when the filtered liquid is inverted, and so make the dextrin determination slightly too high.

[1]

A paper read before the Society of Chemical Industry, London, 1891. From the Journal