----------+------+--------+--------+-----------+------------+-----------+
Gum |Weight| Weight | Weight |[alpha]_{J}|[alpha]_{J} |[alpha]_{J}|
used. | Gum | Alcohol| Gum | Original | Alcohol | Filtrate. |
|Waken.| Precip-|Filtrate| Gum. |Precipitate.| |
| | itate. | | | | |
----------+------+--------+--------+-----------+------------+-----------+
| | Grms. | | | | |
/a......| 5 | 2.7940 | 1.9415 | | +58.4 | +53.7 |
3{ | | | | +66.2 | | |
\b......| 5 | 3.5805 | 0.8910 | | +57.4 | -52.5 |
| | | | | | |
/a......| 5 | 2.3315 | 2.3736 | | -20.8 | -67.5 |
9{ | | | | -38.2 | | |
\b......|4.9620| 2.3310 | 2.4180 | | -19.4 | -63.4 |
| | | | | | |
/a.|3.4900| 0.3925 | 2.7920 | | -104.2 | -76.0 |
Ghatti{ | | | | -140.8 | | |
\b.|3.2450| 0.4605 | 2.8385 | | -106.0 | -72.4 |
| | | | | | |
/a.|2.2550| 0.2900 | 1.8078 | | -106.04 | +68.0 |
Ghatti{ | | | | -147.05 | | |
\b.|2.6635| 0.2845 | 2.3360 | | -102.04 | -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 | Starch. | Real Gum.
| absorbed.| |
-----------------------------+----------+--------------+--------------
| | Per Cent. | 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. | BaO
| | absorbed.
----------------------------+---------------|-------------------------
| Per Cent. | 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: