862 
GUM. 
entirely. When mucilage is evaporated the 
gum is obtained unaltered. This mucilagi- 
nous solution may be kept for years without 
undergoing putrefaction. Scarcely any ve- 
getable substance is less liable to decompo- 
sition. At last, however, the odour of ace- 
tic acid becomes perceptible in it. 
When gum is exposed to heat it softens 
and swells, but does not melt ; it emits air- 
bubbles, blackens, and at last, when nearly 
reduced to charcoal, emits a low blue flame. 
This flame appears sooner if a flaming sub- 
stance is held just above the gum. After th? 
gum is consumed, there remains a small 
quantity of white ashes, composed chiefly of 
the carbonats of lime and potass. 
2. It does not appear that gum is acted 
upon by oxygen gas. A solution of gum and 
water, when exposed to the air, soon becomes 
mouldy on the surface, but undergoes no 
farther change for a long time. The action 
of the simple combustibles on gum has 
scarcely been examined. Azotic gas seems 
to have no action on it whatever. 
Gum does not act upon metals ; but it has 
the property of combining with several of the 
metallic oxides, and forming compounds : at 
least, some of the salts occasion precipitates 
when dropt into solutions of gum. The most 
curious effect is that produced by tire oxyinu- 
riat of iron. When this salt, concentrated, 
is dropt into a very strong mucilage, the 
whole becomes a brown semitransparent jelly, 
which is not readily dissolved by water. 
When dried, the jelly becomes lighter-co- 
loured, and assumes nearly the appearance 
of gum. Its taste is that of gum mixed with 
iron. 
Liquid potass first converts gum into a 
substance not unlike curd, and then dissolves 
it. The solution is of a light amber-colour, and 
transparent. When long kept, the gum again 
falls into the state of curd. Alcohol throws 
down the gum in white flakes still soluble in 
water; but .it retains the potass obstinately, 
and is much more friable than before. Lime- 
water arid ammonia likewise dissolve gum, 
and it may be afterwards separated little al- 
tered. 
Charcoal powder, when mixed with a so- 
lution of gum in water, gives it a black co- 
lour, which cannot be removed by filtration, 
unless a very great proportion of the powder 
is added. In that case the water passes clear; 
but the whole of the gum is retained by the 
charcoal. Mr. Lowitz found that not less 
than 30 lbs. of charcoal powder must be mix- 
ed with water containing an ounce of gum 
dissolved in it, before the water is entirely de- 
prived of the gum. 
The vegetable acids dissolve gum without 
alteration; the strong acids decompose it. 
Sulphuric a id converts it into water, acetous 
acid, and charcoal. The same effect is said 
by Fourcroy to be produced by muriatic 
acid. But this is not accurate unless some 
heat is applied. When gum is dissolved in 
strong muriatic acid, a brown solution is ob- 
tained, which becomes perfectly transparent 
when diluted with water, while at the same 
time some charrv matter falls. If the solution 
is now saturated with ammonia, evaporated 
to dryness, and the residue digested in alco- 
hol, the alcohol assumes a deep-brown co- 
lour, and dissolves the whole except a very 
little sal ammoniac. The gum now bears 
seme resemblance to sugar in its properties ; 
at least when heated it melts, and gives out a 
very strong smell of caromel. 
Oxymuriatic acid converts gum into citric 
acid, according to the experiments of Vau- 
quelin. He passed a current of oxymuriatic 
acid gas through a diluted solution of gum in 
water. In a few days almost the whole of the 
gum was acidified; and lie detected citric 
acid by the formation of citrat of lime, soluble 
in water, and decomposable by oxalic acid. 
If nitric acid is slightly heated upon gum till 
it has dissolved it, and till a little nitrous gas is 
exhaled, the solution on cooling deposits sac- 
lactic acid. Malic acid is formed at the same 
•time; and if the heat is continued, the gum is 
at last changed into oxalic acid. Thus no 
less than three acids are developed by the ac- 
tion of nitric acid on gum. We are indebted 
to Mr. Cruikshank for the most precise ex- 
periments on the quantity of oxalic acid ob- 
tainable from gum by nitric acid. By di-' 
gesting 4S0 grains of it with six ounces of 
nitric acid, he obtained 210 grains of oxalic 
acid, and six grains of oxa'at of lime. 
Gum is insoluble in alcohol. When alco- 
hol is poured into mucilage, the gum imme- 
diately precipitates; because the affinity be- 
tween water and alcohol is greater than that 
between water and gum. The gum in this 
case is in the state of soft opaque white flakes. 
Neither is gum soluble in ether. It is not 
soluble in oils; but when triturated with a 
little oil it renders the oil miscible with water. 
The action of the hydrosulphurets, sulphu- 
rets, phosphurets, and of most of the salts on 
gum, has not been examined with any atten- 
tion. 
Gum and sugar readily unite together by 
dissolving both in water. By gentle evapo- 
ration a perfectly transparent solid substance 
is obtained, which does not crystallize. When 
treated with alcohol it becomes white, opaque, 
and soft. The greaterpart of the sugar is dis- 
solved, and the gum remains united to a small 
portion. It has a sweetish taste, and very 
much resembles in appearance the substance 
of which the nests of wasps are formed. 
3. When gum is distilled in' a retort, the 
products are water impregnated with a con- 
siderable quantity of pyro-mucous acid, or 
acetic acid combined with oil, a little empy- 
reumatic oil, carbonic acid gas, and carbu- 
reted hydrogen gas. When the pyromucous 
acid obtained .by this process is saturated 
with lime, a quantity of ammonia is disen- 
gaged with which that acid had been com- 
bined. The charcoal which remained in the 
retort leaves behind it, after incineration, a 
little lime and phosphat of lime. Mr. Cruik- 
shank, to whom we are indebted for these 
facts, gradually heated 480 grains of gum 
arabic to redness in a coated glass retort. 
The products were, 
Pyromucous acid mixed with some 
oil 210 gr. 
Charcoal - - - 96 
Lime and a little phosphat of lime 1 0 
Carburated hydrogen and carbonic 
acid gas - - - - 164 
Total 480. 
The pyromucous acid liquid contained 
less acid than what was obtained lrom an 
equal weight of sugar, in the proportion o 
i 1 8 to 150. The gases consisted of 93 ounce 
measures of carbonic acid, and 180 of carbu 
reted hydrogen, composed of 5 parts eha** . 
coal to' l of hydrogen. When the pyroniu’ 
cous acid was saturated with lime, ammonia 
was disengaged. 
From these experiments it follows, that 
gum contains oxygen, hydrogen, carbon, 
azote, and lime. The lime 'may be detected 
by dropping nitric acid into a solution of 
gum; needlefonn crystals of sulphat of lime 
are slowly deposited. If we compare the 
products obtained by the distillation of sugar 
with those obtained from gum, we can 
scarcely doubt that the latter contains the 
greater proportion of carbon. As it yields 
less pyromucous acid, it is not improbable 
that it'also contains less oxygen. Sugar is a 
triple compound; but gum contains five 
constituents. 
4. The species of gum at present known 
amount to four ; though it is likely that a 
more rigid examination of the vegetable 
kingdom will discover a greater number. 
These are gum arabic, gum tragaeanth,. 
cherry-tree gum, and the mucilage which 
is contained in the roots and leaves of many 
plants. 
Gum arabic exudes from the mimosa ni- 
lotica, and other species of mimosa. It is 
the species described in the preceding part 
of this article. 
5. Gum tragaeanth is the produce of the 
astragalus tragacantha, a thorny shrub which 
grows in Candia and otlier islands of the Le- 
vant. The gum is said to exude about the 
end of June from die stem and larger 
branches, and soon dries in the sun. It is in 
the state of whitish vermiform pieces, not 
nearly so transparent as gum arabic. It is 
much stronger than gum arabic, not so easily 
dissolved in water, and is said to go farther. 
When Mr. Cruikshank distilled 4S0 grains- 
of it in a glass retort, he obtained the follow- 
ing products: 
Pyromucous acid - * 245 gr. 
Charcoal 93 
Lime with some phosphat - 12 
Carbonic acid gas and carbureted 
hyrogen gas - - - - 130 
480 
When the pyromucous acid was saturated, 
with lime, a considerably greater proportion 
of ammonia was disengaged than from the 
pyromucous acid ot gum arabic.. I he gases 
.were 78 ounce-measures of carbonic acid, 
and 91 of carbureted hydrogen. Hence we 
see that gum tragaeanth contains more azote 
and lime, and perhaps more oxygen and less- 
carbon, than gum arabic. 
6. The prunes avium, tire common cherry 
and plum trees, and the almond and apricot, 
likewise yield a gum which exudes in great 
abundance from natural or artificial openings, 
in the stem, it is of a reddish-brown colour, 
in large masses, and much setter and more 
easily "melted than gum arabic. But no pre- 
cise set of experiments has been made to as- 
certain how far its properties coincide with, 
those of the last two species. 
7. Mucilage is contained in the roots and 
leaves of a vast number of plants. Almost 
all the bulbous roots and fleshy leaves yield 
it. For example, the roots of thehyacinthus 
nondescriptus, and the althea officinalis ; the 
leaves of the althaea, of the malva sylvestris 
of many of the luci, and of the greater mun. 
