QUALITIES. _Form_, white columnar masses; _Odour_ and _Taste_, none.
CHEMICAL COMPOSITION. Fecula is one of the proximate principles of
vegetable matter, and _Starch_ is the fecula of wheat.[371] SOLUBILITY.
It is soluble in boiling water, forming with it a semi-transparent,
insipid, inodorous, and gelatinous paste, very susceptible of
mouldiness, but which is retarded by the addition of alum; it is
insoluble, but falls to powder in cold water; nor is it dissolved by
alcohol or ether; although potass dissolves starch, yet the solution of
it is not disturbed by potass, carbonate of potass, nor ammonia, but an
alcoholic solution of potass produces a precipitate; acetate of lead,
and infusion of galls occasion also precipitates. Starch is susceptible
of several interesting and important changes; thus, if it be exposed to
heat until its colour becomes yellow, its properties are so far altered
that it is no longer insoluble in cold water; and according to the
experiments of Saussure, if it be mixed with water, a spontaneous
decomposition takes place, and a quantity of sugar is formed, amounting
in weight to one half of the starch employed, in addition to which a
peculiar gummy matter results, and a substance intermediate between gum
and starch, to which the name of _Amidine_ has been given. Starch
moreover is convertible into saccharine matter by the agency of
sulphuric acid. USES. Being demulcent it is generally employed as a
vehicle for the exhibition of opium in the form of enema. The ordinary
blue starch is coloured by a solution of smalt and alum, and is unfit
for medicinal use; formerly it was tinged yellow with saffron or
turmeric, but this went out of fashion on the execution of the famous
midwife Mrs. Collier, who was hanged in a ruff starched with that
colour. OFFICINAL PREPARATIONS. _Mucilago Amyli._ L.E.D. _Pulvis
Tragacanth: comp:_ (=B=) L. _Pil: Hydrargyri_ (=M=) E. _Troschisi
Gummos:_ E.
It has been lately observed that _Iodine_ is a delicate test of the
presence of starch; if a drop or two of a solution of this substance in
alcohol be added to an aqueous solution of starch, a blue compound is
formed which eventually precipitates. Iodine may therefore be employed
for ascertaining the goodness of starch, a test which is very important,
for much of what is sold under the name of starch, does not possess its
peculiar characters; it ought however to be stated, that the blue
indication is prevented from taking place by a variety of different
bodies, as _Arsenious acid_, _corrosive sublimate of mercury_, &c. &c.