C H E M I 
enables us to underftand why the cruciform plants are fo 
readily decompofed. Dried in the air, this fubftance 
affirmed the duftility and tranlparency of glue. 
If the water ufed tofeparate the farina from the gluten 
of wheat be filtered, and afterwards expofed to heat, 
white flocks of a concrete matter will be feparated, which 
pofiefs all the properties of albumen. So that wheat con¬ 
tains a gluten fimilarto the fibrous part of animals, and 
another fimilar to the albumen. In diftillation, vegeta¬ 
ble albumen affords ammoniacal carbonat, a red fetid 
oil, and carbonated hydrogen gas. 
The Mucous Extradite Par: of Flour .—This fubftance, 
denominated by its difeoverer the viucofo-faccharine mat¬ 
ter, exhibits all the phenomena of fugar in its combuflion 
and diftillation. It is this which excites the acid fermen¬ 
tation in the water that floats above the ftarch ; for, as 
Macquer well obferves, the latter is not at all l'oluble in 
cold water. It cannot be doubted, notwithstanding the 
finall quantity of this fubftance contained in the farina 
of wheat, but that it is materially concerned in the fer¬ 
mentation by which pafte is leavened and made to rife. 
This inteftine motion, fonecelfary in the making of good 
bread, is not yet well underffood. It may perhaps con- 
lift in the commencement of fermentation, which is pu¬ 
trid in the gluten, acid in the ftarch, and perhaps vinous 
in the mucofo-faccharine matter: and from tliefe- three 
incipient fermentations mutually impeding each other, 
the lighter compound, which by baking forms bread, 
may probably arife. At all events, it is certain, that 
the three fubltances we 'ipeak of are fo combined and al¬ 
tered in bread, that they can no longer be feparated. 
The ablion of heat is fufficient, without fermentation, 
to produce this intimate combination ; for unleavened 
bread, according to Malouin and Pouiletier, does not 
afford the glutinous matter. From the foregoing fabts 
we fee, how greatly other kinds of flour differ from that 
of wdieat, and ftill more thole leguminous and farinace¬ 
ous feeds, fuch as beans, peafe, chefnuts, &c. are far from 
poffeffing the qualities neceflary to make good bread. 
Of VEGETABLE COLOURING MATTERS. 
The colouring parts of vegetables have hitherto been 
examined by philosophers, with reference to the dobtrine 
of colours, as a part of optics; but the dyers, who con- 
fider only the manner of extracting and fixing them for 
commercial purpofes, have followed a different route. 
Hellot, Macquer, and Berthollet, have been fucceflively 
employed in making chemical obfervations on the art of 
dying, and on the colouring principle ; and they have 
proved; that a number of the phenomena of dying, were 
eafily explained upon chemical principles. 
Macquer diftinguifhed two kinds of colours, the ex¬ 
tradite, and the extrado-refmous. The firft are obtained 
from vegetables by decoblion, and are tranfparent. The 
Second are obtained alfo from plants ; but they are preci¬ 
pitated in' cooling. The fame chemilt fpeaks of two 
other forts of colours, the oily, and the refmo-oily. But 
Berthollet, from a feries of experiments and oblervations, 
has demonftrated, that the colouring principles are not 
fuch as are indicated by Macquer; that the greateft part 
of colouring matters have a great affinity with earths ; 
that moll colours have alfo a great affinity for the white 
metallic oxyds; and that the colouring principles have a 
ltrong affinity for oxygen. The art of bleaching is 
founded upon this property, becaufe, after this abforp- 
tion of oxygen, the colouring matters are foluble in al¬ 
kalis; the different (hades, for inftance, which the leaves 
•take in the courfe of the Summer, are occafioned merely 
by the abforption of oxygen, which changes their colour 
to yellow or brown. Laftly, the colouring matters have 
a ftrong affinity with animal fibrous matters. 
Fourcroy, in his leblures at the Polytechnical School, 
has divided colours in a manner more exadtly agreeing 
with vegetable analyfis than Macquer. He divides co¬ 
lours into extrablive, extractive oxygenated, carbonated, 
S T R Y. 343 
and oily. Yet Berthollet’s divifion is more ufeful for 
praClice. But we cannot here be expeCted to enter into 
details of the procelies ufed in dying ; we are not, under 
Chemistry, to teach the art, but the principles of the 
art; and thefe we (hall divide into three parts: i. Of 
mordants, a. The fubltances ufeful in dying. 3. Exam¬ 
ples, by which ftudents may be enabled to make experi¬ 
ments themfeives. 
Of Mordants. —The name of mordant, is given to 
the intermedium between the colouring matter and the 
ftuff to be dyed, whether to facilitate their combination, 
or to modify it. By the ufe of mordants, the colours 
are occafionally changed, receive brightnefs, are fixed, 
and made lading. The mordant is not always a fimple 
agent; but, in the mixture whereof it is compofed, it 
fometimes forms frelh combinations; fo that the fubltances 
employed do not aft immediately of themfeives, but by 
the combinations which refult from them. 
The chemical agents ufed in dying are, the fulphuric, 
nitric, and muriatic, acids; the muriats of tin, lime, 
magriefia, foda, ammoniac, and the oxygenated muriat 
of mercury: the muriatic falts in general have the pro¬ 
perty of rendering colours darker, and of increafmg 
their ftrength. 
The oxygenated muriatic acid, is ufed where feveral 
colours are required, and to have them clear. It is em¬ 
ployed alio to prove the ftrength of colours, and to com¬ 
pare the ftrength of colouring matters of the lame kind. 
It is ufeful in whitening the ground work of painted 
cloths ; and in difeharging the colours of pieces which 
have met with lome accident in dying, or winch are faded 
through age. 
The ilitro-muriatic acid is much ufed in metallic folu,- 
tions, as of tin, bifmuth, See. The folution of tin is 
very ufeful in the art of dying. The oxyd of tin is 
much difpofed to abandon its folvent, to combine either 
with the ftuff or with the colouring matters; it gives to 
colours a white bafe, not fiibjebl to change ; it produces 
no combuflion in the colouring matters 1110ft difpofed to 
change their colour from that caufe, fuch as flnimach and 
nut-galls. Purified tartar, and vinegar, are of ufe 
alfo in dying. The acidulous oxalat of potafli produces 
very good effects in certain fituations. 
Alum is of very extenfive ule in dying. The appli¬ 
cation of alum mull beconfidered as a general and indif- 
penfible operation of filk-dying, as without it the greateft 
part of the colours will have neither beauty nor ftrength. 
The proportion of alum to colouring matter fhould be as 
four to Sixteen, or one fourth. Diffolve the alum in a 
kettle of warm water, ftirring it to prevent the alum 
from cryftallizing ; add a lolution of foda in the propor¬ 
tion of a fixteenth part of foda to one whole part of 
alum; IbmS add befides a very fmall quantity of tartar 
and arfenic. The raw material mull be well impregnated 
with this folution; take each time about one pound of 
the filken thread, pour the remainder of the liquor over 
the whole of the filk collebled in another veffel; let it 
foak four-aild-twenty hours ; then expofe it to a running 
ftream for an hour and a half or two hours, and wafh it. 
Soaking in a decoblion of nut-galls is as neceflary an 
operation for cotton or flax. 
Sulphatof iron is greatly ufed in dying, efpecially for 
black colours, and grey, and other (hades which are 
darkened by it means. Solutions of iron by the acetous 
acid, and by fome other vegetable acids, are employed 
alfo in dying; but thefe preparations are generally made 
on the fpot where they are uled. 
Sulphatof copper is employed occafionally, but not often. 
The oxydof copper combineswith moll colouring matters, 
which precipitate it from acids ; it often communicates 
an agreeable colour; but, as its own natural colour, 
which influences'that of its combination, is foon chang¬ 
ed by the air, it can only produce colours variable and 
not lafting. It has been remarked alfo that the fuiphat 
of copper corrodes the fluffs more than that of iron ; 
