D I A
DYE
i
faces of a rhomboidal prism, the angles ofwhich are alternately 130° and 50 s / Its hard-ness is somewhat superior to that of glass. Sp.gr. 3.4.
A small portion of this mineral when ex-posed for a few seconds to the flame of a candleis dispersed in fragments with a brisk decrepi-tation. If heated in a close crucible it assumesthe appearance of boracic acid. It is composed,according to Vauquelin, of80. Alumine17. Water3. Oxyd of iron
100 .
Its gangue is a ferruginous clay.
Is not this mineral a mere variety of theWavf.li.ite, first discovered by Dr. Wavelnear Barnstaple in Devonshire , and since foundin Brazil by Mr. Mawe ?
DIOPTASE. See Coffer.
An enquiry of considerable extent and appa-rently conducted with much pains on the modein which some of the most usual mordants unitewith animal and vegetable fibre has been madeby Messrs. Thenard and Roard, of which weshall give a short abstract. a
Of Alum Mordants.
When wool or other fibre is macerated witha solution of alum, either with or without otheradditions, this salt is commonly considered tobe decomposed, the alumine alone remainingunited with the animal or vegetable fibre, andthus serving as an intermede for the fixation ofthe colouring matter. The fact of this sup-posed decomposition was first examined.
Some silk, thoroughly cleansed and washed,was digested for six days at a common tem-perature with a solution of 100 grammes ofalum in 4 litres of water. It was then takenout and washed in distilled water; the remainingliquor with the washings being then evaporated,yielded pure crystals of alum to the very last.The silk itself was then boiled with twelve suc-cessive portions of water, all of which werealso evaporated, and gave nothing but purealum. The entire quantity of alum thus re-covered from the silk and from the remaining so-lution was within of the quantity oi alumoriginally employed. The silk thus deprived ofalum had no greater power of fixing dyes thanif the alum-bath had never been used.
)
From this it is inferred that alum is not de*-composed in the fibres of the silk.
Some wool was boiled repeatedly with weakmuriatic acid to separate the carbonat of limewhich naturally adheres to it. It was thenboiled with many portions of fresh distilledwater till no trace either of muriatic acid or oflime was left in the washings, (which was a.very long and tedious operation) and the woolwas left quite pure. Some of it war, then boiledin a hot f solution of a given quantity of alum,and thus prepared it took colouring matter withgreat ease. But after being repeated.ly boiledin water, all the alum was washed out andmost of this salt was recovered in the samestate in which it was at first employed, thoughsome of it remained mixed with an animalmatter extracted from the wool, and couldhardly be made to crystallize. The experimentwas therefore repeated with the difference ofusing a cold alum-bath in the first instance,and then, by successive washing, the whole of.the alum to -y-^th part was recovered in crys-tals unaltered. Alum therefore combines withpure wool without undergoing any change ordecomposition.
Cotton and thread gave similar results, onlythat the subsequent extraction of the alum wasmuch less difficult, as the union between alumand vegetable fibre is much weaker than withanimal fibre.
The authors then examined the operation ofthe alum-bath on wool in its common state andnot previously purified of the carbonat of limewhich adheres to it. In all the common pro-cesses of aluming wool the alum liquor inwhich it is steeped becomes very turbid, and oncooling lets fall an abundant white precipitate.This was found to consist of suiphat of lime, of :neutral suiphat of alumine, and sometimes of a.little alumine. The clear liquor above the pre-cipitate, on the other hand, contained alum and.acid sulphate of potash with a little animalmatter. The wool itself when boiled with watergave up merely alum with a very trifling pro-portion of this white precipitate. As this pre-cipitate never appears when the wool lias beencarefully purified from the carbonat of lime, theauthors examined the operation of this earthupon alum alone. Alum long boiled with car-bonat oi- lime is totally decomposed by a suf- ■ficient proportion of it, into a solution of ecidsuiphat of. iime and a deposit of -ulpnat of limewith subsulphat of alumine and potash.
Other earthy or alkaline salts have a similar
» Aa. Chim. tom. 74,