C H E M I S T R V. 



113 



f'Uen.ica! 



Uxamini- 



tun, of 



Nature. 



the reason that sulphurous acid, the vegetable acids, 

 sulphate of iron, give sulphate of indigo a green colour. 



From these experiments, we see also that the colour of 

 indigo is destroyed by the addition of those substances 

 which part with oxygen very readily, as the black oxide 

 of manganese. In that case the indigo is destroyed, for 

 its colour cannot be again restored. When the sulphate 

 of indigo is poured into boiling water, it affords a green- 

 coloured solution ; but with cold water a deep blue so- 

 lution. What is called smoking sulphuric acid dissolves 

 indigo much more readily than the pure acid, and evolves 

 much more heat during the solution. Bucholz has shown, 

 that by boiling sulphur in pure sulphuric acid, it acquires 

 the property of dissolving indigo as readily as the smo- 

 king acid. 



Nitric acid attacks indigo with great violence ; with 

 th.> evolution of abundance of heat and nitrous gas. 

 Wheti of the specific gravity 1.52, it even sets fire to in- 

 digo. When the acid is diluted, the action is still violent, 

 unless the proportion of water be considerable. Mr 

 Hatchett poured upon 100 grains of indigo an ounce of 

 nitric acid, diluted with an equal quantity of water. The 

 action was so rapid, that he found it necessary to add 

 another ounce of water. When the effervescence had 

 nearly subsided, the liquid was placed on a sand bath for 

 some days, and evaporated to dryness. Water poured 

 upon the residuum dissolved a considerable portion of it, 

 and formed a beautiful deep yellow solution of an intense 

 bitter taste. This solution contains only a very small 

 portion of oxalic acid j but with a solution of isinglass it 

 forms a copious yellow insoluble precipitate, and of course 

 contains a portion of artificial tannin. With ammonia 

 crystals precipitate, consisting of Litter principle combi- 

 ned with ammonia. 



When four parts of nitric acid are boiled upon one 

 part of indigo, the pigment soon loses its colour, and is 

 dissolved. The solution becomes yellow, and a thin 

 layer of a resinous matter appears on the surface. If 

 the process be now stopt, the resinous matter becomes 

 thin by cooling. If this matter be removed, and the so- 

 lution evaporated to the consistence of honey, redissolved 

 in hot water and filtered, potash throws down yellow 

 spicular crystals, consisting of titter principle combined 

 with potash. These crystals have the curious property 

 of detonating with a purple light, when wrapt up in pa- 

 per, and struck with a hammer ; the resin, by treating 

 it with fresh nitric acid, may be converted into the same 

 bitter principle. If the process be stopt sooner than the 

 point mentioned above, yellow crystals are obtained, 

 which are more soluble in water, and which sublime in 

 white needles, having all the properties of benzoic acid. 



Muriatic acid docs not act upon indigo in its common 

 state, but it readily dissolves indigo precipitated from 

 the sulphate, and forms a blue-coloured solution. The 

 same phenomena are exhibited by the phosphoric, ace- 

 tic, Uitaric acids, and probably by all except the acid 

 tapporters. 



Chlorine destroyi the colour of indigo as readily as ni- 

 tric acid. 



Alcohol dissolves a small portion of indigo, but it 

 gradually precipitates again unless red matter be present, 

 in which case the solution is permanent. 



Indigo is not acted upon by ether or oils, at least if 

 the experiments of Bergman be accurate. 



When indigo is mixed with bran, woad, and other si- 

 milar substances which readily undergo fermentation, it 

 assumes a green colour during the fermentation, and is 

 then easily dissolved by lime or potash. It is by thi$ 

 process that it is usually rendered proper for dyeing. 



VOL. vi. PART I. 



SECT. XII. Of Gluten. 



Chemical 



Kxumiiu. 



tion of 



If wheat-flour be kneaded into paste with a little v_. ^ ^_j 

 water, it forms a tenacious, elastic, soft, ductile mass. Gluten. 

 This is to be washed cautiously, by kneading it under 

 a small jet of water, till the water no longer carries off 

 any thing, but runs off colourless ; what remains behind 

 is called gluten. It was discovered by Beccaria, an Ita- How ob- 

 lian philosopher, to whom we are indebted for the first tained. 

 analysis of wheat-flour. 



Gluten, when thus obtained, is of a grey colour, ex- j ts proper- 

 ceedingly tenacious, ductile, and clastic, and may be ex- ties. 

 tended to twenty times its original length. When very 

 thin, it is of a whitish colour, and has a good deal of re- 

 semblance to animal tendon or membrane. Its smell is 

 peculiar. It has scarce any taste, and does not lose its 

 tenacity in the mouth. 



When exposed to the air, it gradually dries ; and when 

 completely dry, it is pretty hard, brittle, slightly trans- 

 parent, of a dark brown colour, and has some resem- 

 blance to glue. 



Fresh gluten imbibes water, and retains a certain quan- 

 tity of it with great obstinacy. To this water it owes 

 its elasticity and tenacity. When boiled in water it 

 loses both these properties. 



When fresh gluten is macerated for a considerable 

 time in cold water, the liquid becomes opake, and con- 

 tains small films suspended, which do not soon subside. 

 By repeated filtrations it becomes transparent ; but it 

 holds in solution gluten, which renders it frothy, and 

 gives it the property of precipitating when mixed with 

 oxymuriatic acid or the infusion of nutgalls. Thus glu- 

 ten is to a certain extent soluble in cold water. When 

 the water is heated, the gluten separates in the state of 

 yellow flakes. 



When kept moist, it very soon begins to decompose, 

 and to undergo a species of fermentation. It swells, 

 and emits air-bubbles, which Proust has ascertained to 

 consist of hydrogen and carbonic acid gases. It emits 

 also a very offensive odour, similar to what is emitted by 

 putrefying animal bodies. Cadet kept gluten in a -vessel 

 for a week in a damp room. Its surface became covered 

 with byssi, the fermentation just mentioned had commen- 

 ced, and the odour was distinctly acid. In 2-t days, on 

 removing the upper crust, the gluten was found convert- 

 ed into a kind of paste, of a greyish- white colour, not 

 unlike bird-lime. In that state he gave it the name of 

 fermented gluten. If the gluten be still left to itself, it 

 gradually acquires the smell and the taste of cheese. This 

 curious fact was first ascertained by Rouelle junior. In 

 that state it is full of holes, and contains the very same 

 juices which distinguish some kinds of cheese. Proust 

 ascertained that it contains ammonia and vinegar ; bodies 

 which Vauquelin detected in cheese ; and ammonia robs 

 both equally of their smell and flavour. 



Fresh gluten does not sensibly dissolve in alcohol, 

 which even throws down fresh gluten from water j yet in 

 certain cases this liquid forms^ solution of gluten in very 

 small proportion. 



Whrti the fermented gluten of Cadet is triturated with Fermented 

 a little alcohol into a mucilage, and then mixed with a gluten, 

 sufficient quantity of that liquid, a portion of it" is dis- 

 solved. This solution constitutes an excellent varnish, 

 possessed of considerable elasticity. It may be spread 

 over paper or wood ; and when dry resists other bodies, 

 as well as most varnishes. In this state, too, it may be 

 employed to cement china ; and triturated with paints, 

 especially vegetable colours, it forms a very good groun 



