CHEMISTRY. 



Chemical stinct proportKM. But hitherto these varieties hive not 

 been examined with uch attention as to enable ui to give 

 a detailed description of each. 



Iimi of 

 Nature. 



: .;.< 



How ob- 

 tained. 



:opcr. 



Stcr. XI. Of Indigo. 



'1 ins valuable pigment, one of the capital manufacture* 

 of America, it obtained from the leavei of different spe- 

 cies of plants : the indigoftra argentea or wild indigo, 

 the indigufrra dispenna or Gualimala indigo, and the 

 indigfiffra tintloria or French indigo, which yields the 

 greatest quantity of indigo, and is therefore preferred by 

 the planter, though its quality is said to be inferior to 

 that of the indigo obtained from the two first species. In 

 the Welt Indies the seed* are sown in March, in trenches 

 about a foot asunder, and the plant comes into blossom, 

 and is fit for cutting down in May. But in South Ame- 

 rica, six months elapse before it can be cut. In the West 

 Indies, four cuttings are often obtained from the same 

 plant in the course of a year ; but in America, never 

 more than two, and often only one. The produce con- 

 stantly diminishes after the first cutting, so that it is ne- 

 cessary to renew the plants for seed every year. 



The plants are cut down with sickles, and laid in stra- 

 ta in the stecjx-r till it is about three pans full. This is 

 a large cistern of wood or mason work about 16 feet 

 square. Here they are pressed down with planks, and 

 loaded to prevent them from swimming, and covered 

 with water to the height of four or five inches. Here 

 they ferment, and the utmost attention is required to the 

 process. It they be allowed to remain too long, the pig- 

 ment is spoiled ; and if the water be drawn off too soon, 

 much of the indigo is lost. The temperature of 80 is 

 said to answer best. The water acquires a green colour, 

 a smell resembling that of ammonia is exhaled, and bub- 

 bles of carbonic acid are emitted. When the fermenta- 

 tion has continued long enough, the liquor is let out into 

 a second cistern placed lower than the first ; this cistern 

 i called the battery, and is commonly above 12 feet 

 square, and four and a half deep. Here it is agitated 

 tor 15 or '20 minutes, by means of levers driven by ma- 

 chinery, till the flocculi beginning to separate give it a 

 curdled appearance. A quantity of lime-water is now 

 poured in, and the blue tlocculi are allowed to subside. 

 The water is then drawn off, and the pigment put to be 

 drained in small linen bags, after which it is put into little 

 square boxes, and allowed to dry in the shade. 



Chevreul has shown, that the indigo exists in the plant 

 i:hiefly in the state of a v.-hite matter, which becomes 

 blue when it combines with oxygen. Indigo may be 

 obtained also from other plants, the wrium tincloriiim, 

 for example, and the iwtii tincloria, or :coa(l, a plant 

 common enough in Britain. But the quantity obtained 

 from this plant does not exceed one-tenth of what may 

 be procured from the iudijoflra. 



Indigo is a tine light friable substance of a deep blue 

 colour. Its texture is very compact, and the shade of 

 its surface varies according to the manner in which it has 

 been prepared. The principal tints are copper, violet, 

 and blue. The lightest indigo is the best ; but it is al- 

 ways mixed with foreign substances, scarcely one half 

 even of the best indigo of commerce consisting of the 

 pure pigment. The following substances were extracted 

 by Chevreul from 100 parts of Guatimala indigo : 



Ammonia, a trace 

 Diioxygenized indigo 

 Green matter .... 

 Bitter matter, a trace 



Red matter 



Carbonate of lime . . . 

 Oxide of iron and alumina 

 Silica . 



12 

 SO 



6 

 2 

 2 

 3 



Pure indigo 45 



100 



Pure indigo has neither taste nor smell. It is insolu- 

 ble in water in its usual state, but disoxygeni/ed indigo is 

 soluble in that liquid, as are likewise some of the foreign 

 bodies with which indigo is usually mixed. When 

 heated, indigo sublimes in a purple smoke, and may be 

 obtained unaltered crystallized in needles. This purple 

 smoke is characteristic of indigo. 



Neither oxygen nor the simple combustibles have any 

 action on common indigo ; but disoxygenised indigo 

 readily combines with oxygen, and may be separated 

 again from it without decomposition. In this respect it 

 differs from almost all other vegetable substances, and ap- 

 proaches the properties of the simple combustibles and 

 metals. 



The fixed alkaline solutions have no effect upon indi- 

 go, except it be newly precipitated from a state of solu- 

 tion. In that case they dissolve it with facility. The 

 solution has at first a green colour, which gradually dis- 

 appears, and the natural colour of the indigo cannot be 

 again restored. Hence we see, that the alkalies when 

 concentrated decompose indigo. Pure liquid ammonia 

 acts in the same way. Even carbonate of ammonia dis- 

 solves precipitated indigo, and destroys its colour. 



Lime-water has scarcely any effect upon indigo in it 

 usual state, but it dissolves precipitated indigo. The 

 solution is at first green, but it becomes gradually yellow. 



When diluted sulphuric acid is digested over indigo, 

 it produces no effect except that of dissolving the impu- 

 rities ; but concentrated sulphuric acid dissolves it rea- 

 dily. One part of indigo, when mixed with eight parts 

 of sulphuric acid, evolves heat, and is dissolved in 2t 

 hours. The solution of indigo is well known in this coun- 

 try by the name of /iqnid Hue. Bancroft calls it *//- 

 phale of indigo. While- concentrated, it is opake and 

 black ; but when diluted, it assumes a fine deep blue co- 

 lour, and its intensity is such, that a single dn p of the 

 concentrated sulphate is sufficient to give a blue colour 

 to many pounds of water. Bergman ascertained the ef- 

 fect of different reagents on this solution with great pre- 

 His experiments threw light not only on the 



cision. 



properties of indigo, but upon the phenomena that take 

 place when it is used as a dye-stuff. 



From his experiments, it is obvious that all those sub- 

 stances which have a very strong affinity for oxygon 

 give a green colour to indigo, and at last destroy it. 

 Hence it is extremely probable, that indigo becomes 

 green by giving out oxygen. Of course it owes its blue 

 colour to that principle. This theory was first suggested 

 by Mr Haussman, and still farther confirmed by Ber- 

 thollet. Now it is only when green that it is in a state 

 capable of being held in solution by lime, alkalies, &c. 

 in which state it is applied as a dye to cloth. The cloth, 

 when dipt into the vat containing it thus dissolved, com- 

 bines wilh it, and the blue colour is restored by exposure 

 to the atmosphere. It may be restored equally by plun- 

 ging the cloth into oxymuriatic acid. Hence the resto- 

 ration cannot but be ascribed to oxygen. Hence then 



