849 



INDICTION. 



INDIGO. 



850 



denser, is recorded. The indications of the pressure of steam in the 

 boiler are marked by the pressure gauge ; the indications of the useful 

 amount of power transmitted to the first motion-wheel are recorded by 

 the dynamometer ; whilst the meaning of the generic term is limited 

 to the sense above given. Different constructors have introduced 

 varieties in the form and working details of their indicators ; but they 

 all are identical in principle. They are all self-registering, and show 

 (by means of a curve traced on a slip of paper by a pencil attached to 

 the part of the indicator immediately connected with the steam in the 

 interior of the cylinder, or with the vacuum), the difference between the 

 pressure of the atmosphere around the cylinder, and the pressure within 

 the cylinder, at every part of the up and down stroke of the piston, for 

 that end of the cylinder to which the indicator is attached. In order 

 to effect this object, a small cylinder is connected with the larger one in 

 which a steam and air tight piston works, and the rod of this piston 

 passes into another small cylinder, in the interior of which is placed a 

 spiral spring, which in its turn presses upon the rod of the indicator 

 piston in such a manner as to keep the latter in the centre of its 

 cylinder, when the pressure of the atmosphere and of the vapour in 

 the cylinder of the engine balance one another. In proportion as the 

 relative pressure increases on either side of the cylinder of the in- 

 dicator, it will be found that the piston will rise or fall ; for spiral 

 springs (as Messrs. Hann and Gener observe), are extended and com- 

 pressed through equal spaces by equal and opposite pressures. There 

 is a stud passing from the top of the piston-rod through a slot in the 

 spring cylinder, and an index pointer shows on a scale attached to the 

 side of the slot the pressure acting upon the piston. The description 

 and use of this instrument are well explained in Hann and Gener's 

 book, above quoted, entitled ' The Steam Engine,' 1854, 4c. &c. 



INDICTION ; CYCLE OF INDICTION. [PERIODS OF REVO- 

 LUTION.] 



INDICTMENT is a written accusation of one or more persons of a 

 crime or misdemeanour presented by a grand- jury. The sheriff 

 returns to every session of the peace and every commission of oyer and 

 terminer and jail delivery at least twenty-four freeholders of the 

 county, twelve of whom, at least, and not more than twenty-three, are 

 sworn upon the grand jury. They are instructed in the articles of 

 their inquiry by the judge, and then withdraw to sit and receive bills 

 of accusation, which are presented to them in the name of the crown, 

 but at the suit of any private person. The decision of the grand jury 

 is not in the nature of a verdict as to the guilt of the accused, but 

 merely the' expression of their opinion that from the case made by the 

 prosecutor the matter is fit to be presented to the common jury. In 

 conducting the inquiry the evidence in support of the accusation only 

 is heard. If the grand jury think the accusation groundless, they 

 indorse on the bill " not a true bill," or " not found ; " if the contrary, 

 " a true bill ; " and in finding a true bill twelve at least of the grand 

 jury must concur, because no subject can be put to answer an 

 accusation except such as is credited by such a number of hia fellow- 

 citizens. Anciently the words " ignoramus " and " billa vera " were 

 used. When a bill is found to be a true bill, the trial of the accused 

 takes place in the usual form ; when the bill is " ignored," the accused 

 ia discharged, but a new bill may be preferred against him before the 

 same or another grand jury. Sometimes, when the bill is ignored on 

 account of some slip or error, the judge will direct the accused to be 

 kept in custody, in order to prevent him from escaping from justice. 



INDIGESTION. [DTSPEPSU.) 



INUIGO. This well known colouring matter is a product of the 

 vegetable kingdom, being derived from the cellular tissue of the leaves 

 of certain plants. For the names and habitats of these plants, see 

 ISDIUOFEHA, in NAT. HIST. Div. 



Indigo was well known to the ancients. Bands of cloth, evidently 

 dyed with this material, are met with in the wrappings of Egyptian 

 mummies ; and Dioscorides and Pliny mention it as an Indian product 

 under the names of tvtimov and indicnm. It began to be employed in 

 Europe about the middle of the 16th century, but owing to the oppo- 

 sition of the cultivators of the native woad, its importation into Eng- 

 land was prohibited ; and it was not till the time of Charles II. that 

 these prejudices were overcome. At the present time there are sent 

 into this country from 60,000 to 70^000 cwts. of indigo per annum. 



The cultivation of the indigo plants is extensively carried on in 

 various parts of India and America. In the spring the seeds are sown, 

 at the rate of about a dozen pounds per acre ; the plant* grow rapidly, 

 commencing to blossom in three months time, when they are cropped 

 and* again allowed to grow till they are sufficiently mature to admit of 

 another cutting. Occasionally a third and even a fourth cropping is 

 made, but each of these contains successively less and less of the 

 matter that yields the colouring principle. 



The indigo plants present no appearance when growing that would 

 lead an observer to suppose they contained any matter capable of pro- 

 ducing colour ; nor is it yet satisfactorily determined in what form the 

 indigo pre exists in the vegetable tissues. From the recent experi- 

 ments of Schunk (' Manchester Memoirs,' 1855, vol. xii., p. 177), it 

 would teem that a principle, termed by him Indican, of yellow colour, 

 and having the composition (C 5 ,H,,NO,,?) ) occurs naturally in the 

 plant*, and that it is from the decomposition of this body, under cir- 

 cumstances immediately to be described, that the indigo is produced. 



The manufacture of indigo in the East Indies is conducted some- 



ABT8 AXD SCI. I1Y. VOr,. IV. 



what after the following plan. The recently cut plants are placed in 

 large stone cisterns, called fermenting vats, or steepers, and covered 

 with water; fermentation soon commences, large quantities of gas rise 

 in bubbles to the surface, and the whole mass becomes covered with a 

 copper-coloured scum. After about fifteen hours fermentation ceases, 

 and the liquor is then run off into another vat placed on a lower level. 

 Here it is brought well into contact with the air by beating with 

 wooden paddles for about an hour and a half. During the beating the 

 contents of the vessel darken in colour, and the indigo separates out in 

 flocks; these are allowed to subside, heated to boiling to effect the 

 removal of certain impurities, then collected on woollen strainers, and 

 finally pressed, dried, and the mass cut up into blocks for sale. At 

 some places the plants are, after cutting, spread out to dry in the sun, 

 it being supposed that a better quality of indigo is thus obtained. 



Indigo is usually met with in commerce under the form of cubical 

 masses or cakes, which are brittle and of characteristic colour, a colour 

 that ranges from a deep blue containing a little purple, to a dark purple 

 containing a little blue. When burnished it acquires a beautiful, 

 glossy, copper tint. It is generally thought that its specific gravity 

 should be less than that of water, though the method of preparation 

 and adulteration with foreign matters often cause it to weigh more 

 than an equal bulk of that liquid. Many methods for estimating the 

 absolute amount of colouring matter indigqtin in a specimen of 

 indigo have been proposed, but they all require experience in chemical 

 manipulation for their accurate performance : they will be found de- 

 scribed in the various analytical text- books. 



The great value of indigo as a dyeing material, and the enormous 

 extent to which it is used, have procured for it a large share of the 

 attention of scientific chemists. The result is that its application as a 

 pigment has been greatly extended, and many important and inter- 

 esting derivatives obtained from it. The following list will exhibit at 

 a glance the relations which the chief of the.se derivatives bear to each 

 other. Starting with pure indigo itself (indigotin), which may be 

 looked upon as a positive radical, and which we may represent by the 

 symbol (ln = indyt), we have : 



Indigo-blue . 

 White indigo 

 Sulphindigotic acid . 

 Inatin 

 Is.itic acid 

 Isathvd 

 Indin . . 

 Hydrindin . 



C..H.NO, 

 C^H^NO,, S 



C,.H 7 NO. 

 C 16 H.N0 4 

 C,,H, n N,0. 



= In 

 = In, H 

 = In, S 2 O 4 

 = InOj 

 = HO, InOj 



= In,H 



Indigo-Wue or indigotin (C 10 H S NO.,), may be obtained in a very 

 beautiful form by mixing commercial indigo with about half its weight 

 of plaster of paris and sufficient water to make a thick cream ; this is 

 evenly spread over an iron plate so as to form a stratum about one- 

 eighth of an inch in depth, and set aside to dry. A spirit or gas flame 

 is then applied to the under surface of the plate when the indigo sub- 

 limes out from the mixture and condenses in brilliant little purplish- 

 brown crystals, on that part of the plaster immediately over the source 

 of heat. By gradually moving the lamp the indigo is sublimed in 

 successive portions that readily admit of being removed by a spatula 

 from the hard cement below. Indigo-blue thus produced is very 

 beautiful, but not pure. To obtain it in the latter state, it is necessary 

 to dissolve it in a dilute alkaline solution, and as indigo itself is not 

 soluble hi such a menstruum, it is brought into intimate contact with 

 some substance that is capable of furnishing hydrogen to it, hydride of 

 blue indigo being thus formed which is quite soluble. One of the best 

 methods of accomplishing this is the following. Four parts of indigo 

 in 'powder, four parts of grape sugar, and ten parts of a saturated solu- 

 tion of caustic soda, are placed in a flask of such a size that it will 

 hold about twenty times the volume of the mixture. The vessel is 

 now filled up with boiling alcohol, well stoppered, briskly agitated, and 

 set aside. The liquid soon becomes clear, and is then poured out into 

 shallow vessels and exposed to the air, when it rapidly absorbs oxygen 

 and deposits crystals ; these when washed, first with alcohol and then 

 with hot water, form perfectly pure indigo blue. 



Indigo-blue is a neutral body, tasteless and inodorous. It is insoluble 

 in water, alcohol, ether, oils, dilute acids, or alkalies. It fuses and 

 sublimes in purple-coloured vapours at about 550 Fahr. ; at a higher 

 temperature it is decomposed, hydrocyanate and carbonate of ammonia, 

 aniline, and an empyreumatic oil being*among the the volatile products, 

 while much carbonaceous residue remains. 



\\'liile~lndiyo, Indigogen, or (improperly) Reduced Indigo (C, e H N0 3 

 = C 10 H 5 NO. ! ,H) is a combination of hydrogen with indigo blue, and 

 may be prepared by several processes : all of which, however, depend 

 upon the deoxidising effect they have upon water, the hydrogen of that 

 compound being thus furnished to the indigo. Grape sugar is such a 

 deoxidising or hydrogenising agent, and if to the dilute alkaline solu- 

 tion (already referred to in the description of the method for obtaining 

 pure indigo blue) there be added acetic acid, dirty white flocks of 

 indigogen are precipitated. White indigo is insoluble in water, slightly 

 soluble in alcohol and in ether, and when exposed to the air rapidly 

 absorbs oxygen, forming water and indigo-blue. 



3 I 



