68

DYEING AND CALICO PRINTING.

acid is added, and starch (not green starch) previously made into a paste withwater is poured into this liquid, which must be kept boiling, the starch is firstconverted into dextrine, and afterwards into glucose; (4) The peculiarly-modified nitrogenous principle met with in malted barley, generally calleddiastase (though Professor Mulder proved some years ago in his excellent workon Beer that diastase individually does not exist at all, and that the incipientgermination of barley or any other grain has the effedt of greatly modifyingthe composition and adtion of the protein compounds met with in grain) hasthe power of converting starch into dextrine and glucose at a temperature notexceeding 167° F. (75° C.), and to adt thus even in relatively small quantity,since one part of the nitrogenous principle readily converts a thousand partsof dry starch into dextrine, provided sufficient water be present ; saliva,pancreatic juice, bile, and intestinal mucus of men and animals adt in a similarmanner, and are very rapid in their adtion. Iodine in free state gives to starchpaste a beautiful deep blue colour, which disappears on the application ofheat, but the adtion of this well-known reagent is either entirely or in partprevented by the presence of some substances, among which, according toGoppelsroeder, are many soluble sulphates. If, when dry starch and strongsulphuric acid are mixed at the ordinary temperature of the air, an amylo-sulpho-acid is formed, at a higher temperature the starch is decomposed andbecomes black; it is converted into xyloi'dine at the ordinary temperature bythe adtion of very strong nitric acid, and, at a higher temperature, into oxalicacid. Starch-paste, when exposed to the adtion of air, is gradually convertedinto ladtic acid. Schweitzer’s reagent does not affedt starch. When ofgood quality starch contains but little ash, for the incombustible matter thatmay be present being only that which was found as solid residue after theevaporation of water, is, therefore, attributable to the water employed in themanufadture of starch. It sometimes happens that starch is purposelyadulterated with gypsum, sulphate of baryta, or prepared chalk, but suchfrauds are readily detedted by incineration. Starch (fecula) is very largelyused, especially in the dressing of goods ; it is applied either with or withoutdextrine to thicken colours for printing ; sago, wheaten flour, wheaten starch,farina, and salep are all thickeners of the same class.

Dextrine.

We have already alluded to the method by which dextrine is manufactured,but it is necessary to mention here that since the colour of what is now calledcalcined farina was, in many instances, an objection to its being used in calicoprinting, it was very desirable to find a process for its conversion at so low atemperature as to leave the converted farina nearly colourless. This was firsteffected in 1838 by M. Payen, who found that by adding 400 parts of dryfarina to 1 part of nitric acid at a specific gravity of i'4, diluted with sufficientwater to form with the farina a hard paste, this being dried slowly and heatedin a close chamber for twenty hours at a temperature of 392° F. (200° C.), anearly white farina was obtained. It is interesting to observe how so smalla quantity as a few thousandths of acid can effedt this great molecular change.Since that time many processes have been devised to attain the same end;such, for instance, as the employment of ladtic acid by Mr. E. Hunt, ofoxalic and tartaric acids by others, and a process by Mr. Charles O’Neill, which