COTTON.

33

which produced an abundant light brown flocculent precipitate, consisting ofimpure pedlate and parapedtate of soda. The liquid, which was brown, con-tained a considerable quantity of colouring matter, and was filtered. Theprecipitate, after being washed with alcohol, was again dissolved in water, andto the solution there was added a quantity of strong caustic soda-lye, whichproduced a precipitate of a much paler colour than that with alcohol, whilethe liquid retained in solution another portion of colouring matter. The pre-cipitate having been allowed to settle, the liquid was decanted, and theprecipitate was stirred up with a fresh quantity of caustic soda-lye, the processbeing repeated until the supernatant liquid had become colourless. The pre-cipitate was now treated with warm dilute hydrochloric acid, which took upthe alkali, leaving a quantity of pale brown flocks tindissolved. These werefiltered off and washed for some time with cold water, the latter being exchangedfor alcohol as soon as the principal part of the acid and chloride of sodium hadbeen removed, in order to prevent a loss of parapedtic acid, which is soluble inpure water, though insoluble in water containing strong acids or salts. Assoon as the liquid ceased to give a precipitate with nitrate of silver, the masswas taken from the filter, pressed between folds of blotting-paper, in order totake up the alcohol, and then treated with boiling water for some time. Amilky liquid was thus obtained, which was set aside for some time in order toallow the flocculent matter suspended in it, which prevented it from filteringreadily, to settle. The liquid, having been decanted from the deposit, wasfiltered and evaporated, when it left an amorphous light brown residue, resem-bling gum or gelatine, and having the properties and composition of parapedticacid. The flocculent deposit consisted of pedtic acid ; but it was contaminatedwith a considerable quantity of colouring matter, from which it could, in mostcases, not be separated without undergoing a change. If, for instance, it wasdissolved again in caustic soda, and the process just described was repeated,it was usually converted for the most part into a substance soluble in water—that is, into parapedtic acid.

The second method of preparing the acid is far less tedious, and yields aprodudt much freer from colour than the one just described. It differs, how-ever, from the latter in one particular only, viz., in the use of a bleaching agentinstead of caustic soda as a precipitant. To the watery solution of impurepedtate and parapedtate of soda, obtained in the same manner as before, therewas added a clear solution of chloride of lime, which produced an abundantprecipitate consisting mostly of pedtate and parapedtate of lime. By adding anexcess of the bleaching solution, the colour of the precipitate, which was atfirst brown, became gradually lighter. As soon as it had lost all its colour, orretained only a yellowish tinge, it was filtered off, washed with water, andtreated with dilute hydrochloric acid. The white flocks left undissolved bythe acid were filtered off, washed at first with water and then with alcohol,and lastly treated as before with boiling water, which left the pedtic acidundissolved. The filtered liquid left, on evaporation, a residue of parapedticacid, having only a faint yellowish tinge like that of the purest gum. It didnot differ in composition from that prepared by the process first described,proving that the chloride of lime produced no decomposing effedt, but merelyserved to destroy and remove the colouring matter.

The following is a short account of the properties and composition of thevarious substances the preparation of which has been just described :—