'400 PURPLE DYEING, ANCIENT AND MODERN. 



produced volatile alkali, aud pliospLoretic ammonire natron. The last named 

 salt was probably known to the ancients, aud used by them in soldering metals. 



The uric acid which now is so importantly utilized in viriuc, or rather in uric 

 calculi, was found by Scheele, in the year 177G, which are, for the most part, 

 composed of that acid, itself a component part of the urine of all carnivorous 

 animals, and perhaps of all animals having a renal secrcition ; * being in the ex- 

 crement of birds, snakes, and even iu that of caterpillars, snails, &c. 



Scheele remarked, that a solution of uric acid in acid of saltpetre left, Avlien 

 evaporated, a red sediment, and would stain the skin a fine red color. In 1818, 

 Prout, by the action of ammoniaj on a solution of uretric matter in acid of salt- 

 petre, discovered a material which he called " purple acetic amraonia^um," on 

 account of its splendid color. He thus describes the process of producing it : 

 Pure acetic acid and acid of saltpetre are mixed with an equal volume of water 

 and gently warmed till solution takes place and strong fermentation is produced. 

 The superfluous acid of saltpetre is then diluted w4th ammoniajum and the 

 whole reduced by evaporation. During the operation the color gradually 

 changes from purple to red, and through numerous shades of dark red. Green- 

 ish granular crystals are precipitated, which consist of purpuric acid and am- 

 moniajum. This reaction is remarkable, and so positive that chemists have 

 long availed themselves of it to detect the presence of uretric acid in any organic 

 substance, for this characteristic coloring only takes place in the way mentioned 

 and where uretric acid is present. 



Liebig and Woehler, in 1837, also produced this brilliant colored matter 

 while exjierimenting on the changes of uretric acid under the influence of oxyd- 

 ating matter. It appeared iu the form of small crystals, or short four-sided 

 prisms, Avhich Avhen held up to the sunlight appear of a rich garnet-red color, 

 changing, under a reflected light, to a greenish metallic splendor not unlike that 

 of the wing of a rose-chaflfer. Those chemists believing Prout to be mistaken 

 as to the chemical composition of this beautiful colored material gave it the 

 name of "murexid," from murex, the purple snail. It is not formed directly 

 from the uretric acid, W'hich is first converted into aloxan and aloxantine by 

 means of acetic snltpetre. These are two colorless combinations of little. dura- 

 bility, but, acted upon by ammoniajum, they exhibit the purple-red coloring. 



Prout produced sevoral compositions fi-om purpuric acid and other bases, such 

 as lime, quicksilver, and oxyde of zinc, aud all such compositions were remark- 

 ably beautiful in color. lie also claimed that some of those compositions can 

 be utilized not only in painting, but also in the dyeing of wool and other tex- 

 tiles, but his statement could not immediately be acted upon. In the first 

 place, his description Avas so vague and general that experiments often failed 

 when based xipon it. Then the temperature, not less than the concentration 

 of the fluid, is of great importance iu producing the result, which often is very 

 different even when the accurate prescription of Liebig and Woehler is followed. 

 Moreover, in Prout's time the raw material was insufficient for the production 

 of a large and constant supply of this dyestuff. It is true that, as we have 

 stated, the uretric acid is furnished by many species of animals, but it is fur- 

 nished only in very small quantities. Man, for instance, secretes only about 

 one-third of a drachm of it in twenty-four hours. The excrement of birds is dis- 

 tinguished for its great proportion of uretric acid ; it is yL part of the Aveight of 

 dried |)ig(!on's dung. But that could not be prodiiced in large quantities any 

 more than the excrement of snakes, which consists chiefly of uretric-acidical 

 salts. 



* Millions of dollars are annually paid for guano by the farmers on both sides of the Atlantic, 

 yet tlicy, for the most part, suffer the urine of tlioir live stock to sink uselessly into the ground 

 or to ])nlliite and empoison the air, forgetting, if they ever knew, that guano is only more 

 valuable than the manure of the farm-yard or the stable because birds have no urinary pas- 

 sage, and therefore their faecal excrement contains all the uretric salts. — Translator. 



