CHEMISTRY. 



61 



of 



Its proper- 

 ties. 



containing muriate of lime, andthis with a third empy 

 The flasks were surrounded wi;h a mixture of two 

 ^emutrj. p arts snoW) an d one p ar t ga lt, A. gentle heat being 



applied to the retort, the prussic acid passed over, 

 and swam above the muriate of lime. By gently 

 heating the first flask, the acid is made to pass into 

 tb second flask, and by this means is rectified. 



Prussic acid, thus obtained, is a limpid and colour- 

 less liquid. It is extremely volatile and pungent ; 

 boiling at the temperature of 80, and at the tem- 

 perature of 50 it supports a column of mercury ra- 

 ther more than ten inches high. At 68 it quintuples 

 the bulk of any gas with which it is mixed. It freezes at 

 2. In consequence of this extreme volatility and easy 

 congelation, a curious phenomenon takes place. If a 

 drop of it be let fall upon a sheet of paper, it imme- 

 diately freezes, in consequence of rapid evaporation. 

 The specific gravity of this liquid is 0.70583 at the 

 temperature of *4.5. It reddens vegetable blues. 



It is capable, when dry, of withstanding a red heat 

 without decomposition ; but when water is present, it 

 very readily undergoes change. 



It combines with the salifiable bases, and forms a 

 class of bodies called prussiates. But they have very 

 little permanency, being decomposed by all other 

 acids, and even by exposure to tm? atmosphere. 



It is also capable of forming triple compounds, in 

 which it is combined with two bases at once, oneofthem 

 an alkali or earth, the other a metallic oxide. These 

 compounds are much more permanent, and are there- 

 fore usually employed by chemists. The one in 

 most frequent use is the triple pnittiafe of potash, a 

 yellow coloured salt crystallizing in cubes, and com- 

 posed of prussic acid, potash, and oxide of iron. 



Scheele succeeded in forming prussic acid, by cau- 

 sing a current of ammoniacal gas to pass through red 

 hot charcoal ; and the experiment has been since re- 

 peaud successfully by others. Hence it is obvious, 

 that this acid is composed of the constituents of am- 

 monia and charcoal united together, or of hydrogen, 

 azote, and carbon. This has been further confirmed 

 by Berthollet. Chlorine has the property of alter- 

 ing the nature of prussic acid, and renders it capable 

 of throwing down iron from solutions green instead 

 of blue. To the acid thus altered, Berthollet gave 

 the name of oxyitrussie acid. When heat is applied 

 to it, the whole is converted into carbonate of am- 

 monia. 



Prussian blue may be formed, by calcining a mix- ' 

 ture of potash and dried blood in a covered crucible, 

 in a heat gradually raised to redness. The mass is ' 

 dissolved in watt r, and poured into a solution of sul- 

 phate of iron. A green coloured precipitate falls, 

 which becomes prussian blue when digested in muri- 

 atic acid. The triple prussiate of potash was for- 

 merly called phlugisticated alkali. It is still useful 

 in detecting different metals in solutions, by the co- 

 lour of the precipitate which it occasions, especially 

 iron, which it throws down of a deep blue. 



SECT. II. Of Gallic Acid. 



Gallic acid. Thi acid forms one of the constituents of the 

 tubstance called iiujgitlis, a concretion formed on the 

 oak m consequence of the puncture of insects, Nut- 



Element 

 of 



Scheele'* 

 method of 

 forming 

 Pruuie 

 acid. 



Prussian 

 blue. 



galls come to this country chiefly from the Levant. 



They vary a good deal in their appearance. Scheele 



first separated gallic acid from nutgalls. An infusion 



of nutgalls left to itself for some time, becomes moul- 



dy on the surface, and lets fall small crystals. These How 06. 



crystals being picked out, dissolved in water, and tam 



obtained again by evaporation, constitute gallic acid. 



The acid obtained by this process is never quite 

 pure. If the infusion of nutgalls be evaporated to 

 dryness, and the powdered residue be digested in 

 pure alcohol, the alcohol, when cautiously distilled 

 to one-eighth, leaves a residue behind it nearly co- 

 lourless, which is soluble in water, and yields by 

 evaporation gallic acid in needles, much lighter co- 

 loured and purer than that obtained by the first de- 

 scribed process. 



Gallic acid is white, usually with a shade of brown 'Iti proper- 

 or yellow. It is crystallized in needles or transpa- ties. 

 rent plates. Its taste is acid, and somewhat astrin- 

 gent ; and when heated, it exhales a peculiar and 

 rather unpleasant aromatic odour. It is soluble in 

 1^ parts of boiling, and in 12 parts of cold water. 

 When the solution is heated, the acid is decomposed. 

 When long kept, it becomes darker coloured, and the 

 acid is likewise altered in its properties. 



When heated, it sublimes, but its properties are 

 somewhat altered. When distilled, it yields, like 

 other vegetable acids, carbonic acid gas, and heavy 

 inflammable air. Water is also formed, and a por- 

 tion of the acid escapes, slightly modified in its nav 

 ture. 



It is not altered by exposure to the air. Neither 

 oxygen gas, the simple combustibles, or incombus- 

 tibles, seem to produce any effect upon it. The ac- 

 tion of the metals is unknown. 



The compounds of this acid, with the salifiable 'Gallatesf, - 

 bases, are called gallatet They have scarcely been 

 examined. When gallic acid is dropf into lime, ba- 

 rytes, or strontian water, it strikes a bluish red co- 

 lour, and occasions a flaky precipitate. It occasions 

 a precipitate likewise when poured into the solutions 

 of yttna, glucina, or zirconia in acids. Upon me- 

 tallic solutions it acts with considerable energy, chan- 

 ging their colour, and occasioning precipitates in many ' 

 of them. Thus with iron it strikes a dark blue, or 

 almost black colour. When it precipitates metallic 

 oxides, it seems to bring them nearer to the metallic 

 state, and sometimes reduces them altogether. Thus 

 gold is precipitated by it in the metallic state. 



SECT. III. Of Tannin. 



Nutgalls, besides gallic acid, contain several con- Tannin. 

 stituenu; one of the most curious and important of 

 these is tannin, which is to occupy our attention in 

 this Section. 



Tannin was first pointed out in vegetables by Dey- 

 eux, though somt ot its properties had been noticed 

 long before by Lewis. Seguin first pointed out ill 

 great itrp jrtance in tanning, and hence the name was 

 given it, by which it is at present known. Proust 

 endeavoured to obtain it in a separate state. Mr 

 l)avy added to our knowledge of its properties. But 

 it is to Mr Hatcheti that we are indebted for the most 

 important set of new facts. He pointed out a rue 

 8 



