Spontaneous Changes of the Injufion of Tan. 275 



As this precipitate by Its infolubllity approaches to the nature of wood, and as It falls down 

 even when the lixivium of tan is preferved in well clofed velTcls entirely filled, it might be 

 fuppofed that it is merely a gradual and fuccelTive depolition of the remains of the woody 

 and parenchymatous parts of the bark difleminated and fufpended in the fluid ; it might be 

 imagined that the contad of air has nothing to do with this phenomenon. Eut we mud 

 obferve, that this precipitate is found in the moft limpid folution, even after it has been fil- 

 tered ; that it is moft abundant when the infufion has been prepared from bark ground long 

 before, and preferved without care ; and that it is more readily depofited when the liquor pre- 

 fents a great furface to the contaft of the air. In this cafe the furface of the fluid becomes 

 tarmfhed by an extremely fine pellicle, which covers it, and becomes thicker and firmer the 

 longer the fluid remains thus expofed and at reft. If it be divided by agitating the liquor. 

 It IS again renewed, and in this manner produces the precipitate we fpeak of. We may de- 

 termine, in fome refpeft, at pleafure, the formation of this pellicle and precipitate, by ex- 

 poGng the liquor to the contaft of oxygene gas, or by pouring into it the oxygenated mu- 

 riatic acid. It is fufficient even to bring a bottle containing the oxygenated muriatic acid 

 into the vicinity of the infufion of tan. A pellicle is inftantly formed, refembling a flight 

 gauze, which floats and fpreads inftantly over the whole furface, afTuming in a regular 

 order the moft beautiful colours of the iris, and at laft becomes brownifli. If the experi- 

 ment be continued for a few minutes, the pellicle thickens, and the precipitate begins to 

 fall. The formation of this precipitate, and the properties it acquires, depend, therefore, 

 on the combination of oxygene, which operates either during the infufion and filtration of 

 the lixivium, or after its having been drawn off" clear with the contaft of atmofpheric oxygene. 

 Hence are ftiewn the neceffity and advantage of ufing tan recently prepared *, and of pre- 

 ferving the tan as well as the iiifufions defended from the contaft of the air. 



Thefe phenomena are common to all the preparations of vegetables made by means of 

 ■water, whether by infufion, decodion, or even diftillation. We have feen thefe precipi- 

 tates gradually formed in the infufion or decoaion of nut-gajl, fumac, gentian, quinquina, 

 and even of the moft infipid as well as of the moft aromatic and acrid plants. The ex- 

 preflbd juice of frefh plants, fuch as hemlock, forrel, anil, kc. prefents the fame phenomena, 

 and affbrds, more particularly when the aQion of air is promoted by agitation, a coloured 

 pulverulent depofition which has long been diQinguifiied by the name of fecula. Laftly, 

 all the diftilled waters which are kept for feveral years become turbid, by the formation of 

 fdaments and whitifli flocks, more or lefs abundant, but always infoluble. It is to a com- 

 bination of oxygene, as Fourcroy in his excellent anaiyfis of quinquina has fliewn, that we 

 ought to attribute the formation of thefe precipitates, the infolubility they acquire, and their 

 approach to the nature of the ligneous fibre. 



Ncverthelcfs, the fpontaneous precipitation which is cfl^'eftcd by the contad of atmofpheric 

 oxygene in the Hxivium of tan does not change its properties, at leaft in any fenfible manner; 

 and as in a lixivium confiderably faturated the precipitate is moft abundant on the firft 

 days, it might perhaps be of advantage to wait until this firft precipitate was formed before 



• Thc.'J obfcrvations are applicable 10 M vegetable fubftanccs whicli arc reduced 10 powder. They not only 

 lofc their peculiar aroma, but likcwife change their nature hy ilie contatt of light and armorphericair. Tlieir 

 propcriiesbcingthusconfiderably altered, the dofc at which it is proper to employ them becomes uncertain. 

 The plij fician ought not th<>rcfore to prefcribc in hij formulae any powilcik but I'ufh ai arc recenily prepare,!. C, 



N " 2 immcrfing 



