59 



was equally unsuccessful in procuring that acid in a pure form by 

 the other processes usually had recourse to. It was either combined 

 with minute portions of tan, or, when obtained by sublimation, was 

 empyreumatically tainted. 



In conclusion, it is remarked, that the Chinese galls differ from 

 other analogous vegetable substances in the absence of extractive 

 matter, whence they may be regarded as the most promising source 

 of pure tan and gallic acid ; that the same circumstance renders 

 them peculiarly fitted for the basis of a black dye, and of writing-ink, 

 while it at the same time renders them ill calculated for the produc- 

 tion of leather, which without extractive matter is brittle and im- 

 perfect. 



Some Researches on Flame. By Sir Humphry Davy, LL.D. F.R.S. 

 V.P.R.I. Read January 16, 1817. [Phil. Trans. 1817, p. 45.] 



This communication is subdivided into four sections, of which the 

 first treats of the effect of rarefactions of the air, by diminished pres- 

 sure, upon flame, and explosion. An inflamed jet of hydrogen was 

 placed in the receiver of an air-pump, and the flame was observed to 

 enlarge during exhaustion, till the gauge indicated a pressure of one 

 fourth or one fifth ; it then diminished in size, but was not extinguished 

 till the pressure was reduced to between one seventh and one eighth. 

 A somewhat larger jet burned until the rarefaction amounted to one 

 tenth, and rendered the glass tube whence the gas issued white hot. To 

 this circumstance the author refers the long-continued combustion of 

 the gas, and thinks the conclusion confirmed by the following experi- 

 ment. A platinum wire was coiled round the jet tube, so as to reach 

 into and above the flame, and it became white hot during the exhaus- 

 tion, and continued red hot even when the pressure was only one 

 tenth. The lower part of the flame was now extinguished, but the up- 

 per part in the contact of the wire continued to burn till the pressure 

 was reduced to one thirteenth. The flame; therefore, of hydrogen is 

 extinguished in rarefied atmospheres, whenever the heat it produces is 

 insufficient to communicate visible redness to platinum wire. Sir Hum- 

 phry Davy was thus led to infer, that those combustibles which require 

 least heat for combustion would burn in rarer atmospheres than those 

 requiring more heat ; and that bodies which produce much heat in 

 combustion would burn in rarer air than those producing little heat, 

 and experiments are detailed proving this to be the case : thus, an in- 

 flamed jet of light carburetted hydrogen, which produces little heat in 

 combustion, and requires a high temperature for its ignition, was ex- 

 tinguished whenever the pressure was below one fourth, even though 

 the tube was furnished with a wire. Carbonic oxide burned under a 

 pressure of one sixth ; sulphuretted hydrogen of one seventh. Sul- 

 phur, which burns at a lower temperature than any other ordinary 

 combustible, except phosphorus, had its flame maintained in an atmo- 

 sphere rarefied 15 times, and phosphurctted hydrogen was inflamed 

 when admitted into the best vacuum of an excellent air-pump. 



