8 Dr. Scliweigger on Prof. Doebereiner's 



the tourmaline. But every thing which facilitates the ap- 

 pearance of the electricity will be favourable to those appear- 

 ances of light and caloric in the compression of the gases, ac- 

 cording to my theory : and that this is really the case, I shall 

 now proceed to show. The paper on " The Light produced by 

 the Discharge of an Air-gun " by Mr. John Hart, inserted 

 in the Journal of Science, vol. xv. p. 64-, affords an interesting 

 confirmation of the theory now proposed, and shall therefore 

 be given as an Appendix to the present essay. All the ex- 

 periments made by Mr. Hart, in order to produce light by 

 the discharge of an air-gun, failed, until he accidentally dis- 

 charged some paper wadding, when he perceived a faint light, 

 which, however, he could not reproduce by a repetition of 

 the experiment. He therefore tried other bodies: but neither 

 clean dry silk, woollen, paper, nor wood, would answer, and 

 even shell-lac only succeeded occasionally ; whilst sugar, but 

 especially narrow slips of glass, never failed in producing the 

 effect. It appeared at length that even some particles of lime 

 or sand produced a luminous discharge, and that when sand 

 or fragments of sugar were held at the muzzle of the air-gun, 

 while the stream of air was rushing forth, they appeared lumi- 

 nous. We find this paper translated in the Annates de Chimie, 

 torn. xxii. p. 436-439 ; and the Editors, in confirmation of 

 Mr. Hart's results, observe, that when we blow with a clean 

 pair of bellows on even the most delicate electrometer, no sign 

 of electricity is obtained ; whereas when the bellows contain a 

 little powder or ashes, the electricity produced is very strong. 



Mr. Hart explains these phaenomena by the electrical fric- 

 tion of the bodies. But he will hardly be able to state, why 

 the bodies which so easily become electric, as silk, woollen, 

 and shell-lac, would not answer in his experiments; while it 

 was only necessary to throw the silk on the floor, so that it 

 might become dusty, for a luminous discharge to take place. 

 With these experiments the following observation may be 

 connected. I have received from General von Hellwig (who 

 long since intended to make experiments on the compression 

 of air by an hydraulic-press, like those lately instituted in 

 England) a glass apparatus for producing fire by compression, 

 constructed in a most convenient manner, like that repre- 

 sented by M. Thenard in his Traite de Chimie, tab. xxii. 

 fig. 3. The polished bore of the tube in which the accurately 

 fitting piston moves, is scarcely half an inch in diameter, 

 whilst the glass forming the tube measures about an inch 

 in thickness. The appearances of light upon the violent 

 compression of the air are very easily perceptible in this in- 

 strument; and when a small piece of amadou is affixed to 



the 



