66 On Light produced by the Discharge of an Air-gun. 



and that the sand adhering to the split lath, saw-dust, silk, &c., 

 might be the real cause of the light. We next tried pieces of 

 very clean and dry silk, wool, feathers, and cylinders of wood, 

 carefully freed from sand, and found that no light could be 

 excited by their means. 



Finally satisfied that attrition was the sole cause of the 

 luminous appearance, we tried siliceous and other hard bodies, 

 which emit light on being rubbed together, such as quartz, 

 fluor-spar, &c., and found them all to be luminous. From 

 bodies of an opposite nature no light could be elicited. To 

 ascertain whether the light from these hard substances 

 might arise from small particles of iron torn from the sides of 

 the barrel, like sparks from a cutler's wheel, we held sand, 

 fragments of spar and sugar successively in our hands, at the 

 muzzle of the gun, and discharged it at them. In this way 

 they all appeared luminous, though not so bright as when dis- 

 charged from the barrel. To see whether it might not be an 

 electrical appearance, arising from the air being violently 

 blown against these crystalline bodies, we formed a small 

 grating of clean and well-dried thermometer tubes, which we 

 held as before, opposite to the muzzle of the gun ; but could in 

 this case perceive no luminous appearance whatever from dis- 

 charges of condensed air passed through them. 



Hence it may be concluded that light emitted on the dis- 

 charge of an air-gun arises solely from attrition, occasioned by 

 sand or other hard substances adhering to the wadding, or 

 getting by accident into the barrel; and, that no light can be 

 produced from the sudden expansion of the air from a con- 

 densed magazine, or from its impulse on the still atmosphere*. 

 By introducing sugar into the gun and discharging it against 

 a wall in the dark, a flash of light is seen to proceed from the 

 sugar, as it strikes the wall. 



(Signed) John Haut. 



* The ligbt which 51. Biot says is extricated wheu we cause a glasa 

 globe filled with air to burst in vacuo, must be ascribed tn the friction of 

 the particles of the broken glass on each other. 



