'266 Mr, MoRGAN*s Obfervations and Experiments on 



brilliantly over the infide of a tube, merely by fp reading fome 

 pitch very thinly over the outfide of the fcime tube. 



EXP. XIII. I would now give another fa6l, whofe fingulari- 

 ties depend v^ery much on the influence of the medium through 

 which the electrical light is made to pafs. If into a Torricel- 

 lian vacuum, of any length, a few drops of sther are conveyed, 

 and both ends of the vacuum are flopped up with metallic con- 

 dudors, fo that a fpark may pafs through it, the fpark in its 

 paflage will affume the following appearances- When the eye 

 is placed clofe to the tube, the fpark will appear perfectly 

 white. If the eye is removed to the diilance of two yards, it 

 will* appear green ; but at the diilance of fix or feven yards, 

 the colour of the fpark will be reddlfh. Thefe changes evi- 

 dently depend on the quantity of medium through which the 

 the light paffes ; and the red light more particularly, which 

 we fee at the greatell: diftance from the tube, is accounted for 

 on the fame principle as the red light of a diftant candle or a 

 beclouded fun. 



EXP. XIV. Dr. Priestley long ago obferved the red ap- 

 pearance of the fpark when pafling through inflammable air. 

 But this appearance is very much diverfified by the quantity of 

 medium, through which you look at the fpark. When at a 

 very confiderable diftance, the red comes to the eye unmixed; 

 but, if the eye is placed clofe to the tube, the fpark appears 

 white and brilliant. In confirmation, however, of fome of 

 my conclufions, I would obferve, that by increafing the 

 quantity of fluid which is conveyed through any portion 

 ef inflammable air, or by condenfing that air, the fpark may 

 be entirely deprived of its red appearance, and made perfe6lly 

 brilliant. I have only to add, that all weak exploiions and 



fpark s, 



