VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. 677 



strikes very brilliantly on one side of the piece of deal, on the other side it will 

 appear very red. In like manner a red appearance may be given to a spark 

 which strikes brilliantly over the inside of a tube, merely by spreading some 

 pitch very thinly over the outside of the same tube. 



Exper. 13. I would now give another fact, whose singularities depend very 

 much on the influence of the medium through which the electrical light is made 

 to pass. If into a Torricellian vacuum, of any length, a few drops of ether 

 are conveyed, and both ends of the vacuum be stopped up with metallic con- 

 ductors, so that a spark may pass through it, the spark in its passage will assume 

 the following appearances. When the eye is placed close to the tube, the spark 

 will appear perfectly white. If the eye be removed to the distance of 1 yards, it 

 will appear green ; but at the distance of 6 or 7 yards, the colour of the spark 

 will be reddish. These changes evidently depend on the quantity of medium 

 through which the light passes ; and the red light more particularly, which we 

 see at the greatest distance from the tube, is accounted for on the same prin- 

 ciple as the red light of a distant candle or a beclouded sun. 



Exper. 14. Dr. Priestley long since observed thered appearance of the spark when 

 passing through inflammable air. But this appearance is very much diversified by 

 the quantity of medium, through which you look at the spark. When at a very con- 

 siderable distance, the red comes to the eye unmixed ; but, if the eye be placed 

 close to the tube, the spark appears white and brilliant. In confirmation however 

 of some of my conclusions, I would observe, that by increasing the quantity of fluid 

 which is conveyed through any portion of inflammable air, or by condensing 

 that air, the spark may be entirely deprived of its red appearance, and made 

 perfectly brilliant. I have only to add, that all weak explosions and sparks, 

 when viewed at a distance, bear a reddish hue. Such are the explosions which have 

 passed through water, spirits of wine, or any bad conductor, when confined in a 

 tube, whose diameter is not more than an inch. The reason of these appear- 

 ances seems to be, that the weaker the spark or explosion is, the less is the light 

 which escapes ; and the more visible the effect of any medium which has a power 

 to absorb some of that light. 



The preceding observations concerning electrical light were the result of my 

 attempts to arrange, under general heads, the principal singularities attending it. 

 They may perhaps assist others in determining how far they may have led my mind 

 astray in giving birth to a theory which I would now briefly describe in a few 

 queries. 



1 . If we consider all bodies as compounds, whose constituent parts are kept 

 together by attracting each other with different forces, can we avoid concluding, 

 that the operations of that attractive force are regulated, not only by the qua- 

 lity, but the quantity also of those component parts ? If a union of a certain 



