(JJ8 PHILOSOPHICAL TRANSACTIONS. [ANNO 1785. 



number of one kind of particles, with a certain number of a 2d and 3d 

 kind of particles, forms a particular body, must not the bond which keeps 

 that body together be weakened or strengthened by increasing or diminishing 

 any one of the different kinds of particles which enter into its constitution ? 

 1. When, to the natural share of the electric fluid already existing in the 

 body, a fresh quantity of the same fluid is added, must not some of the component 

 parts of that body escape; or must not that attractive force which kept all toge- 

 gether be so far weakened as to let loose some constituent parts, and among these 

 the particles of light in particular ? 



3. Must not this separation of parts be great in proportion to the quantity of 

 extraneous particles which are added to the body ? Or, agreeable to the 4th observa- 

 tion, must not the spark be more splendid and brilliant, the more the electrical 

 fluid is concentred in any given space. 



4. In the diminution or alteration of that attractive force on which depends 

 the constitution of bodies, may there not be a gradation which, in the present 

 case, as well as in that of burning bodies, will cause the escape of some rays 

 sooner than others ? 



Observations on phosphoric light. — It is obvious, from Mr. B. Wilson's expe- 

 riments, that there are many curious diversities in the appearances of phosphori. 

 Some shells, prepared agreeably to his directions, after exposure to the sun or to 

 the flash of a battery, emit a purple, others a green, and others a reddish light. 

 If, with Mr. Wilson, we suppose that these shells are in a state of slow com- 

 bustion, may we not conclude, that some are just beginning to burn, and there- 

 fore, agreeably to what I have observed on combustible bodies, emitting the 

 most refrangible rays ; while others are in a more advanced state of combustion, 

 and therefore emitting the least refrangible. If this conclusion be right, the 

 shells which are emitting the purple, or the green, must still retain the yellow, 

 the orange, and the red, which will also make their appearance as soon as the 

 combustion is sufficiently increased. 



Exper. 15. Place a shell, while emitting its green rays, on a warm shovel, 

 and the appearance of the shell will be soon changed into that of a yellow 

 mixed with red. To Mr. Wilson's theory of slow combustion, the following 

 objections may be opposed. 



1°. If phosphoric shells owe their light to this cause, we must consider the 

 word combustion, when applied to them, as implying in its signification all those 

 circumstances which are the usual attendants of a body while on fire. Among 

 other necessary consequences in such a case, the increase of heat must increase 

 the decomposition of the combustible ; whereas we discover an effect the very 

 opposite to this in the appearance of a phosphoric body, which never fails to lose 

 its light entirely in a certain degree of heat, without losing the power of 



