408 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



satisfactorily proved, &c." Beccaria also has adopted the same opinion, saying, 

 " That a quantity of excessive fire cannot be introduced into one surface, but 

 inasmuch as an equal dose of natural fire can quit the other surface." 



These assertions Dr. G. apprehends directly contrary to what really happens. 

 Instead of which, he believes, we may safely assert, that glass, and every other 

 known substance, may have its natural quantity of electric fluid either increased 

 or diminished to a certain limited degree ; which degree bears no proportion to 

 the quantity of matter contained in a body, but is, caeteris paribus, in propor- 

 tion to the extent of its surface. This law, which is perhaps without exception, 

 he thinks, may be considered as one of the fundamental laws of electricity, and 

 one on which many of its principal phenomena. depend. At present he only 

 considers it so far as it is the cause of what is commonly called the charge of a 

 coated jar. Suppose such a jar insulated, and connected by its knob to the 

 prime-conductor of an electric machine ; if then the machine be put in action, 

 a certain quantity of electric fluid, agreeable to the above-mentioned law, is 

 added to the natural quantity belonging to the inner surface of the jar. After 

 which, if the finger, or any other conducting substance, be presented to the 

 outer coating of the jar, a quantity of electric fluid, nearly equal to that thrown 

 in, comes from it. But this departure of electric fluid from the outside of the 

 jar, cannot be, as Dr. Franklin supposes it, the cause which permits the addition 

 of fluid to the inside, but is merely the consequence of the action of that super- 

 fluous quantity which was thrown in. And the operator may, if he pleases, in- 

 stead of taking electric fluid from the outside of the jar, take out again, by 

 touching the knob, nearly the whole of what he had thrown in, which he could 

 not do if an equal quantity had already gone from the outside of the jar.* When 

 the quantity already spoken of has been taken from the outside of the jar (the 

 equilibrium being nearly restored) another quantity like the first may again be 

 added to the inner surface : after which a similar quantity may again be taken 

 from the outside : thus, by the succession of a sufficient number of the quanti- 

 ties allowed by the before- mentioned law, the jar may at length be completely 

 charged. 



There are other ways of charging coated glass ; but if it be allowed that the 

 charge, in the foregoing instance, is produced in the manner I have supposed, 

 it will not, I think, says Dr. G., be disputed, that all other charges are pro- 

 duced by a similar alternation of small quantities. This however will appear 



* Much dispute has arisen among electricians respecting the degree of charge which may be 

 given to an insulated jar ; but no one, that I know of, has taken notice of a deception which will 

 happen, if care be not taken that the same side, by which the jar is attempted to be charged, be 

 first touched in trying whether it be charged or not ; whereas it is clear, from what has been said, 

 that if the contrary surface be first touched, a small charge will, from that very circumstance, be 

 produced. — Orig. 



