VOL. LX.] PHILOSOPHICAL TRANSACTIOKS. 4^ 



affixed fine needles to the bodies he made use of. Tlie lightest pith balls placed 

 near the extremities of these needles, were not in the least affected by the 

 explosion. When he placed a number of brass balls, one behind another, the 

 lateral explosion passed through them all, being visible in the intervals between 

 each of them, and returned the same way, leaving them all in the same state in 

 which it found them ; and a great number of lateral explosions might be taken 

 at the same time, in different parts of the circuit, some of them very near one 

 another. It made no difference, whether the lateral explosion was received on a 

 fiat smooth surface, or the points of fine needles. In both cases the spark was 

 equally long and vivid. 



Dr. P. had no sooner completed these experiments on the lateral explosion, 

 but he had a curiosity to see what kind of an appearance it would make in 

 vacuo; since no other phenomenon in electricity resembles it. In all other 

 cases, the electric matter rushes in one single direction ; whereas, in this, it goes 

 and returns in the same path, and, as far as can be distinguished, at the same 

 instant of time; so that all the difference of the two electricities, which are so 

 conspicuous in vacuo, must here be confounded. Accordingly he found, 

 though the pump was not in good order, that he could perceive this explosion in 

 vacuo, at the end of rods, placed several inches asunder; and when they were 

 brought within about 2 inches, they seemed to be joined by a thin blue or purple 

 light, quite uniform in its appearance. As these rods were made to approach, 

 this light grew denser; but still exhibited no such variety, as is observed between 

 the bodies that give and receive electricity, in the common experiments in vacuo. 

 Dr. P. was pretty soon convinced, that uncoated jars could not be used to any 

 more advantage in these experiments, than those that were coated; since the want 

 of coating only operated as an interruption in the circuit, occasioning a difficulty 

 in the admission of the charge on the outside of the jar. And, in all cases, the 

 greater this difficulty of passage was made, provided the discharge was made at 

 once, the more considerable was the lateral explosion, and the greater shock 

 was given to the hand that held the discharging rod; which shock was 

 nothing more than one of these lateral explosions, issuing from the rod as part 

 of the circuit. He concludes the account of these experiments with 

 observing, that they may possibly beof some use in measuring the conducting 

 power of different substances; since, the greater is the interruption 

 in the electric circuit, occasioned by the badness of its conducting power, 

 the more considerable, caeteris paribus, is the lateral explosion. 



XIX. Experiments and Observations on Charcoal. By Joseph Priestley, LL.D., 



F.R.S. p. 211. 



May be consulted in the author's collected works on different kinds of air. 



