VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. (373 



wires introduced, as in the preceding experiment, whose points shall be very 

 near each other. It is a well known fact, that the discharge of a small Leyden 

 phial in passing over a strip of gold, silver, or Dutch metal leaf will appear very 

 luminous. By conveying the contents of ajar, measuring 2 gallons, over a strip 

 of gold leaf ■$. of an inch in diameter, and a yard long, I have frequently given 

 the whole a dazzling brightness. I cannot say, that a much greater length might 

 not have been made very splendid, nor can I determine to what length the force 

 of a battery might be made luminous in this manner. We may give this experi- 

 ment a curious diversity, by laying the gold or silver leaf on a piece of glass, 

 and then placing the glass in water: for -the whole gold leaf will appear most 

 brilliantly luminous in the water by exposing it, thus circumstanced, to the 

 explosion of a battery. 2. The difficulty of making any quantity of the elec- 

 trical fluid luminous in any body increases as the conducting power of that body 

 increases. 



Exper. 1. To make the contents of a jar luminous in boiling water, a much 

 higher charge is necessary than would be sufficient to make it luminous in cold 

 water, which is universally allowed to be the worst conductor. 



Exper. 2. I have various reasons for believing the acids to be very good con- 

 ductors. If therefore into a tube, filled with water, and circumstanced as I 

 have already described, a few drops of either of the mineral acids are poured, 

 it will be almost impossible to make the fluid luminous in its passage through 

 the tube. 



Exper. 3. If a string,* whose diameter is \ of an inch, and length 6 or 8 

 inches, be moistened with water, the contents of a jar will pass through it lumi- 

 nously : but no such appearance can be produced by any charge of the same jar, 

 provided the same string be moistened with one of the mineral acids. To the 

 preceding instance we may add the various instances of metals which will con- 

 duct the electrical fluid without any appearance of light, in circumstances the 

 same with those in which the same force would have appeared luminous in passing 

 through other bodies whose conducting power is less. But I proceed to observe, 



3. That the ease with which the electrical fluid is rendered luminous in any 

 particular body is increased by increasing the rarity of the body. The appear- 

 ance of a spark, or of the discharge of a Leyden phial, in rarefied air is well 

 known. But we need not rest the truth of the preceding observation on the 

 several varieties of this fact ; similar phenomena attend the rarefaction of ether, 

 of spirits of wine, and of water. 



Exper. 4. Into the orifice of a tube, 48 inches long, and ■§- of an inch in 

 diameter, I cemented an iron ball, so as to bear the weight which pressed on it 

 when I filled the tube with quicksilver, leaving only an interval at the open end, 



* The thickness and diameter of the string should be regulated by the force we employ. — Orig. 

 VOL. XV. 4 R 



