6/1 PHILOSOPHICAL TRANSACTIONS. [ANNO 1785. 



which contained a few drops of water. Having inverted the tube, and plunged 

 the open end of it into a basin of mercury, the mercury in the tube stood nearly 

 half an inch lower than it did in a barometer at the same instant, owing to the 

 vapour which was formed by the water. But through this rarefied water the 

 electrical spark passed as luminously as it does through air equally rarefied. 



Exper. 5. If, instead of water, a few drops of spirits of wine be placed on 

 the surface of the mercury, phenomena similar to those of the preceding experi- 

 ment will be discovered, with this difference only, that as the vapour in this case 

 is more dense, the electrical spark, in its passage through it is not quite so lumi- 

 nous as it is in the vapour of water. 



Exper. 6. Good ether substituted instead of the spirits of wine will press the 

 mercury down so low as the height of l6 or 17 inches. The electrical fluid in 

 passing through this vapour, unless the force be very great indeed, is scarcely 

 luminous. But if the pressure on the surface of the mercury in the basin be 

 gradually lessened by the aid of an air-pump, the vapour will become more and 

 more rare, and the electric spark in passing through it more and more 

 luminous. 



Exper. 7. I could not discover that any vapour escaped from the mineral acids 

 when exposed in vacuo. To give them therefore greater rarity or tenuity, I 

 found different methods necessary. With a fine camel hair pencil, dipped in 

 the vitriolic, the nitrous, or the marine acid, I drew on a piece of glass a line 

 about i of an inch broad. In some instances I extended this line to the length 

 of 27 inches, and found that the contents of an electric battery, consisting of 

 10 pint phials coated, would pass over the whole length of this line with the 

 greatest brilliancy. If by widening the line, or by laying on a drop of the acid, 

 its quantity was increased in any particular part, the charge, in passing through that 

 part, never appeared luminous. Water, spirits of wine, circumstanced similarly to 

 the acids in the preceding experiment, were attended with similar, but not equal 

 effects, because, in consequence of the inferiority of their conducting power, 

 it was necessary to make the line through which the charge passed considerably 

 shorter. 



4. The brilliancy or splendour of the electric fluid, in its passage through any 

 body, is always increased by lessening the dimensions of that body. I would 

 explain my meaning by saying, that a spark, or the discharge of a battery which 

 we might suppose equal to a sphere -]- of an inch in diameter, would appear much 

 more brilliant if the same quantity of fluid be compressed into a sphere J- of an 

 inch in diameter. This observation is the obvious consequence of many known 

 facts. If the machine be large enough to afford a spark whose length is Q or 10 

 inches, this spark may be seen sometimes forming itself into a brush, in which 

 state it occupies more room, but appears very faintly luminous. At other times 



