ELECTRICITY. 



43 



using oil instead of water in this ex- 

 periment, the ball was projected with 

 still greater force. 



( 1 55 .) If two wires be introduced into 

 a soft piece of tobacco-pipe clay, so that 

 their ends be near each other, and 

 a shock passed through them, the clay 

 will be curiously expanded in the inter- 

 val between the wires. The experiment 

 will not succeed if the clay be either too 

 dry or too moist. If the clay be too dry, 

 or' the shock too powerful, the mass will 

 be shivered into innumerable fragments. 

 If the clay be placed in the tube of a to- 

 bacco-pipe, or in a glass tube, the ex- 

 pansion of the clay will be so consider- 

 able as to shatter "the tube which con 

 tains it. 



(156.) The expansion of air by the 

 passage of the electrical fluid, either in 

 the form of sparks or shocks, is shown 

 in the following experiment of Kinners- 

 ley, the apparatus for which has been 

 called the Electrical Air Thermometer. 

 It consists of a glass tube closed at both 

 ends by air-tight brass caps, through 

 which two wires slide in the direction of 

 the axis of the tube. These wires are 

 terminated by brass balls, which are 

 made to approach within the striking 

 distance. To an aperture in the bot- 

 tom of the lower cap is fitted a bent 

 tube of glass which turns upwards, and 

 is open at both ends; the bent partis 

 filled with mercury, or with a coloured 

 fluid, which may indicate by its rising 

 or falling in the tube any dilatation or 

 contraction that may take place in the air 

 within the vessel. It is found that every 

 time a spark passes between the brass 

 balls, the fluid suddenly rises, but de- 

 scends again to its former level imme- 

 diately after each explosion ; thus show- 

 ins: that the dilatation of the air, produced 

 by the abrupt passage of electricity, is but 

 of momentary duration. 



(157.) When a strong electrical charge 

 is sent through a very confined portion 

 of air, the explosive effects produced by 

 it are as considerable as those we have 

 seen exhibited by denser fluids. Thus if 

 a piece of plate glass of the size of a 

 square inch, and half an inch in thick- 

 ness, be laid flat upon the small table of 

 Henley's universal discharger, ($ 136,) 

 and pressed down by a weight, and the 

 points of the sliding wires be set oppo- 

 site to each other and against the under 

 edge of the glass, so that the electricity 

 may pass beneath it, the charge of a 

 large jar transmitted in this w r ay will 

 break the glass into innumerable frag- 



ments, and even reduce a portion into an 

 impalpable powder. If the mouth of a 

 small mortar made of ivory, with a ca- 

 vity of half an inch diameter and an inch 

 deep, be stopped by a cork, fitted so as 

 to close the aperture accurately, yet 

 without much friction, and if two wires 

 be inserted through the sides of the mor- 

 tar so that their points within the cavity 

 be separated by an interval of about a 

 quarter of an inch, a strong charge 

 being sent through the wires will expand 

 the air within the cavity so suddenly as 

 to project the cork to some distance. 



(158.) Solid bodies of a porous tex- 

 ture, such as wood, are easily torn 

 asunder by an electric charge. If two 

 holes be drilled in the opposite ends of 

 a piece of wood, about half an inch 

 long, and a quarter of an inch thick, 

 and the ends of two w T ires inserted in 

 the holes, so that their points may be at 

 the distance of a quarter of an inch ; on 

 passing a strong charge through them, 

 the wood will be split in pieces. Stones, 

 loaf-sugar, and other brittle and im- 

 perfectly conducting substances, may be 

 broken in a similar way. 



Place a piece of dry writing paper 

 upon the table of the universal dis- 

 charger, and having removed the balls 

 from the ends of the sliding wires, press 

 the points of the wires against the paper 

 at the distance of two inches from each 

 other ; if a powerful shock be now sent 

 through the wires, the paper will be 

 torn in pieces. If a number of wafers 

 be placed on the table, instead of paper, 

 they will be dispersed in a curious man- 

 ner, and many of them broken into 

 small fragments. 



(159.) A singular result is obtained 

 by the following variation in the circum- 

 stances of the last experiment, which 

 was made by Mr. Lullin. Suspend a 

 varnished card by silk threads, (see fig. 

 38,) in such a manner that two blunt 



Fig. 38. 



wires proceeding from the two sides of 

 a jar or battery, may be in contact with 



