230 Analyses of Books. [March 



He placed a brass ball about two inches in diameter in the centre of 

 a large receiver, and connected it with an electroscope by means of 

 a brass rod passing right through a collar fixed in a glass plate and 

 socket. A quantity of electricity was communicated to the ball, suf- 

 ficient to cause a divergence of 40° in the electroscope. This effect 

 was not influenced by removing fifty-nine sixtieths of the air in the 

 receiver. • 



4. In reference to the transmission of electricity between con- 

 ductors, it appears that when the attracting force operating between 

 two conductors can overcome the atmospheric pressure, a discharge 

 ensues between the nearest points of the opposed surfaces. In these 

 points the force appears to become at length indefinitely great in re- 

 spect of points more remote, so that the whole quantity accumulated 

 is finally determined through them. Thus the precise points of con- 

 tact between two spheres being found, and the spheres subsequently 

 separated by given distances measured between these points, it may 

 be shewn that the respective quantities requisite to produce a dis- 

 charge will vary with the distances directly. The distance at which 

 electricity can be discharged in air of a given density is an accurate 

 measure of the comparative quantity contained in a unit of space, or 

 of the tension (by which is to be understood the elastic force of a given 

 quantity accumulated in a given space, and is directly as the density 

 of the stratum,) and the attractive force discovered by the electrome- 

 ter, or the intensity is directly as the square of the quantity con- 

 tained in a unit of space. 



5. The effect of an atmosphere varying in density and temperature 

 in restraining electrical discharges, is as follows : 



1 st. The respective quantities requisite to pass a given interval, 

 varied in a simple ratio of the density of the air. When the density 

 was one half as great, the discharge occurred with one half the quan- 

 tity accumulated, that is to say, with one fourth of the attractive 

 force indicated by the electrometer. 2nd. The distance through 

 which a given accumulation could discharge was found to be in an 

 inverse simple ratio of the density of the air, the intensity or free ac- 

 tion being constant. In air of one half the density, the discharge 

 occurred at twice the distance, or the resistance of air to the passage 

 of electricity is as the square of the density directly, and if as the den- 

 sity of the air be decreased, the distance between the points of action 

 be increased, the electrical accumulation will still remain complete. 



6. Heated air is not as is frequently stated a conductor of electri- 

 city, and heat does not facilitate electrical transmission through air 

 in any other way than by diminishing its density. Supposing heat 

 to be material, it is a non-conductor of electricity, because the incor- 

 poration of a conducting with a non-conducting substance is found to 

 impair the insulating power of the latter, as in the case of air charged 

 with free vapour, whereas in the intimate union of two non-conduc- 

 tors the insulating power remains perfect. 



7. Sir Humphry Davy has well illustrated the effect of heat in 

 imparing the conducting power of metals, and the same fact has been 

 observed by Mr. Christie. Dr. Ritchie, however, has lately brought 

 forward an objection ; for, in transmitting electricity over a forked 

 iron rod, one of the legs of which he heated to redness, he found 



