no Mr. S. T. Preston on the Nature of what 



r =^ 1*500 centim., 



R = 3-000 centims., 



d = 2-250 „ 

 and a = 0*125 centim. 



Hence F = 396 x 10^ grammes per square centimetre, 



^ ~" ' J- absolute electrostatic C.G.S. units. 



V= 722-7 J 



Changing the units to the ordinary ones in practical use, we 

 find 



14Q = 16*86 microfarads, 



Y = 216810 volts. 



Assuming the sparking-distance to increase as the square 

 of the potential, it can be calculated from the experimental 

 results obtained by Messrs. Warren De La Rue and Miiller 

 (Proc. Roy. Soc. Jan. 1876), namely that 1000 rod-chloride- 

 of-sih'cr cells give a spark 0*009166 inch, that a difference of 

 potentials of 216,810 A^olts would produce a spark in air be- 

 Ween two electrodes at a distance of about 36 feet apart. 

 This is, of course, a relatively very short distance ; but it must 

 be remembered that we have only taken into consideration 

 that portion of the energy of the discharge which was em- 

 ployed in breaking the fourteen insulators, and have neglected 

 all that was spent in heat, light, &c. 



XV. On the Nature of what is commonly termed a '' Vacuum,'''^ 

 By S. ToLVER Preston. 



1 . "T T may perhaps not be uninteresting to consider what 

 J- light the researches of Professor Maxwell on mole- 

 cular distances in connexion with the kinetic theory of gases, 

 and those of Sir William Thomson and others, are capable of 

 throwing upon the physical condition of what is ordinarily 

 termed a ^' vacuum." This inquiry might have an additional 

 interest at the present time, when experiments w^ith extremely 

 rarefied media in connexion with the radiometer and the dis- 

 charge of electricity in evacuated tubes are going on, and 

 certain inferences regarding the condition of "vacua" would 

 lead one to infer that a prevalent idea may exist regarding 

 the state of a " vacuum " (so termed) which is not consistent 

 with facts, as indeed I am not aware that any special investi- 

 gations have been made on this subject. I make no pre- 



* Communicated by the Author. 



