The theory of a single electric Jiuid 353 



density beyond which the fluid cannot be accumulated, and that therefore the 

 stratum of the electric fluid -collected at the surface of electrified bodies has a 

 finite thickness. 



No experimental evidence, however, has as yet been obtained of any limit to 

 the quantity of electricity which can be collected within a given volume, or any 

 measure of the thickness of the electric stratum on the surface of conductors*, 

 so that if we wish to maintain the doctrine of a maximum density, we must 

 suppose this density to be exceedingly great compared with the density of the 

 electric fluid in saturated bodies. 



A difficulty of far greater magnitude arises in the case of undercharged 

 bodies. It is a consequence of the theory that there is a stratum near the 

 surface of an undercharged body which is entirely deprived of electricity, the 

 rest of the body being saturated. Hence the electric phenomena of an under- 

 charged body depend entirely upon the matter forming this stratum. Now, 

 though on account of our ignorance of the electric fluid we are at liberty to 

 suppose a very large quantity of it to be collected within a small space, we 

 cannot make any such supposition with respect to ordinary matter, the density 

 of which is known. 



In the first place, it is manifestly impossible to deprive any body of a greater 

 quantity of the electric fluid than it contains. It is found, indeed, that there 

 is a limit to the negativef charge which can be given to a body, but this limit 

 depends not on the quantity of matter in the body but on the area of its surface, 

 and on the dielectric medium which surrounds it. Thus it appears from the 

 experiments of Sir W. Thomson and those of Mr Macfarlane, that in air at the 

 ordinary pressure and temperature a charge of more than 5 units of electricity 

 [per cm. 2 ,] positive or negative, can exist on the surface of an electrified body 

 without producing a discharge. In other media the maximum charge is different. 

 In paraffin oil, and in turpentine, for instance, it is much greater than in air J. 

 In air of a few millimetres pressure it is much less, but in the most perfect 

 vacuum hitherto made, the charge which may be accumulated before discharge 

 occurs is probably very great indeed. 



Now this charge, or undercharge, whatever be its magnitude, can be accu- 

 mulated on the surface of the thinnest gold leaf as well as on the most massive 



* [The modern molecular theories of the phenomena of galvanic polarization 

 touch on this subject.] 



t [The phenomena of discharge in dielectrics, especially of negative electrons as 

 indicated in the text, are of course now much more thoroughly understood. Of. 

 Sir J. J. Thomson's treatise on Conduction of Electricity through Gases, or Prof. J. S. 

 Townsend's book on The theory of ionization...by collision.] 



J By Messrs Macfarlane and Playfair's experiments the maximum electromotive 

 intensity is 364 for paraffin oil and 338 for turpentine. For air it is 73, between 

 disks one centimetre apart. (Trans. R. S. Ed. 1878.) They have since found that 

 the electric strength of the vapour of a certain liquid paraffin at 50 mm. pressure is 

 1-7 times that of air at the same pressure, and that the electric strength of a solid 

 paraffin which melts at 22-^ C. is 2-5 when liquid and 5 when solid, that of air 

 being i. 



c. p. i. 23 



