336 SCIENCE PROGRESS 



Nevertheless it was not till twenty years had passed, in 

 1896, that this mode of experimentation was taken up by his 

 pupil, Angel Gallardo, whose previous training as a civil 

 engineer had doubtless made him eager to utilise practical 

 tests. Like Giard, Gallardo must have felt unconsciously the 

 inadequacy of the abstract lines of force; but the only advantage 

 that he claimed for his experiment (utilising the electrostatic 

 field, and obtaining therein " chains " of sulphate of quinine 

 suspended in turpentine), was that it represented the cell-field 

 in three dimensions, instead of in two only, as in the magnetic 

 models of dust on rough surfaces previously utilised. 



To analyse the nature of the dust-lines or the quinine threads 

 we must consider what is meant by relative " permeability " to 

 such forces as magnetism or statical electricity. 



If in front of a magnetic pole we interpose a block of soft 

 iron, we find that the attractive force on a small piece of iron 

 some distance off is increased, just as if it had been brought 

 nearer to the magnet in air (or vacuum). In the same way, if 

 we place a block of sulphur, or still better, glass, between the 

 knob of a charged Leyden jar and a suspended pith-ball, we 

 find the attraction of the pith-ball is increased, just as if it had 

 been approximated in air (or vacuum). We say that the glass 

 and sulphur, and the iron have greater permeability, electro- 

 static or magnetic respectively, than air. We cannot enter here 

 into quantitative details, but if we take vacuum {i.e. ether) as 

 having a permeability of 1 in both cases, soft iron has a 

 magnetic permeability of several hundred or several thousand, 

 glass an electric permeability of 6, an insulated conductor a 

 permeability of infinity. Gravity alone among the forces we 

 have considered is equally permeant to vacuum (ether) and to 

 all known substances. 



A permeable particle of negligible volume tends to orient 

 itself in a field, so that its longest axis lies along a line of 

 force, and if free to travel to pass along it to the nearer pole. 



A permeable body in a field tends to pass to the position 

 where it will enclose the maximum number of unit lines of 

 force — i.e. to the strongest part of a field : thus a soft iron 

 rod is sucked into a solenoid of wire through which a current 

 is passing. 



If a body of high permeability be introduced into a field 

 occupied by a medium of lower permeability, the lines of force 



