556 Dr. M. Hartog. { Dec. 1, 
their sag, persisted for three days; but for such results we must, of course, 
use dust of special fineness. In such a medium the chains may be seen 
to sag without rupture under the influence of gravity; they can be pushed 
aside by contact with a glass rod, or swayed by currents; or, if the vessel 
that contains them be moved, they may be deflected by the shifting of their 
position relative to the poles before they rupture, through the lag imposed 
by the viscidity of the medium (fig. 14). Owing to the relatively high 
permeability of their substance, they retain in all these cases of deviation, 
despite their elongation, the majority of their lines of force: for, under the 
conditions of the experiment, the elongation is of a lower order of magnitude 
than the ratio of their average permeability to that of the medium. 
VIII. 
Under the microscope a chain is seen to be spongy, composed of loosely 
laid strands joining at very acute angles. Its constitution is conditioned 
by three factors: (1) The relative permeabilities of the powder of the 
medium; (2) the viscosity of the medium; (3) the intensity of the 
magnetising force: as the last increases, the texture becomes more serried 
by the squeezing out of the intervening liquid. As we have just seen, the 
chains are composed of fine strands meeting at acute angles, so in the 
segregation to form the chains themselves two of them may meet and unite 
at an acute angle—“anastomose,” in fact. Such junctions are to be found 
in almost all published figures of dust in air on paper from the time of 
Faraday (figs. 10, 13, 15); bat they are especially easy to observe when 
the dust is suspended in liquid (figs. 6—9). Once formed, the magnetic 
stresses hardly tend to rupture them, and they are nearly as resistent as 
any other part of the chains (if not quite): they are often sufficiently 
abundant to give the whole figure the aspect of a network with rhomboidal 
meshes. Of course, lines of force cannot anastomose in this way, and the 
existence of anastomoses in the cell-figure was necessarily a difficulty 
in the way of explaining that figure by centred forces, so long as the 
distinctive characters and properties of the chains of force were not 
recognised. 
In a viscid medium, moreover, the chains of force are often seen to cross 
one another at slightly different levels—to interlace in fact—simulating 
such true networks as we have just seen. Here is another point of 
difference between chains of force and lines of force in a uniform medium. 
Now crossings as well as networks are of frequent occurrence in the cell- 
figure, notably when it is in process of formation, and before the spindle 
has reached its full development. At this stage the astral rays growing 
