which gives uniformity to the course of the magnetic machine, might be considerably weakened by employing hollow tubes (18).
29.
The free magnetism of the inner surface of hollow tubes magnetized by an electric helix is very feeble, although the magnetization of the external surface is very energetic. I shall return to this subject subsequently, but shall confine myself at present to the remark, that this magnetism does not appear to belong properly to the inner surface, but that it should be attributed to the external layer which acts at a distance, on the testing needle, or on the iron filings which have been introduced into the interior (36). If we employ a hollow tube for the armature of a horse-shoe, the magnetism of the interior is much more decided; but the distribution at the surface is effected in so complicated a manner that I have not yet been able to account in any way for it. If we break the voltaic circuit which serves to magnetize the horse-shoe, the armature remains adherent to the poles; unless it is a hollow tube, every trace of free magnetism disappears. When the armature is solid, it still retains some traces of magnetism, which appear to be diffused rather at the lower surface than at the upper part. The force with which the armature still adhered, after having interrupted the voltaic circuit, was for the cylinder D of one line in thickness 1138lb., and for the solid cylinder only 713lb., their own weight included. These numbers are the arithmetical means of five closely related experiments.
30.
A number of interesting questions are naturally connected with researches of this kind, among which, those respecting the state of magnetic saturation of which soft iron is capable, occupy an important place. It is customary to use the expression "magnetized to saturation," chiefly when it relates to the rendering of steel magnetic. But can such a state also exist in soft iron magnetized by {he influence of currents, whose energy and quality may be increased at will? Is there a limit above which magnetic development cannot be forced? In fact we may expect from experiments undertaken on this subject, brilliant and fertile results standing in closer connexion with the essence of these problematic forces. But whoever has devoted himself to expensive and wearisome researches of this kind, will
