With respect to my views and experiments, I do not think that anything shorter than the papers (and they will run to a hundred pages in the “Transactions”) will give you possession of the subject, because a great deal depends upon the comparison of observations in different parts of the world with the facts obtained by experiment, and with the deductions drawn from them; but I will try to give you an idea of the root of the matter. You are aware that I use the phrase _line of magnetic force_, to represent the presence of magnetic force, and the direction (of polarity) in which it is exerted; and by the idea which it conveys one obtains very well, and I believe without error, a notion of the distribution of the forces about a bar magnet, or between near flat poles presenting a field of equal force, or in any other case. Now, if circumstances be arranged so as to present a field of equal force, which is easily done, as I have shown by the electro-magnet, then if a sphere of iron or nickel be placed in the field, it immediately disturbs the direction of the lines of force, for they are concentrated within the sphere. They are, however, not merely concentrated, but _contorted_, for the sum of forces in any one section across the field is always equal to the sum of forces in any other section, and therefore their condensation in the iron or nickel cannot occur without this contortion. Moreover, the contortion is easily shown by using a small needle (one-tenth of an inch long) to examine the field, for, as before the introduction of the sphere of iron or nickel, it would always take up a position parallel to itself. Afterwards it varies in position in different places near the sphere. This being understood, let us then suppose the sphere to be raised in temperature. At a certain temperature it begins to lose its power of affecting the lines of magnetic force, and ends by retaining scarcely any. So that as regards the little needle mentioned above, it now stands everywhere parallel to itself within the field of force. This change occurs with iron at a very high temperature, and is passed through within the compass, apparently, of a small number of degrees. With nickel it occurs at much lower temperatures, being affected by the heat of boiling oil.