(_a_) Monopolar field of diffusion. A drop of blood in a saline solution of higher concentration. (_b_) Bipolar field of diffusion. Two poles of opposite signs. On the right a grain of salt forming a hypertonic pole of concentration, on the left a drop of blood forming a hypotonic pole of dilution. ] _The Field of Diffusion._--Just as Faraday introduced the conception of a field of magnetic force and a field of electric force to explain magnetic and electrical phenomena, so we may elucidate the phenomena of diffusion by the conception of a field of diffusion, with centres or poles of diffusive force. If we consider a solution as a field of diffusion, any point where the concentration is greater than that of the rest may be considered as a centre of force, attractive for the molecules of water, and repulsive for the molecules of the solute. In the same way any point of less concentration may be regarded as a centre of attraction for the molecules of the solute, and a centre of repulsion for the molecules of water. A field of diffusion may be monopolar or bipolar. A bipolar field has a hypertonic pole or centre of concentration, and a hypotonic pole or centre of dilution. By analogy with the magnetic and electric fields we may designate the hypertonic pole as the positive pole of diffusion, and the hypotonic as the negative pole. {57} The positive and negative poles and the lines of force in the field of diffusion may be illustrated by the following experiment. A thin layer of salt water is spread over an absolutely horizontal plate of glass. If now we take a drop of blood, or of Indian ink, and drop it carefully into the middle of the salt solution, we shall find that the coloured particles will travel along the lines of diffusive force, and thus map out for us a monopolar field of diffusion, as in Fig. 3 a. Again, if we place two similar drops side by side in a salt solution, their lines of diffusion will repel one another, as in Fig. 4. [Illustration: FIG. 4.--Two drops of blood in a more concentrated solution, showing a field of diffusion between two poles of the same sign.] Now let us put into the solution, side by side, one drop of less concentration and another of greater concentration than the solution. The lines of diffusion will pass from one drop to the other, diverging from the centre of one drop and converging towards the centre of the other (Fig. 3 _b_). In this manner we are able to obtain diffusion fields analogous to the magnetic fields between poles of the same sign and poles of opposite signs.