in which were ranged the unknown and ungraspable causes of caloric, luminous, electric, etc., phenomena.  Gradually, the “fluid” has vanished, and we are left (or rather, we were a short time ago) at the notion of forces—­a precise and mathematically graspable notion, but yet an essentially mysterious one.  We see this conception gradually disappearing to leave finally only the elementary ideas of matter and motion—­ideas, perhaps, which are not much clearer philosophically than the others, particularly that of matter taken per se, but which, at least, are necessary, since all the others supposed them.

Among those notions that study and time are reducing to other and simpler ones, that of electricity should be admitted; for it presents itself more and more as one of the peculiar cases of the general motion of matter.  It will be to the eternal honor of Fresnel for having introduced into science and mathematically constituted the theory of undulations (already proposed before him, however), thus giving the first example of the notion of motion substituted for that of force.  Since the principle of the conservation of energy has taken the eminent place in science that it now occupies, and we have seen a continual transformation of one series of phenomena into another, the mind is at once directed to the aspect of a new fact toward an explanation of this kind.  Still, it is certain that these hypotheses are difficult of justification; for those motions that are at present named molecular, and that we cannot help presuming to be at the base of all actions, are per se ungraspable and can only be demonstrated by the coincidence of a large number of results.  There is, however, another means of rendering them probable, and that is by employing analogy.  If, by vibrations which are directly ascertainable, we can reproduce the effects of electricity, there will be good reason for admitting that the latter is nothing else than a system of vibration differing only, perhaps, in special qualities, such as dimensions, direction, rapidity, etc.

Such is the result that is attained by the very curious experiments that are due to Mr. Bjerknes.  These constitute an ensemble of very striking results, which are perfectly concordant and exhibit very close analogies with electrical effects, as we shall presently see.

[Illustration:  FIG. 1.]

They are based on the presence of bodies set in vibration in a liquid.  The vibrations produced by Mr. Bjerknes are of two kinds—­pulsations and oscillations.  The former of these are obtained by the aid of small drums with flexible ends, as shown to the left in Fig. 1.  A small pump chamber or cylinder is, by means of a tube, put in communication with one of these closed drums in which the rapid motion of a piston alternately sucks in and expels the air.  The two flexible ends are successively thrust outward and attracted toward the center.  In an apparatus