"an ultimate analysis brings us down," and on which "a rational synthesis must build up." From this first principle come as consequences two correlative principles, viz.: uniformity of law, which is simply the persistence of the relations between forces, manifested under identical forms and conditions; and the principle of the equivalence of forces, inductively established within the last twenty years. The researches which resulted in the establishment of this principle rest implicitly on the persistence of force, inasmuch as they measure all the precedent forces, which have disappeared, and all the consequent forces, which have been produced, by the aid of a unit supposed to be constant. If we add two other corollaries, the one relating to the direction of motion in the line of least resistance, the other to the form of motion, which is always rhythmic, we have, with the principles of the continuousness of motion and of the indestructibility of matter (these representing under two correlative forms the principle of the persistence of force), the sum total of the primary truths which serve as a basis for knowledge in general. But these principles, however general, are only analytical truths; though they are essential to a philosophy, they do not constitute a philosophy. They are the laws of the action of forces separately considered. The universal synthesis which is to constitute philosophy must express the total operation accomplished by the coöperation of these factors. The law which shall formulate this synthesis must be a law of the changes in forces under the two phases, matter and motion, by which they are manifested to us: it must be a principle of dynamics holding good both for the whole of the cosmos, and for its every detail. The changes of an object are all produced by new arrangements of the matter constituting it, and by a new distribution of the forces which belong to it. Their necessary direction is given in evolution in virtue of two principles, both of them corollaries of the primary principle of the persistence of force: the law of the instability of the homogeneous and the law of the multiplication of effects.

Every body tends to pass into a more heterogeneous state, because each of the units that constitute it is of necessity differently affected from the others by the combined action of the others upon it; because the resulting difference places each unit in different relations with the incident forces; finally, because these units, owing to their respective positions, cannot all receive the action of an external force in the same direction and with the same intensity. This law, which accounts for the commencement of the changes, accounts also for its continuance.

At the same time a uniform external force, acting on a body, is there dispersed; acting on unlike parts, it breaks up into forces differing in quality and intensity in proportion to the number and diversity of these parts. The same is to be said of each fraction of the force; the process of dispersion goes on increasing, and the result is expressed by the law of the multiplication of effects.

By another law, flowing from the same primary principle, the parts of a whole diverge from one another in proportion to their diversity, and group themselves together in proportion to their resemblances. Motions that are alike in direction or intensity, acting on these parts, drive them in the same direction, and with the same velocity, whence results an integration of these parts, while those driven by motions unlike in direction or intensity go in different directions with different velocities, separate from one another, are disintegrated. This is the law of segregation, the application of which brings into prominence the heterogeneous character of the products of change, by giving to their heterogeneity a clearer and more definite nature.

Finally, we note another consequence of the persistence of force. Every change in an aggregation of sensible parts is conditioned by opposing forces, the one representing action, the other reaction; the one the tendency to change, the other resistance; their antagonism can end only when equilibrium has been established, by the dissipation of the excess of the one force over the other. A body subject to any disturbance whatever, owing to a modification of its circumstances, tends toward equilibrium with its new circumstances; and, as the different forces acting on it have not the same intensity, those which are weaker soon find their equilibrium, while those which are stronger continue to give motion to the body, and then the latter presents the spectacle of an aggregate whose parts are in an invariable ratio to each other, while the total aggregate is ever changing its relations to external objects. This is equilibrium mobile, unstable equilibrium, and it serves as a transition to a more perfect equilibrium, or else to a renewal of the internal movements which have already found equilibrium.

The action of these laws of change of objects and their parts leads to two contrary results, according to the mode of distribution of the forces in action. We have evolution, i. e., change with integration of matter, dissipation of internal motion, increase of the number and diversity of the parts, whenever the external forces are not such as to break the bond which unites them; we have dissolution, continuous or discontinuous, i. e., a change with disaggregation of matter; absorption of motion (which, becoming internal, drives the constituent units with greater velocity) and diminution both of the numbers and of the diversity of the parts, whenever the external forces are sufficiently intense to destroy the cohesion of the aggregate and to restore to its parts their original independence.

The work of Mr. Spencer in his "Biology" consists in referring to these general laws the generalizations obtained in the various parts of the domain of biology, and in discerning those which possess the character of necessity. This course has the twofold advantage of giving to these generalizations greater authority, and of introducing into a coördinated system of philosophy the science whose general

ized truths they are. The "Principles of Biology "is thus an attempt to explain the phenomena called vital, by general laws common to phenomena of every kind.

[To be continued.]

LESSONS IN ELECTRICITY.'

HOLIDAY LECTURES AT THE ROYAL INSTITUTION.

BY PROFESSOR TYNDALL, F. R. S.

I.

ECTION 1. Introduction.-Many centuries before Christ, it had been observed that yellow amber (elektron) when rubbed possessed the power of attracting light bodies. Thales, the founder of the Ionic philosophy (B. c. 580), imagined the amber to be endowed with a kind of life.

This is the germ out of which has grown the science of electricity, which takes its name from the substance in which this power of attraction was first observed.

It will be my aim, during six hours of these Christmas holidays, to make you, to some extent, acquainted with the history, facts, and principles, of this science, and to teach you how to work at it.

The science has two great divisions; the one called "Frictional Electricity," the other "Voltaic Electricity." For the present, our studies will be confined to the first, or older portion of the science, which is called "Frictional Electricity," because in it the electrical power is obtained from the rubbing of bodies together.

SEC. 2. Historic Notes.-The attraction of light bodies by rubbed amber was the sum of the world's knowledge of electricity for more than 2,000 years. In 1600 Dr. Gilbert, physician to Queen Elizabeth, whose attention had been previously directed with great success to magnetism, vastly expanded the domain of electricity. He showed that not only amber, but various spars, gems, fossils, stones, glasses, and resins, exhibited when rubbed the same power as amber.

Robert Boyle (1675) proved that a suspended piece of rubbed amber, which attracted other bodies to itself, was in turn attracted by a body brought near it. He also observed the light of electricity, a diamond, with which he experimented, being found to emit light when rubbed in the dark.

Boyle imagined that the electrified body threw out an invisible, glutinous substance, which laid hold of light bodies, and, returning to the source from which it emanated, carried them along with it.

1 A course of six lectures, with simple experiments in frictional electricity, before juvenile audiences during the Christmas holidays.

Otto von Guericke, Burgomaster of Magdeburg, contemporary of Boyle, and inventor of the air-pump, intensified the electric power previously obtained. He devised what may be called the first electrical machine, which was a ball of sulphur, about the size of a child's head. Turned by a handle and rubbed by the dry hand, the sulphur-sphere emitted light in the dark.

Von Guericke also noticed that a feather, having been first attracted toward his sulphur globe, was afterward repelled, and kept at a distance from it, until, having touched another body, it was again attracted. He also heard the hissing of the "electric fire," and observed that a body, when brought near his excited sphere, became electrical and capable of being attracted.

The members of the Academy del Cimento examined various substances electrically. They proved smoke to be attracted, but not flame, which, they found, deprived an electrified body of its power.

They also proved liquids to be sensible to the electric attraction, showing that when rubbed amber was held over the surface of a liquid, a little eminence was formed, from which the liquid was finally discharged against the amber.

Sir Isaac Newton, by rubbing a flat glass, caused light bodies to jump between it and a table. He also noticed the influence of the rubber in electric excitation. His gown, for example, was found to be much more effective than a napkin. Newton imagined that the excited body emitted an elastic fluid which penetrated glass.

Dr. Wall (1708) experimented with large, elongated pieces of amber. He found wool to be the best rubber of amber. "A prodigious number of little cracklings" was produced by the friction, every one of them being accompanied by a flash of light. "This light and crackling," says Dr. Wall, "seem in some degree to represent thunder and lightning." This is the first published allusion to thunder and lightning in connection with electricity.

Stephen Gray (1729) also observed the electric brush, snappings, and sparks. He made the prophetic remark, that "though these effects are at present only minute, it is probable that in time there may be found out a way to collect a greater quantity of the electric fire, and, consequently, to increase the force of that power which by several of those experiments, if we are permitted to compare great things with small, seems to be of the same nature with that of thunder and lightning."

2

SEC. 3. The Art of Experiment.-We have thus broken ground with a few historic notes, intended to show the gradual growth of electrical science. Our next step must be to get some knowledge of the facts referred to, and to learn how they may be produced and extended. The art of producing and extending such facts, and of inquiring into them by proper instruments, is the art of experiment. 1 "Philosophical Transactions," 1708, p. 69. Ibid., vol. xxxix., p. 24.

were,

It is an art of extreme importance, for by its means we can, as it converse with Nature, asking her questions and receiving from her replies.

It was the neglect of experiment, and of the reasoning based upon it, which kept the knowledge of the ancient world confined to the attraction of amber for more than 2,000 years.

Skill in the art of experimenting does not come of itself, it is only to be acquired by labor. When you first take a billiard-cue in your hand, your strokes are awkward and ill-directed. When you learn to dance, your first movements are neither graceful nor pleasant. By practice alone, you learn to dance and to play. This also is the only way of learning the art of experiment. You must not, therefore, be daunted by your clumsiness at first; you must overcome it, and acquire skill in the art by repetition.

By so doing you will come into direct contact with natural truth -you will think and reason not on what has been said to you in books, but on what has been said to you by Nature. Thought springing from this source has a vitality not derivable from mere book-knowledge.

SEC. 4. Materials for Experiment.-At this stage of our labors we are to provide ourselves with the following materials:

a. Some sticks of sealing-wax.

b. Two pieces of gutta-percha tubing, about eighteen inches long and three-quarters of an inch outside diameter.

c. Two or three glass tubes, about eighteen inches long and threequarters of an inch wide, closed at one end, and not too thin, lest they should break in your hand and cut it.

d. Two or three pieces of clean flannel, capable of being folded into pads of two or three layers, about eight or ten inches square.

e. A couple of pads, composed of three or four layers of silk, about eight or ten inches square.

f. A board about eighteen inches square, and a piece of India-rubber.

g. Some very narrow silk ribbon, and a wire loop, like that shown in Fig. 1, in which sticks

of sealing-wax, tubes of gutta-percha, rods of glass, or a walking-stick, may be suspended. I choose a narrow ribbon because it is convenient to have a suspending cord that will neither twist nor untwist of itself.

I usually employ a loop with the two ends, which are here shown free, soldered together. The loop would thus be unbroken. But you

W

R

FIG. 1.

may not be skilled in the art of soldering, and I therefore choose the

free loop, which is very easily constructed.

VOL. VIII.-39