exception of tidal energy, all the work done in the world is due to the sun, so that we must look to our luminary as the great source of all our energy.

Intimately linked as we are to the sun, it is natural to ask the question, will the sun last for ever? or will it also die out?

Now there is no apparent reason why the sun should form an exception to the fate of all fires, its only difference being one of size and time. It is larger and hotter, and will last longer than an ordinary lamp, but it is nevertheless a lamp, or, to speak more correctly, a very large hot body.

In fine, the principle of degradation would appear to hold throughout; and if we regard not mere matter but useful energy, we are driven to contemplate the death of the universe.

Recapitulation.-It may be desirable, before concluding, to recapitulate the various transmutations of energy.

427. Visible Energy.-Visible Energy of Motion is transmuted into visible potential energy when a stone is projected upwards and lodged on the top of a house (Art. 111), and it is transmuted into heat when friction or percussion stops a body in motion (Art. 113).

It is transmuted into electrical separation when we work the electric machine (Art. 356), and into electricity in motion when a revolving conductor is brought between the poles of a powerful magnet (Art. 396).

Visible Potential Energy is generally converted into visible energy of motion, and through it into the other forms of energy.

428. Heat. This species of energy is converted into visible motion in the heat engine (Art. 245). It is converted into radiant energy when a hot body radiates (Art. 334). It is converted into electrical separation when tourmalines and other gems are heated (Art. 356). It is converted into electricity in motion in the thermo-electric pile (Art. 378). Finally, it is converted into chemical separation when a body is decomposed by heat (Art. 215).

429. Radiant Energy. This species of energy is converted into heat when radiant light or heat is absorbed by a bɔdy (Art. 334), and it is converted into chemical separation when a ray of sunlight decomposes chloride of silver in photography, or carbonic acid in the leaves of plants (Art. 425).

430. Electrical Separation.-The energy of electrical separation is transformed into visible motion when two oppositely electrified bodies approach each other (Art. 356), and it is transformed into the energy of electricity in motion when two such bodies are connected together by means of a wire (Art. 356).

431. Electricity in Motion.-This form of energy is converted into visible motion when currents act on one another, as in Art. 393; it is converted into absorbed heat when a current meets with resistance (Art. 411); and into chemical separation when a current decomposes a compound body (Art. 416).

432. Chemical Separation.-This form of energy is transmuted into heat when a substance burns, or when combustion takes place (Art. 422); into electrical separation when two dissimilar metals are brought into contact (Art. 371); and into electricity in motion in the voltaic battery.

These form some of the chief transmutations of the various forms of energy into one another, but it ought to be borne in mind that the classification of energy into various forms is simply one of convenience, and represents the present state of our knowledge of the subject.