given by this one discovery is to be attributed the immense progress which the science of chemistry has made during the present century, a progress which has been so rapid, and is still increasing to such an extent, that it is now almost impossible for even the most industrious student to keep pace with the development of every department of the science. The immense number of new facts and substances which are discovered each year, in organic chemistry alone, are to be counted by hundreds; and as yet the true relation which exists between these bodies is but imperfectly known. With the discovery of new facts this relation each year is changed, new theories start up, which in their turn are destined speedily to give place to others explaining not only the old but also the more recently observed phenomena. Many books which ten years ago expressed most clearly and fully the views of the time are now perfectly useless. Hence the difficulties which a teacher of chemistry, and especially of organic chemistry, has to contend with are very great. His is a science of immense extent, and vast importance, composed of an almost boundless collection of facts, of which the classification and arrangement is as yet only partially understood. He has in short not to teach a perfect, but an imperfect and rapidly advancing science. Any attempt on my part to give even an idea of the present extent and direction of movement of our science would lead us far beyond the limits of my present address; these considerations must be left to the more extended courses of the lecture room. I will merely state that notwithstanding the enormous amount of knowledge already gained, chemistry is still in her early infancy; and it is natural that it should be so. Chemistry is the most complicated of the sciences of inorganic nature, and its general laws will therefore be the last which experiment unfolds. The cultivators of chemistry may with truth quote the grand words of Newton, when late in life he was congratulated on the magnitude of his discoveries, "To myself I seem to

have been as a child playing on the sea shore, while the immense ocean of truth lay unexplored before me.”

We must now proceed to glance at the next great principle enunciated as forming the basis of true science, a principle which has only in the last few years been recognised by the scientific world, and is as yet almost unknown to the public at large. In order to be able to understand this truth in all its bearings, we must consider the subject from a mechanical point of view. The old alchemists spent their lives in endeavouring to fine the Elixir of Life and the Philosopher's Stone. The possession of either of these most valuable articles would of course give to the fortunate owner that to which most men still devote their energies-untold wealth. The good men who followed the alchemists saw that their strivings were vain, but thought that they would obtain the same end by different means, and tried to construct a "perpetuum mobile," or an arrangement which should produce motion, effect, or force without the expenditure of any exertion.

This would, of course, be a possession quite as much to be prized as either of the valuables so coveted by the alchemists, as it would solve the great question so much discussed by the knowing of all ages, namely, the possibility of making money out of nothing. By degrees men's eyes became opened, and they saw, in special cases, that in order to produce mechanical effects a corresponding quantity of power of some kind must be consumed; that, for instance, a water-wheel moved by the weight of a certain quantity of water falling a given distance, can only effect a limited amount of work, whether in pumping water, grinding corn, or hammering iron. It became, in fact, soon evident, and was afterwards demonstrated with mathematical accuracy, that the mechanical contrivances merely effect a transference of power; that there is no such thing in mechanics as a creation of force. We know, it is true, that by a system of levers or pulleys we may make a weight of one pound raise a hundred, and it may appear at first sight as if

we here gained power, yet if we consider the question more nearly we see that this is not the case, for we find that in order to raise the hundred pounds one inch, we must pull the cord with the force of a pound through 100 inches. Here we come upon the true expression of the power exerted, namely, the weight multiplied into the distance through which it falls. This is called the "vis vive," or "labouring force; " the force which produces results, which overcomes resistances; and the great principle in mechanics is expressed in the maintenance of this law, that by means of any machine we cannot have effects produced which exceed the labouring force of the motive power. Hence, of course, the existence of a perpetuum mobile in mechanical contrivances becomes impossible.

The question next arises, how does this relation hold with the so called imponderable forces of nature, heat, magnetism, and electricity; can we not, by means of these less tangible forces, produce a perpetuum mobile? Every one knows that heat may be produced by mechanical means; iron may be made red hot by mere hammering, and the Indians procure fire by rubbing together two dry sticks. Can we make use of this heat, generated by motion, to reproduce the motion? Can we, for instance, make a steam engine which shall, by rubbing together two iron plates, produce heat enough to keep the engine going? If we can do this our object has been gained. We find, however, that this is also impossible, and we come at last to the conclusion that with whatsoever forces of nature we operate, a perpetuum mobile cannot be constructed; that we cannot, by any means whatever, produce an effect without a consumption of some kind of power.

Let us now consider what follows from this most important conclusion. What, does this mean that a perpetuum mobile is impossible? It means that there is no such thing in nature as a creation of force; that all the changes which we can effect, or which we

see going on around us, are produced solely by the transference of force. Hence force cannot disappear or be destroyed any more than matter-and this is the second great landmark in general science. We may

sum up these results in the words of Mr. Grove, an able expounder of these views. "In all phenomena, the more closely they are investigated the more we are convinced that, humanly speaking, neither matter nor force can be created or annihilated, and that an essential cause is unattainable. Causation is the will, creation

the act of God."

We may find many cases which are seeming exceptions to this general law. When, for instance, a leaden bullet is fired from a gun against an iron plate, the motion, and apparently with it, the force ceases instantaneously. What has in this case become of the power; it is surely annihilated? Not so, for on examining the discharged bullet we find that it is in the first place flattened to a thin disc, and that in the second place it is almost red hot. The mechanical energy which the ball possessed during its flight has been changed into energy of another kind; the physical condition of the lead has undergone a change, the mechanical force has been transferred into heat.

Now the law of the preservation of the labouring force recurs to us, and the thoughts strike one, is it possible that such an equivalency as we found in the case of mechanical effect, exists between two different kinds of force? Can we represent a given amount of heat by mechanical force? Can we find the quantity of mechanical force which when transferred into heat will effect a certain elevation of temperature? Thanks to the labours of your talented townsman, Mr. Joule, such a determination has been made. Mr. Joule has shown that a weight of 772 pounds falling through the space of one foot produces an effect capable of raising the temperature of one pound of water one degree Fahrenheit. That, in fact, the two amounts of force are equivalent or capable of producing the same result,

and hence this is called the mechanical equivalent of heat. Science is as yet not sufficiently far advanced to allow us to determine quantitatively the equivalency of all the other natural forces. There is, however, no doubt that this great principle applies to all the forms of energy with which we are acquainted, and in the last few years some of the other forces have been reduced to their mechanical equivalent, witness the immortal labours of Gauss, who has expressed the intensity of the earth's magnetism in absolute measure as mechanical force; and the later admirable researches of Weber, Kohlraasch, and Kirchoff, who have similarly reduced electricity to an absolute mechanical measure.

Such then is a brief outline of what is meant by this great principle of the indestructibility, or the conservation of force as it has properly been called. I should have wished to have brought before you the lofty generalizations which the minds of such men as Helmholtz and William Thomson have drawn from this great principle; at present, however, I must confine myself to a single reflection. In his life of George Stephenson, Mr. Smile relates that George one day asked his friend Dr. Buckland what force propelled the train which they saw rushing along in the distance. "Probably the arm of some stalwart north-country driver," was Buckland's reply. "No," says Stephenson, "It is the sun's rays." This pithy remark of Stephenson's, alluding to the growth of vegetation in bygone ages which now forms the coaljbeds of England, we may apply more extensively. Every motion, every action of our bodies, in fact, the whole inanimate creation, is indirectly dependent on the light of the sun. It is his rays alone which enables the plant to grow, to decompose the carbonic acid, and thus derive its nourishment from the air; we only sustain life and power of action by means of these plants, or of other animals which have lived on these plants, hence we are dependent on the solar light for our very existence, and may with equal truth, with the celestial Emperor, style ourselves Children of the Sun.