by disconnecting the wire from the battery at one end of the room the needle is made to move at the other at the very same moment. This action would take place even if the wires connected with the poles were Carried 100 or even 1,000 miles away before they were joined together. If a magnetic needle were placed beside the wire conveying the current, even though the wire should be 1,000 miles from the battery, it would be deflected, but as soon as the other extremity of this wire 1,000 miles away was disjoined from the pole of the battery, the current would cease to pass, and the magnetic needle would return to its usual position. You thus see how it is possible, by making and breaking contact of a wire with the pole of a battery, to move a magnetic needle 1,000 miles away.

In fact we have here the principle of the electric telegraph, which performs such wonders in the way of information, telling us what takes place in America a few seconds after it happens. I cannot, however, enter more fully into the subject, but at least you see that it is possible to agitate a magnetic needle 1,000 miles away, and, just as in the alphabet for the deaf and dumb, these signals may be made the means of conveying information.

90. Conclusion.-You have now learned what the electric current can do. How, in the first place, it can heat a fine wire through which it passes; how, secondly, it can decompose water and other compounds; how, thirdly, it can make a piece of soft iron into a powerful though temporary magnet; how, fourthly, it can make a piece of hard steel into a permanent magnet; and fifthly

and lastly, how it can deflect the compass needle, rendering it thereby possible to telegraph to great distances.

We cannot enter more fully into this very interesting subject, but in conclusion let me remind you that you have now learned something about the active moods of matter. We spoke first of all about moving bodies, then about vibrating bodies, then about heated bodies, and lastly about electrified bodies; and we have tried throughout to show you that the activity which a body may possess is never really lost. It may, no doubt, pass to some other body, or it may change its form, going from visible energy into sound, or into heat, or into electricity, or changing about in many different ways, but it is really lost no more than a particle of matter is lost.

Indeed just as the science of Chemistry is built upon the principle that matter only changes form, going from one combination to another, and does not absolutely disappear, so the science of Physics is founded upon the principle that activity or energy only changes form, and never absolutely disappears. This, however, is a principle the full development of which must be reserved for a future stage.

THINGS TO BE REMEMBERED.

A POUND avoirdupois is equal to 7,000 grains. If a stone be dropped from the hand, it will fall through 16 feet during the first second of time.

Steel is the strongest metal, but gold is the most malleable; for a cubic inch of gold can be beaten out so as to cover the floor of a room 50 feet long and 40 feet wide.

The diamond is the hardest solid; that is to say, it can scratch everything else, but nothing else can scratch it.

A cubic inch of water weighs nearly 252 grains ; and, therefore, four cubic inches weigh nearly 1,000 grains.

100 cubic inches of air weigh 31 grains.

100 cubic inches of carbonic acid weigh 47 grains. 100 cubic inches of hydrogen only weigh 2 grains.

The pressure of the atmosphere will support a column of mercury 30 inches high, and a column of water more than 30 feet high.

Sound travels through air at a velocity of about 1,100 feet in one second of time.

If a musical string vibrates 50 times in one second, it emits a deep, low note; if it vibrates 10,000 times in one second, it emits a shrill, high note.

The heat required to melt a pound of ice would heat 79 pounds of water one degree. The heat required to boil away a pound of boiling water would heat 537 pounds of water one degree.

Light travels through space nearly at the rate of 190,000 miles in one second of time.

The spark from a Leyden jar lasts only the twenty-four-thousandth part of one second.

INSTRUCTIONS REGARDING APPARATUS.

THE apparatus to be used should be set up on the table before the lesson, and the teacher should make sure that he can perform without difficulty the various experiments. After the lesson the apparatus ought to be put away carefully into its appropriate place.

Care must be taken that the piston of the air-pump is rendered tight in its cylinder by means of lard. Care must also be taken that the receiver fits well upon its bed-plate, and for this purpose it must be well greased with lard. When this is done, the receiver ought to move smoothly and without noise on its bed-plate; but if there is a grinding noise it shows that some hard substance is present, and the bottom of the receiver must then be carefully cleaned and greased anew. This remark applies to the hemispheres (fig. 15), as well as to the glass receivers.

In order to fill the box of Experiment 28 with carbonic acid gas, the tube conveying the gas should descend very nearly, but not quite, to the bottom of the box.

To fill the same box (Experiment 29) with hydrogen, the tube conveying the gas should ascend very nearly to the bottom of the box, which is now uppermost.

The whole apparatus for Experiment 45 must be placed in a cool room some hours before the experiment is made.