waves. The electric light, therefore, shall be employed in our experimental demonstrations.

40. From this source a powerful beam is sent through the room, revealing its track by the motes floating in the air of the room; for were the motes entirely absent the beam would be unseen. It falls upon a concave mirror (a glass one silvered behind will answer) and is gathered up by the mirror into a cone of reflected rays; the luminous apex of the cone, which is the focus of the mirror, being about fifteen inches distant from its reflecting surface. Let us mark the focus accurately by a pointer.

41. And now let us place in the path of the beam a substance perfectly opaque to light. This substance is iodine dissolved in a liquid called bisulphide of carbon. The light at the focus instantly vanishes when the dark solution is introduced. But the solution is intensely transparent to the dark waves, and a focus of such waves remains in the air of the room after the light has been abolished. You may feel the heat of these waves with your hand; you may let them fall upon a thermometer, and thus prove their presence; or, best of all, you may cause them to produce a current of electricity, which deflects a large magnetic needle. The magnitude of the deflection is a measure of the heat.

42. Our object now is, by the use of a more powerful lamp, and a better mirror (one silvered in front and with a shorter focal distance), to intensify the

nedion here rendered so sensible. As before, the focus rendered strikingly visible by the intense illuminaon of the dust particles. We will first filter the beam so as to intercept its dark waves, and then permit the purely luminous wares to exert their utmost power on a small bundle of gun-cotton placed at the focus,

43, No effect whatever is produced. The gun-cotton might remain there for a week without ignition. Let

how permit the unfiltered beam to act upon the ton. It is instantly dissipated in an explosive flash. This experiment proves that the light-waves are incomstent to explode the cotton, while the waves of the till beam are competent to do so; hence we may conslide that the dark waves are the real agents in the waplosion.

44. But this conclusion would be only probable; for it might be urged that the mixture of the dark waves and the light-waves is necessary to produce the result. Let us then, by means of our opaque solution, isolate our dark waves and converge them on the cotton. It caplodes as before.

h. Honco it is the dark waves, and they only, that wca soncerned in the ignition of the cotton.

. At the same dark focus sheets of platinum are Latout to vivid redness; zine is burnt up; paper inStuthy blazon; magnesium wire is ignited; charcoal within a receiver containing oxygen is set burning; a

dianiond similarly placed is caused to glow like a star, being afterwards gradually dissipated. And all this while the air at the focus remains as cool as in any other part of the room.

47. To obtain the light-waves we employ a clear solution of alum in water; to obtain the dark waves we employ the solution of iodine above referred to. But as before stated (32), the alum is not so perfect a filter as the iodine; for it transmits a portion of the obscure heat.

48. Though the light-waves here prove their incompetence to ignite gun-cotton, they are able to burn up black paper; or, indeed, to explode the cotton when it is blackened. The white cotton does not absorb the light, and without absorption we have no heating. The blackened cotton absorbs, is heated, and explodes.

49. Instead of a solution of alum, we will employ for our next experiment a cell of pure water, through which the light passes without sensible absorption. At the focus is placed a test-tube also containing water, the ful force of the light being concentrated upon it. The water is not sensibly warmed by the concentrated waves. We now remove the cell of water; no change is visible in the beam, but the water contained in the test-tube now boils.

50. The light-waves being thus proved ineffectual, and the full beam effectual, we may infer that it is the dark waves that do the work of heating. But we clench

C

and in a moment a path of the Sun fir as urge as half-a-crown is melted Eve, usuot as the fil beam produces this effect, and the luminous part of the beam does not produce it, we fx span the luck portion the melting of the frost.

58. As before, we elench this inferenze by concentrating the dark waves alone upon the fish. The frost is dissipated exactly as it was by the full beam.

54. These effects are rendered strikingly visible‘by darkening with ink the freezing mixture within the flask. When the hoar frost is removed, the blackness of the surface from which it had been melted comes out in strong contrast with the adjacent snowy whiteWhen the flask itself, instead of the freezing mixture, is blackened, the purely luminous waves being absorbed by the glass, warm it; the glass reacts upon

the frost, and melts it. Hence the wisdom of darkening, instead of the flask itself, the mixture within the flask.

55. This experiment proves to demonstration the statement in paragraph (36): that it is the dark waves of the sun that melt the mountain snow and ice, and originate all the rivers derived from glaciers.

There are writers who seem to regard science as an aggregate of facts, and hence doubt its efficacy as an exercise of the reasoning powers. But all that I have here taught you is the result of reason, taking its stand, however, upon the sure basis of observation and experiment. And this is the spirit in which our further studies are to be pursued.

§ 6. Oceanic Distillation.

56. The sun, you know, is never exactly overhead in England. But at the equator, and within certain limits north and south of it, the sun at certain periods of the year is directly overhead at noon. These limits are called the Tropics of Cancer and of Capricorn. Upon the belt comprised between these two circles the sun's rays fall with their mightiest power; for here they shoot directly downwards, and heat both earth and sea more than when they strike slantingly.

57. When the vertical sunbeams strike the land they heat it, and the air in contact with the hot soil becomes. heated in turn. But when heated the air expands, and when it expands it becomes lighter. This lighter air