THE FORMS <jF WATER 



33. We shall learn presently how to detach 

 the one class of waves from the other class, 

 and to prove that v/aves competent to light a 

 fire, fuse metal, or burn the hand like a hot 

 solid, may exist in a perfectly dark place. 



34. Supposing, then, that we withdraw, in 

 the first instance, the large heat-waves, and 

 allow the light- waves alone to pass. These 

 nviy be concentrated by suitable lenses and 

 sent into water without sensibly warming it. 

 Let the light -waves now be withdrawn, and 

 tiie larger heat-waves concentrated in the 

 same manner , they may be caused to boil 

 the water almost instantaneously. 



35. This is the point to which I wished to 

 ead you. and which without due preparation 

 could not l>e understood. You now per- 

 ceive the important, part played by these 

 large darkness-waves, if I may use the term, 

 in the work of evaporation. When they 

 plunge into seas, lakes, and rivers, they are 

 intercepted close to the surface, and they 

 heat the water at the surface, thus causing 

 it to evaporate ; the light-waves at the samd 

 time entering to great depths without sensibly 

 heating the water through which they pass. 

 Not only, therefore, is it the sun's fire which 

 produces evaporation, but a particular con- 

 stituent of that lire, the existence of which 

 you probably w r ere not aware of. 



30. Further it is these self-same lightless 

 waves which, falling upon the g'.aei .TS of 

 the Alps, melt the ice and prot'-.ico all Hie 

 rivers flowing from the glaciers ; f;>i* I shall 

 prove to you presently that the light-waves, 

 even when concentrated to the uttermost, are 

 unable to melt the most delicate hoar-lro^t ; 

 much less would they be able to produce tlu 

 copious liquefaction observed upon the glo- 

 ciers. 



37. These large lightless waves of the sun, 

 as well as the heat-waves issuing 1 _>m non- 

 luminous hot bodies, are frequently called 

 obscure or invisible heat. 



We have here an example of the nv.nner 

 in which phenomena, apparently remote, are 

 connected together in this wonderful system 

 of things that we call Nature. You cannot 

 study a snow-Hake profoundly wiihout being- 

 led back by it step by step to the constitution 

 of the sun. It is thus throughout Nature. 

 All its parts are interdependent, and tho 

 study of any one part completely would really 

 involve the study of all 



g 5. EXPERIMENTS TO PROVE VIIE FORE- 

 GOING STATEMENTS. 



38 Heat issuing from any source not visi- 

 bly red cannot be concentrated so as to pro- 

 duce the intense effects just referred to. To 

 produce these it is necessary to employ the 

 obscure heat of a body raised to the highest 

 possible state of incandescence. The sun is 

 such a body, and its dark heat is therefore 

 suitable for experiments of this nature. 



39. But in the atmosphere of London, and 

 for experiments such as ours, the heat-waves 

 emitted by coke raised to intense whiteness 

 by a current of electricity are much more 

 manageable than the sun's waves. The elec 



trie light has also the advautage that rts dark 

 radiation embraces a larger proportion of the 

 total radiation than the dark heat of tiie sun. 

 In fact, the force 01 energy, if 1 may use the 

 term, of the daik Avaves of the electric light 

 is fully seven times that of its lightwaves. 

 The electric light, therefore, shall be em- 

 ployed in our experimental demons! rat ions. 



40. From Ibis source a powerful beam u 

 sent throuirh the room, revealing its track by 

 the motes floating in the air of the room ; for 

 were the mo'es entirely absent the beam 

 would be unseen. It falls unon a concave 

 mirror (a glass one silvered behind will an- 

 swer) and is gathered up by the mirror into 

 a cone of reflected rays ; the luminous apex 

 of the cone, which is the/ocw of the mirror, 

 being about fifteen inches distant from its 

 reflecting SMI face. Let us uiaik the* focus 

 accurately by a pointer. 



41. And now let us place in the path of 

 the beam a substance perfectly opaque tr 

 light. This substance is iodine dissolved m 

 a liquid called bisulphide of carbon. Tho 

 light at the focus instantly vanishes when 

 the dark solution is introduced. But the so 

 iution is intensely transparent to the dark 

 waves, and a focus of such waves remains m 

 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 electric- 

 ity, which Reflects a large magnetic needle. 

 The magni' .do of the deflection is a measure 

 of the htMVi 



42. Cm n \jeel now is, by the use of a 

 more power, til lamp, and a better mirror (one 

 silvered in front and with a shorter focal dis- 

 tance), to intensify the action here rendered 

 so sensible. As before, the focus is rendered 

 strikingly visible by the intense illumination 

 of the dust particles. We will first filter the 

 beam so as to intercept its dark waves, and 

 then permit the purely luminous waves to 

 sxert their utmost power on a small bundle 

 of gun-cotton placed at the focus. 



4;j. No effect whatever b produced. The 

 gun-cotton might remain there for a week 

 without ignition Let us now permit the 

 uunitered beam t:> act upon the cotton. It 

 is instantly dissipated in an explosive flash. 

 This experiment proves that the light-waves 

 are incompetent to explode the cotton, while 

 th3 waves of the full beam are compel ent to 

 do so ; hence we may conclude that the dark 

 waves are the real airents in- the explosion 1 . 



44. But this conclusion would be only 

 probable ; for it might be urged that the 

 mixture of the dark waves an.l 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 explodes as before. 



45. Hence it is the dark waves, and they 

 only, that are concerned in the ignition of 

 the cotton. 



46. At the same dark focus sheets 01 plati- 

 num are raised to vivid redness ; zinc is 



