IN CLOUDS AND RIVERS, ICE AND GLACIERS. 



80 



burned ur> , paper instantly blazes , magne- 

 sium wire is ignited ; charcoal witiim a ra- 

 ceivt- r coutainiTig oxygen is set binning : a 

 diamond similarly placed is caused to glow 

 like a star, being afterward gradually dissi- 

 pated. And all this while the air at the fo- 

 "iis 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 tho solution of 

 iodine above referred to. But as before 

 stated (32), the aluui. is not so perfect a tiller 

 as the iodine ; / : or it uausmits a portion of 

 the obscure her i. 



48. Though the light-waves here prove 

 their incompetence to ignite gun-cotton, they 

 arc 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 with- 

 out sensible absorption. At the focus is 

 placed a test-tube also containing water, thtt 

 full force of the light being concentrated 

 upon it. The water is not sensibly warmed 

 by the conoentiated waves. We now re- 

 move 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 in- 

 effectual, and the full beam effectual, we may 

 infer that it is the dark waves that do the woi k 

 of heating. But we clinch our inference by 

 employing our opaque iodine filter. Placing 

 it on the path of the beam, the light is en- 

 tirely stopped, but the water boils exactly as 

 it did when the full beam fell upon it. 



51. The truth of the statement made in 

 paragraph 34 is thus demonstrated. 



52. And now with regard to the im llinrj of 

 ice. On the surface of a flask contain:^ a 

 freezing mixture we obtain n thick fur of hoar- 

 frost (Par. 14). Sending the beam through 

 a water-cell, its luminous waveo c,n; concen- 

 ( rated upon the surface of too E.a*"^ Not a 

 spicula of the frost is disso'l^ti. ''* e now 

 remove the water-cjll, and in ,'i moment a 

 patch of the frozen fur as le.rge as half-a-crown 

 is melted. Hence, inasmuch as the full beam 

 produces this effect, and the luminous part 

 of the beam does not produce it, we fix upon 

 the dark portion the melting of the frost. 



53. As before, we clinch this inference by 

 concentrating the dark waves alone upon the 

 Mask. 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 cor- ~ st with the adjacent su.wy 

 'vt!t,eness. f^hen the flask itself, instead of 

 the -freezing mixture, is blackened, the purely 

 luminous waves bein absorbed by the glass. 



w&rm it ; the glass reacts upon the frost nnd 

 melts it. Hence the wisdom of darkening, 

 instead of the ikisk itself, the mixture within 

 the flask. 



t 55. This experiment proves to demonstra- 

 tion the statement in paragraph 30 : that it 

 is the dark waves of the sun that nit-It thy 

 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, nnd hence 

 doubt its efficacy as an exercise of the rea- 

 soning powers. But all that I have here 

 taught you is the result of reason, taking its 

 stand, however, upon the sure basis of ob- 

 servation and experiment. And this is the 

 spirit in which our further studies are to be 

 pursued. 



o. OCEANIC DISTILLATION. 



56. IN.e sun, you know, is never exactly 

 overhead in England. But at the equator, 

 and within certain limits north and south of 

 it. the sun &t certain periods of the year is 

 directly overhead at noon. These limits are 

 called the Tropics of Cancer and of Capri- 

 corn. Upon the belt comprised between 

 these two circles the sun's rays fall with their 

 mightiest power ; for here they shoot directly 

 downward, 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- 

 rises, like wood plunged into waler.lh rough 

 the heavier air overhead. 



58. When the sunbeams fall upon the sea 

 the water is warmed, though not so much as. 

 the land. The warmed water expands, be- 

 comes thereby lighter, and therefore continues, 

 to float upon the top. This upper layer of 

 water warms to some extent the air in contact 

 with it, but it also sends up a quantity of 

 aqueous vapor which, being far lighter than 

 air, helps the latter to rise. Thus both from 

 the land and from the sea we have ascending- 

 currents established by the action of the sun! 



59. When they reach a certain elevation in. 

 the atmosphere, these currents divide and 

 flow, part toward the north and part toward 

 the south ; -jvliiie from the north and the south 

 a flow of heavier and colder air sets in to. 

 supply the place of the ascending warm air. 



60. Incessant circulation is thus established 

 in the atmosphere. The equatorial air and 

 vapor flow above toward the north and 

 south poles, while the polar air flows below 

 toward the equator. The two currents of 

 air thus established are called the upper and 

 the lower trade-winds. 



61. But before the air returns from the 

 poles great changes have occurred. For the 

 air as it quitted the equatorial regions was 

 laden with aqueous vapor, which could not 

 subsist in the cold polar regions. It is there 

 precipitated, falling sometimes as rain, or 

 more commonly as snow. The land near the 

 pole is covered with this snow, which gives 



