THE FORMS OF WATER 



and descend at the other side you finally ault 

 the snow, and get upon another glacier called 

 the Trift, from the end of which rushes a 

 river smaller than the Rhone. 



19. You soon learn that the mountain 

 snow feeds the glacier. By some means or 

 other the snow is converted into ice. But 

 whence comes the snow ? Like the rain, it 

 comes from the clouds, which, as before, can 

 be traced to vapor raised by the sun. With- 

 out solar lire AVC could have no atmospheric 

 vapor, without vapor no clouds, without 

 clouds no snow, and without snow no gla- 

 ciers. Curious then as the conclusion maybe, 

 the cold ice of the Alps has its origin in the 

 heat of the sun. 



3. THE WAVES OF LIGHT. 



20. But what is the sun ? We knov/ its 

 size and its weight. We also know that it 

 is a globe of fire far hotter than any fire 

 upon earth. But we now enter upon another 

 inquiry. We have to learn definitely what is 

 the meaning of solar light and solar'heat ; in 

 what way they make themselves known to 

 our senses ; by what means they get from 

 the sun to the earth, and how, when there, 

 they produce the clouds of our atmosphere, 

 and thus originate our rivers and our glaciers. 



21. If in a dark room you close your eyes 

 and press the eyelid with your finger-nail, a, 

 circle of light will be seen opposite to the 

 point pressed, while a sharp blow upon the 

 eye produces the impression of a flash of 

 light. There is a nerve specially devoted to 

 the purposes of vision which comes from the 

 brain to tho back of the eye, and there di- 

 vides into line filaments, which are woven 

 together to a kind of screen called the retina. 

 The retina can be excited in various ways so J 

 ns to produce the consciousness of light ; it 

 m?,y, as we have seen, be excited by the rude^ 

 mechanical action of a blow imparted to the 

 eye. 



22. There is no spontaneous creation of 

 light by the healthy eye. To excite vision 

 tho relLia must be affected by something , 

 coming from without. What is that some- 

 thing? In some way or other, luminous 

 bo:lies have the power of affecting the retina 

 but hoio ? 



23. It was long supposed that from such 

 bodies issued, with inconceivable rapidity, au 

 inconceivably fine matter, which flew 

 through spaca, passed through the pores sup- 

 posed to exist in the humors of the eye, 

 reached the retina behind, and by their shock 

 .-gainst the retina, aroused the sensation of 

 light. 



2i. This theory, which was supported by 

 the greatest men, among others by Sir Isaac 

 Newton, was found competent to explain a 

 great number of the phenomena of light, but 

 it was not found competent to explain all the 

 phenomena. As the skill and knowledge of 

 experimenters increased, large classes of facts 

 were revealed which could only be explained 

 by assuming that light was produced, not by 

 a fine matter Hying through space and hitting 

 the retina, but by the shock of minute waxes 



against the retina. .. 



25. Dip your finger into a basin of water, 

 and cause it to quiver rapidly to and fro. 

 From the point of disturbance issue small 

 ripples which are carried forward by the 

 water, and which finally strike the basin. 

 Here, in the vibrating tinger, you have a 

 source of agitation ; in the water you have a 

 vehicle through which the finger's motion is 

 transmitted, and you have finally the side of 

 the basin which receives the shock of the 

 little waves. 



26. In like manner, according to the warn 

 theory of light, you have a source of agitation 

 in the vibrating atoms, or smallest particles, 

 of the luminous body ; you have a vehicle of 

 transmission in a substance which is sup- 

 posed to fill all space, and to be diffused 

 through the humors of the eye ; and finally, 

 you have the retina, which receives the suc- 

 cessive shocks of the waves. These shocks 

 are supposed to produce the sensation of light. 



27. We are here dealing, for the most part, 

 with suppositions and assumptions merely. 

 We have never seen the atoms of a luminous 

 body, nor their motions. We have never 

 seen the medium which transmits their 

 motions, nor the waves of that medium. 

 How, then, do \vc come to assume their ex- 

 istence ? 



; 28. Before such an idea could have taken 

 any real root in the human mind, it must have 

 been well disciplined and prepared by obser- 

 vations and calculations of ordinary wave- 

 motion. It was necessary to know how both 

 water- waves and sound-waves arc formed 

 and propagated. It was above all thing.-.' 

 necessary to know how waves, passing 

 through the same medium, act upon each 

 other. Thus disciplined, the mind was pre- 

 pared to detect any resemblance presenting 

 itself between the action of light and that of 

 waves. Great classes of optical phenomena 

 accordingly appeared which could be ac- 

 counted for in the most complete and satis- 

 factory manner by assuming them to be pro- 

 duced by waves, and which could not bo 

 otherwise accounted for. It is because of its 

 competence to explain all tho phenomena of 

 light that the wave theory now receives uni- 

 versal acceptance on tho part of scientific 

 men. 



Let me use an illustration. We infer from 

 the flint implements recently found in such 

 profusion all over England and in other 

 countries, that they were produced by men, 

 and also that the Pyramids of Egypt were 

 built by men, because, as far as our expe- 

 rience goes, nothing but men could form such 

 implements or build such Pyramids. In like 

 manner, we infer from the phenomena of 

 light the agency of waves, because, as far as 

 our experience goes, no other agency could 

 produce the phenomena. 



4. THE WAVES OF HEAT WHICH PRODUCE 

 THE VAPOR OF OUR ATMOSPHERE AND 

 MELT OUR GLACIERS. 

 29. Thus, in a general way, I have given 



you the conception and the grounds of tho 



