DUST. 25 



iiig town in which the cliiy, sand, soot and water, that are its chief 

 constituents, resolve, theinselves under the power of crystallizing 

 aflHnity into a sapphire, an opal, a diamond, set in a star of snow. 



Tyndall and others have taught us the hygiene of dust, 

 which includes the germ theory of disease that has revolution- 

 ized the sciences of surgery and medicine. This paper,however, 

 is concerned with the physics of terrestrial dust and its influence 

 as. an essential factor in meteorology, and in making the earth 

 what. it is as a place fit for the existence of animal and vegetable 

 life. \ . • 



The discovery of the undulatory movement of light in straight 

 lines, modified only by. the law of reflection and refrajCtion, wiped 

 the slate of previous notions on this subject. The cauise of its 

 difi'usion required explanation and has been found to be, in the 

 first instance, owing to the presence in the atmosphere;of infinite 

 dust particles, the more finely divided in the upper, strata and 

 the coarser in the lower. Hence the clear blue of the sky, which 

 would otherwise be a black back-ground on which would appear 

 the sun, like the blinding projected search-light of a warship, 

 the other heavenly bodies in less degree and all visible in day 

 time. Thus we would receive streams of light with Idack be- 

 wildering shadows between. The face of the earth would stand 

 out stark like a lunar landscape. Where there is no atmosphere, 

 there is no dust, and. therefore no difi'usion of light. 



But the light of the sun coming to us and passing into our 

 dust-pervaded air is caught and reflected by each particle, and as 

 the finer, particles float higher and are large enough to reflect only 

 the short blue wakes ofthespectrum, hence thecolorofthesky. As 

 the light passes dowi,i its rays are c_-.ught in turn by coarser par- 

 ticles, just as they may be large enough to reflect the longer 

 waves of the other colors of the spectrum, and dift'use them. 



To this vast and vital office of insignificant and hated dust 

 do we owe all the beauties of the. morn, the splendor of noon and 

 the glories of sunset; also the ever varying tints of mountain, 

 tea, and landscape. - The eye would cense to derive pleasure in 

 its use, were there no dust, for then art would be impossible. 

 More important still,, were it not for dust and its efl'ects, vegeta- 

 ble life would be impossible except perhaps in the form of fungi; 

 animal life a query, and comiaon life as .we live it intolerable. 



