274 EVAPORATION. 



moist or liquid on being exposed to the air. Common 

 pearlash is moistened by being exposed for a short time 

 to the action of the atmosphere. Having a strong attrac- 

 tion for water, it absorbs it from the air, and becomes 

 rnoist, and ultimately, if allowed to stand a sufficient 

 length of time, liquefied. On the other hand, some sub- 

 stances have so little attraction for water, that they readily 

 give off to the air that which they at first contained, and 

 thus crumble into powder. These two properties are 

 often distinguished by the terms deliquescence, and efflo- 



INTERESTINQ PHENOMENA RESULTING FROM EVAPORATION. 



Many interesting appearances in nature are connected 

 with evaporation. A few of these we will notice. The 

 reflection of light from vapor is perhaps one of the most 

 interesting of these. Among the Hartz Mountains, in 

 Germany, there are places in which a person may see his 

 own image in the air almost or quite as distinctly as in a 

 mirror. A light breeze of wind will destroy it, but it will 

 appear again, soon after the wind has passed. A curious phe- 

 nomenon, probably resulting from a similar cause, acting 

 under different circumstances, has been noticed in the 

 great American Desert. The ground at a distance ap- 

 peared to be covered with water, in which objects could 

 be distinctly seen reflected, as they generally are from 

 still water, but on approaching, no water was to be found. 

 In the deserts of Arabia such an illusion often mocks the 

 hopes of the thirsty traveller. There is, however, some 

 difference of opinion in regard to the cause of this last 

 phenomenon. It is often observed at sea, that when the 

 air is full of vapor, objects appear very large, or ' loom 

 up,' as the sailors term it. For an explanation of this 

 fact we must refer to the principles of Optics. From 

 these we learn, that when light passes out of a rarer 

 into a denser medium, those rays which do not strike 

 the denser medium perpendicularly, are bent so as to 

 make them nearer perpendicular to the denser medium. 

 Now the air ia thickest, and therefore capable of con- 

 taining most morsture near the earth. The rays of light 

 therefore in passing through the vapor are bent downwards, 



