24.0 WELLS'S NATURAL PHILOSOPHY. 



„ , ., "When water is covered by a stratum of dry air, the evapo- 



Tlow does the •' j i i 



Btate of the air ration is rapid, even when its temperature is low ; whereas it 



oratr'nT '^^'^^' goes on very slowly if the atmosphere contains much vapor, 

 even though the air be very warm. 



Evaporation is far slower in still air than in a current. The air imme- 

 diately in contact witli the water soon becomes moist, and thus a check is 

 put to evaporation. But if the air be removed by wind from the surface of 

 the water as soon as it has become charged with vapo*-, and its place 

 supplied with fresh air, then the evaporation continues on without inter- 

 ruption. 



Evaporation is by no means confined to the surface of liquids ; but takes 

 place from the surface of the soil, and from all animal and vegetable produc- 

 tions. Evaporation takes place to a very considerable extent from the sur- 

 face of snow and ice, even when the temperature of the air is far below the 

 freezing point. 



„„ . , . 553. A very singular circumstance is connected with the 



What singular ,.„, , , , •,, 



circumstance diffusion of vapors throughout the atmosphere, viz. : that the 



'*in ti'""rt''ff ** vapors of aii ».'odic? arise into any space filled with air, in 



Bioa 01 vapors? the same manner as if air were not present, the two fluids 



seeming to be independent of each other. 



Thus as much vapor of water can be forced into a vessel filled with air as 

 into one from which the air has been exhausted. 



554. When a drop of water falls upon a surface highly 

 phenomena of heated, as of metal, it will be observed to roll along the sur- 

 *° " p^J^f"'"^" face without adhering, or immediately passing into vapor, 

 liquids. The explanation of this is, that the drop of water does not in 



reality touch the heated surface, but is buoyed up and sup- 

 ported on a layer of vapor which intervenes between the bottom of the drop 

 and the hot surface. This vapor is produced by the heat which is radiated 

 from the hot substance, before the liquid can come in contact with it, and 

 being constantly renewed, continues to support the drop. The drop generally 

 rolls because the current of air which is always passing over a heated sur- 

 face drives it forward. The drop evaporates slowly, because the layer of 

 vapor between the hot surface and the liquid prevents the rapid transmis- 

 sion of heat. The liquid resting upon a cushion of steam continually evolved 

 from its lower surface by heat, assumes a rounded, or globular shape, as tha 

 result of the gravity of its particles toward its own center. 



The designation which has been given to the condition which water and 

 other liquids assume when dropped upon very hot surfaces, is that of the 

 " spheroidal state." 



If the surface upon which the liquid rests is cooled down to such an ex- 

 tent that vapor ie not generated rapidly, and in sufficient quantity to sup- 

 port the drop, it will come in contact with the surface, and heat being com- 

 municated by conduction, will transform it instantly into steam. 



This is the explanation of the practice adopted by laundresses of touching 

 a flat-iron with moisture to ascertain whether the surface is sufficiently hot. 



