238 WELLS'S NATURAL PHILOSOPHY. 



What is va- ^45. If heat be imparted in sufficient qiian- 



porization? ^-^^^ ^^ ^ ]-,qJ^. -j^ .^ ^-^^^^-^j ^^.^^^^ -^ ^^.-^ ^^^^^g -^^^ 



a state of vapor. Thus, water being heated sufficiently 

 will pass into the form of steam. This change is called 

 Vaporization. 



What is Con- ^46. If a bodj in a state of vapor lose heat 

 densatiou? jj^ siifficicnt quautitv, it will pass into a liquid 

 Etate. Thus, if a certain quantity of heat be abstracted 

 from steam, it will become water. This change is called 

 Condensation. 



The change from a state of vapor to a liquid is termed condensation, be- 

 cause, in so doing, the body always undergoes a very considerable diminution 

 of volume, and therefore becomes condensed. Most solids become liquefied 

 before they vaporize ; but some pass at once, on the application of heat, from 

 the state of a solid to that of a vapor, without assuming the liquid condition. 



547. The meltinc^ of a solid, or its conver- 



Isanyparticu- . . ,..,, , , -., 



lar tempera- siou luto a liquid, ouly occurs when the solid 



ture requisite . , - • r« i • i i 



for the forma- IS hcatcd Up to a Certain fixed point ; but tiie 



tion of vapors? . /• t • i • , , ^ i 



conversion ot a liquid into a vapor takes place 

 at all temperatures. 



If in a hot day we expose a vessel filled with cold water to the open air» 

 we find that the quantity of water rapidly diminishes, that is, it evaporates, 

 which means that it is converted into vapor and ditiused thi-ough the air. 



What ia the ^48. Thc vapor of water, and all other va- 

 vapor^'™'"^ °^ pors, are invisible and transparent. The water 

 which has become diffused through the air by 

 evaporation only becomes visible when, on returning to its 

 fluid condition, it forms mist, cloud, dew, or frost. 



Steam, which is the vapor of boiling water, is invisible, but when it comes 

 in contact with air. which is cooler, it becomes condensed into small drops, 

 and is thus rendered visible. 



The proof of this may be found in examining the steam as it issues from 

 an orifice, or the spout of a boiling kettle : for a short space next to the open- 

 ing no steam can be seen, since the air is not able to condense it ; but as it 

 spreads- and comes in contact with a larger volume of air, the invisible vapor 

 becomes condensed into drops, and is thus rendered visible. 



The visible matter popularly called steam, should be, therefore, distin- 

 guished from steam proper, or the aeriform state of water. The cloud, or 

 smoke-like matter observed, is really not an air or vapor at all but a collec- 

 tion of minute bubbles of water, wafted by a current either of true steam, or, 

 more frequently, of mere moist air. 



