150 THE LAST CRUISE OF THE MIRANDA. 



hand, if the air is cooled below the point of saturation a part 

 of the vapor condenses as cloud, rain, or snow. The tempera- 

 ture at which it begins to condense is called the " dew-point," 

 which is also the point of saturation. If it condenses on a 

 large surface it is obviously liquid, but if condensation takes 

 place throughout the mass of the air it forms a cloud, as 

 always occurs when steam from a jet issues into the outer air. 

 Until lately there has been much uncertainty as to the nature 

 of this cloud. Some considered that the particles were hollow 

 vesicles minute bubbles, as it were each containing a little 

 air, and thus made buoyant. Others held that the particles 

 were simply minute drops of water, so small that they were 

 suspended in the gaseous medium, just as particles of dust are. 

 In laboratory experiments on the matter the formation of cloud 

 was found to be very capricious ; sometimes it formed and 

 sometimes not, and several curious hypotheses were suggested 

 to account for the anomalies. 



It was believed, however, that whenever saturated air was 

 cooled below the dew-point condensation would and must 

 occur, as it certainly did in free air. If it was on a cooled 

 surface, then as dew ; if throughout the whole mass of the 

 air, then a cloud must be formed within the air itself; and if the 

 supply of vapor was continued, or the temperature continued 

 to fall, then the cloud-particles would increase in size and 

 become drops of rain, or, if this occurs below 32 Fahr., then 

 frost or snow. Such was the belief until a very few years ago. 



It is now proved that steam or watery vapor does not 

 readily condense unless it has a surface to condense upon. In 

 the open air the suspended dust-particles furnish this surface. 

 Air in closed vessels may artificially be freed from all dust- 

 particles, and if a jet of steam be admitted into such an atmos- 

 phere no cloud is formed ; the air becomes supersaturated 

 that is, contains much more vapor than it will hold in its 

 natural state. It has thus been made to hold several times 



