116 ANNUAL REPORT OF NEW- YORK 



appears; it evaporates, it becomes invisible vapor or steam, and 

 passes into the air. The higher the temperature to which the 

 water is exposed, the more rapidly is this conversion accomplished. 

 On the other hand, when a glass of cold water is brought into a 

 warm, moist atmosphere, or held over the spout of a boiling tea- 

 kettle, a deposition of water takes place on the cold surface; the 

 vapor condenses, liquefies. Thus, by exposing water to great cold 

 it freezes, solidifies, becomes ice; by elevating the temperature of 

 a piece of ice, it becomes first liquid and then gaseous: by cooling 

 vapor, it passes into the liquid and finally into the solid form. 

 Temperature and pressure are the influences that affect the con- 

 dition of water. The first of these alone needs lengthened consi- 

 deration here 



LIQUEFACTION VAPORIZATION LATENT HEAT. ^ 



When a piece of ice is placed in a vessel, whose temperature is 

 increasing, by means of a lamp, at the rate of one degree of the 

 thermometer every minute, it will be found that the temperature 

 of the ice rises until it attains 32*^. When this point is reached, 

 it begins to melt, but does not suddenly become fluid; the melting 

 goes on very gradually. A thermometer placed in the water, 

 remains constantly at 32^, so long as a fragment of ice is present. 

 The moment the ice disappears, the temperature begins to rise again 

 as before, at the rate of one degree per minute. The time during 

 which the temperature of the ice and water remains at 32*^, is 

 140 minutes. During each of these minutes one degree of heat 

 enters the mixture, but is not indicated by the thermometer — the 

 mercury remains stationary; 140^ of heat have thus passed into 

 the ice and become hidden, latent^ at the same time the solid ice has 

 become liquid water. The difference then between ice and water 

 consists in the heat that is latent in the latter. If we now pro- 

 ceed with the above experiment, allowing the heat to increase with 

 the same rapidity, we find that the temperature of the water rises 

 constantly for 180 minutes. The thermometer then indicates a 

 temperature of 212^ (32-^180,) and the water boils. Proceeding 

 with the experiment, the water evaporates away, but the ther- 

 mometer continues stationary so long as any liquid remains. After 

 the lapse of 972 minutes, it is completely evaporated. Water in 

 becoming steam, renders therefore still another portion, 972*^, of 

 heat latent. The heat latent in steam is indispensable to the exis- 



