214 WRITINGS OF JOSEPH HENRY. [1855- 



cannot be the case with a metallic or glass vessel, and the 

 only source to which we can refer the dew is the atmosphere. 

 The stratum of air immediately around the vessel is cooled 

 by contact with its sides and a portion of its vapor reduced 

 to water. The air thus cooled becomes heavier, sinks down 

 along the side of the tumbler, and gives place to a new por- 

 tion of which the vapor is also condensed ; and in this way 

 the process is continued as long as the temperature of the 

 water is below that of the surrounding air. If the water 

 which trickles down the side of the vessel is chemically 

 examined, it will be found in some cases almost entirely 

 pure, and in others contaminated by animal and other 

 effluvia which are diffused in the atmosphere. If the ex- 

 periment be made on different days and at different seasons 

 we shall find a greater or less reduction of the temperature 

 of the liquid within the tumbler is required in order to 

 produce a deposition of the vapor. The greater the number 

 of degrees of this reduction of temperature the greater will 

 be the evaporation from a given surface of water, and the 

 more intense will be the different efifects which depend on 

 the relative dryness of the air. If the experiment be made 

 in summer we shall frequently find but a small reduction 

 of temperature necessary to produce the deposition of moist- 

 ure on the outside of the tumbler, and if we attend to the 

 state of our feelings at the same time we experience that 

 peculiar sensation which is referred to what is called the 

 closeness or sultriness of the atmosphere, and which is caused 

 by the large amount of vapor with which it is charged. 



The phenomena of vapor by itself in a vacuum. — To under- 

 stand even approximately the effects due to the vapor in the 

 atmosphere it is necessary that we should first carefully study 

 the phenomena of water in an aeriform condition as it exists 

 by itself or separated from the atmosphere; and for this 

 purpose we may employ the ingenious method devised by 

 Dr. Dalton, of Manchester, England, to whose researches in 

 meteorology and other branches of physical science we are 

 more indebted than to those of almost any other individual 

 of the present century. He employed in these researches a 



