346 
PROFESSOR TYNDALL ON THE ACTION OP FREE MOLECULES ON 
Wells, “appeared on the two nights of the greatest cold I have ever observed on the 
surface of the earth, relatively to the temperature of the air, both of them having 
occurred after a long tract of dry weather.”* * * § This evidence is specially valuable in 
view of the fact that Wells knew nothing of the action of water vapour on radiant 
heat. On another occasion he observed a difference of 9-|- degrees between the tem¬ 
perature of the air and that of wool placed upon the earth, without any deposition of 
dew whatever upon the chilled wooht He supplements these observations by one 
equally important in the opposite direction. <( On the night,” he writes, “ which 
afforded the most copious dew ever observed by me, the cold possessed by the grass 
beyond that of the air was for the most part only 3° and 4°. ;, | The smallness of the 
refrigeration in this instance, and the copiousness of the dew, I refer to one and the 
same cause ; namely, the abundance of vapour. Heavy dew implies this abundance ; 
abundant vapour, if not too local, implies checked radiation, and checked radiation 
tends to abolish the difference of temperature between air and soil. 
Wells had a theory of his own to account for the association of moderate refri¬ 
geration and heavy dew. The heat rendered free by the condensation of the vapour 
to liquid “ prevented ” the cold. He tried to determine the effect of condensation 
by the following experiment. § To 10 grains of wool he added 21 grains of water, 
this being the quantity of dew deposited on wool in one of his observations. He 
placed the moistened wool in a saucer on a feather bed in a room, and determined 
the chill produced by its evaporation. After eight hours, while the wool still retained 
2-2 grains of moisture, its temperature was 4° lower than that of a dry saucer placed 
near it on the same feather bed. When the process is reversed, condensation instead 
of evaporation coming into play, the foregoing amount of heat, Wells contended, 
would be liberated on the grass, and thus prevent inordinate refrigeration. 
In thus reasoning Wells went to the limit of the knowledge of his time, and the 
explanation here given is a philosophical one. But I do not think it a sufficient 
explanation. The grass is exposed to the open atmosphere, and the heat developed 
by every successive him of moisture condensed upon its blades, is instantly wasted by 
radiation. Those who are accustomed to work with the thermopile know how rapidly 
the associated galvanometer needle falls from a high deflection to zero, when the heat 
incident upon the pile is suddenly cut off. A similar rapidity of waste would assuredly 
occur during the slow formation of dew. The heat of condensation could not for this 
* Essays, p. 186. 
f Ibicl., p. 183. Wells sometimes found wetted wool to lose weight, while dry wool gained no weight 
though lowered many degrees below the temperature of the air, p. 184. 
+ Ibid., p. 169. From a remark occurring at page 135, it may be inferred that the night here referred 
to was that common to the 29th and 30th of July, 1813. On the two occasions first mentioned, when there 
was but little dew, the grass was in one instance 12°, and in the other 14° colder than the air. 
§ Essays, p. 187. 
