40 PHYSIOGRAPHY. [chap. 



is partially condensed, and therefore ceases to be true 

 vapour, that it appears in those cloud-like forms which are 

 popularly called " steam." Could you look into the interior 

 of a kettle or of a boiler from which clouds of so-called 

 steam are issuing, you would see absolutely nothing in 

 the space above the boiling water. It is only necessary to 

 boil water in a glass vessel, such as a Florence flask, in 

 order to observe that the steam remains invisible until 

 exposed to some chilling influence, such as that of a body 

 of cold air. 



More or less of this watery vapour or steam, in its 

 invisible condition, is constantly present in the surrounding 

 atmosphere. It is drawn up into the air from every 

 exposed piece of water, by means of solar heat, just as 

 steam is generated from the water in the boiler by the aid 

 of artificial heat. Whether it be evolved rapidly, with for- 

 mation of bubbles, as in the ordinary process of boiling, 

 or slowly and quietly, as in the course of evaporation, the 

 product is the same — namely, invisible watery vapour. 

 But let the air thus charged with moisture be sufficiently 

 cooled, and its burden of vapour, previously unseen, 

 makes its appearance as cloud, or mist, or fog. And, 

 under certain atmos]iheric conditions, the condensation 

 proceeds further, until the moisture ultimately falls to the 

 ^arth in the shape of rain. Every one knows that if a 

 cold object, such as a steel knife, be held in a cloud of 

 steam, the surface rapidly becomes covered with drops of 

 condensed water; and the drops of water in a shower 

 of rain have been generated by a similar process of condensa- 

 tion, carried on in nature. 



In most cases, the atmospheric moisture passes through 

 the condition of visible cloud, or mist, before finally con- 

 densing as rain. Yet it sometimes happens that rain falls 



