VAPORIZATION. 47 



introduction of an angular body into the water is sufficient, 

 in either instance, to induce the suspended change. The same 

 irregular deviation of the boiling point in glass vessels, takes 

 place in other liquids as well as water, and in some of them 

 to a much greater extent. 



There is a curious circumstance in regard to boiling, which is 

 a matter of common observation in some shape or other. When 

 a little water (a few drops) is thrown into a metallic cup, hotter 

 than the boiling point of water, the hotter the cup is, the 

 less rapidly does the water boil away. We should expect the 

 reverse, or that the hotter the metallic cup, the more quickly 

 would the water be dissipated. The cause of the phenomenon 

 appears to be this. Water exhibits an attraction for the sur- 

 face of almost alt solids at low temperatures, and wets them. 

 Fluid mercury exhibits the opposite property, or a repulsion for 

 most surfaces. This attraction of water for surfaces brings it 

 into the closest contact with them, and greatly promotes the 

 communication of heat by a heated vessel to the water con- 

 tained in it. But heat appears to develope a repulsive power 

 in bodies, and it is probable that above a particular temperature 

 the heated metal no longer possesses this attraction for water. 

 The water, not being attracted to the surface of the hot metal, 

 and induced to spread over it, is not rapidly heated, and there- 

 fore boils off slowly. A rude method of judging of the degree 

 of heat is founded on the same principle, and is seen fa- 

 miliarly exemplified in the laundry. The heat of the smoothing 

 iron is judged of by its effects upon a drop of saliva let fail 

 upon it. If the drop do not boil, but run along the surface of 

 the metal, the iron is considered sufficiently hot ; but if the 

 drop adheres and is rapidly dissipated, the temperature is con- 

 sidered low. 



The temperature at which any liquid boils is not fixed (like 

 the melting point of solids), but depends entirely upon a parti- 

 cular circumstance, the degree of pressure to which the liquid 

 is at the time subject. Liquids are in general subject to the 

 pressure of the atmosphere ; for although the air is an exceed- 

 ingly light substance, being 815 times lighter than water, yet 

 by reason of its great quantity and height, it comes to weigh 

 with considerable force upon the earth. This is called the 

 atmospheric pressure, and amounts to no less than fifteen 

 pounds upon each square inch of surface. The force with 



