LETTER OF J. HENRY TO REV. S. B. DOD. 161 



This class of investigation also included the study of soap bub- 

 bles, and the establishment of the fact of the contractile power of 

 these films. The curvature of the surface of a bubble tends to 

 urge each particle toward the center with a force inversely as the 

 diameter. ' Two bubbles being connected, the smaller will collapse 

 by expelling its contents into the larger. By employing frames of 

 wire, soap bubbles were also made to assume various forms, by 

 which capillarity and other phenomena were illustrated. This 

 subject was afterward taken up by PLATEAU, of Ghent. Another 

 part of the same investigation was the study of the spreading of oil 

 on water, the phenomenon being referred to the fact that the attrac- 

 tion of water for water is greater than that of oil for oil, while the 

 attraction of the molecules of oil for each other is less than the 

 attraction of the same molecules for water; hence the oil spreads 

 over the water. This is shown from the fact that when a rupture 

 is made in a liquid compound, consisting of a stratum of oil resting 

 on water, the rupture takes place in the oil, and not between the oil 

 and water. The very small distance at which the attraction takes 

 place is exhibited by placing a single drop of oil on a surface of 

 water of a considerable extent, when it will diffuse itself over the 

 whole surface. If however a second drop be placed upon the 

 same surface, it will retain its globular form. 



XVI. Another contribution to science had reference to the origin 

 of mechanical power and the nature of vital force. Mechanical 

 power is defined to be that which is capable of overcoming resist- 

 ance; or in the language of the engineer, that which is employed 

 to do work. 



If we examine attentively the condition of the crust of the earth, 

 we find it, as a general rule, in a state of permanent equilibrium. 

 All the substances which constitute the material of the crust, such 

 as acids and bases, with the exception of the indefinitely thin pellicle 

 of vegetable and animal matter which exists at its surface, have 

 gone into a state of permanent combination, the whole being in the 

 condition of the burnt slag of a furnace, entirely inert, and capable 

 in itself of no change. All the changes which we observe on the 

 surface of the globe may be referred to action from without, from 

 celestial space, 

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