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THE POPULAR SCIENCE MONTHLY. 



out, and all the superior liquid will spread in an infinitely thin 

 layer over the other. This is what happens to a drop of oil when 

 it is thrown upon water. When a liquid is brought in contact 

 with a solid, as when a first drop of water is let fall upon a hori- 

 zontal plate of glass, the inclosing sac is flattened where it is in 

 touch with the glass, and bulges where it is in contact with the air. 

 The form of the sac and the angle of its junction with the glass 

 are determined by the fact that the two tensions of the envelope, 



Fig. 4.— An Ikon Ring having been plunged under Water, holding down the Cork to 



which it is attached. 



the upper and lower, should balance the traction of the exterior 

 glass upon the cordon separating them. In the case of a drop of 

 alcohol, the tensions being much weaker, can not resist the trac- 

 tion of the glass, and the liquid spreads out at once, as also hap- 

 pens with water when the plate has already been moistened. 

 Mercury opposes a very strong tension, and is hardly flattened at 

 all on striking the glass. A drop of water cast upon a hot plate 

 also exhibits a superior tension, and assumes the spheroidal state, 

 which was first analyzed in 1850 by M. Boutigny, of Evreux. He 

 said, " Bodies in a spheroidal state are bounded by a film of mat- 

 ter, the molecules of which are so connected that we can compare 

 them to a solid, transparent, very thin, very elastic envelope, 

 probably less dense than the rest, that protects the liquid within 

 it against any too considerable heating." 



This force of superficial tension exists and is manifest in all 

 liquids, but in different degrees. It is stronger in water than in 

 any other of the common liquids except mercury. Its value has 

 been measured, and is usually expressed, in milligrammes per milli- 



