90 Inquiries into the Principles of Liquid Attraction. 



sidered an element ; but Chemists, by subjecting it to the test of a 

 rigid analysis, have ascertained its composition ; and it is by the help 

 of some appropriate experiments, that we must acquire a knowledge 

 of its attracting powers. - 



The practice of washing wheat, to free it from smut, has present- 

 ed, on this subject, some facts, worthy of attention. In the process 

 of washing, when the tub containing the wheal and water is inclined 

 for the purpose of pouring off the water, many of those grains which 

 are just above the water, float out and cover the surface, and re- 

 quire to be constantly beaten down to make them sink, and to prevent 

 them from running off with tlie water. These grains of wheat, com- 

 pared with the water, are specifically heavier, and yet they float, as 

 in the case of the globules of mercury, abovementioned ; not be- 

 cause, as some have supposed, they are specifically lighter than wa- 

 ter, but because the obtuse angles, which their convex surfaces make 

 with the surface of the water, cause a depression around them. 



Again, when tlie wheat is emptied by inverting the tub, some of 

 the grains usually adhere to the sides of the tub, by means of the 

 aqueous surface which surrounds them ; and thus they are suspended 

 by an attracting force, much greater than their own weight. But if 

 we again fill the tub with water, these grains of wheat sink as soon 

 as the water rises above them. This fact proves that the attracting 

 force which suspended them is destroyed, when the grains are sub- 

 merged. The needle, also, when wet, will adhere to the side of a 

 tumbler, above the water ; but only raise the water above the needle, 

 by inclining the tumbler, and the needle sinks. These facts evince 

 that the surface of the liquid is the seat of the attracting power. 



For the properties of the attracting power, we must look to the 

 soap bubble as it stands on a liquid, the bubble of pure water being 

 too evanescent for inspection. This soap bubble presents to us a 

 thin coat of particles, apparently very uniform in thickness, impervi- 

 ous to air, of uniform strength, and equally contractile in all its parts.* 



* By " contractile," is meant that constantly uniform strain, which may be ob- 

 served between the particles of the surface, and by which the surface has a tendency 

 to shorten itself. 



Thus, by way of illustration, when globules of mercury haA"e been flattened by 

 pressure, and have thus had their surfaces enlarged, their globular form is again re- 

 stored by the contracting power of the surface, as soon as the pressure is removed ; 

 and when drops of wafer, on cabbage leaves, have had their surfaces enlarged in the 

 game way, they will again resume their spherical form, for the same reason that the 

 globles of mercury did, on removing the pressure. 



