8 6 
Dr. Young's Essay 
out of which it is raised ; and the same attraction must operate 
upon an unctuous fluid to cause it to spread on water, the 
fluidity of the water allowing this powerful agent to exert 
itself with an unresisted velocity. An oil which has thus 
been spread is afterwards collected, by some irregularity of 
attraction, into thin drops, which the slightest agitation again 
dissipates : their surface forms a very regular curve, which 
terminates abruptly in a surface perfectly horizontal : now it 
follows from the laws of hydrostatics, that the lower surface 
of these drops must constitute a curve, of which the extreme 
inclination to the horizon is to the inclination of the upper 
surface as the specific gravity of the oil to the difference be- 
tween its specific gravity and that of water: consequently 
since the contractile forces are held in equilibrium by a force 
which is perfectly horizontal, their magnitude must be in the 
ratio that has been already assigned ; and it may be assumed 
as consonant both to theory and to observation, that the con- 
tractile force of the common surface of two substances, is 
proportional, other things being equal, to the difference of 
their densities. Hence, in order to explain the experiments 
of Boyle on the effects of a combination of fluids in capillary 
tubes, or any other experiments of a similar nature, we have 
only to apply the law of an equable tension, of which the 
magnitude is determined by the difference of the attractive 
powers of the fluids. 
I shall reserve some further illustrations of this subject for 
a work which I have long been preparing for the press, and 
which I flatter myself will contain a clear and simple expla- 
nation of the most important parts of natural philosophy. I 
