Mr. Vince's Observations on the Theory , &c. 25 
with their nature. If we suppose their particles to be in con- 
tact, it is extremely difficult to conceive how they can move 
amongst each other with such extreme facility, and produce 
effects in directions opposite to the impressed force without 
any sensible loss of motion. To account for this, the particles 
are supposed to be perfectly smooth and spherical. If we 
were to admit this supposition, it would yet remain to be 
proved how this would solve all the phenomena, for it is by 
no means self-evident that it would. If the particles be not in 
contact, they must be kept at a distance by some repulsive 
power. But it is manifest that these particles attract each 
other, from the drops of all perfect liquids affecting to form 
themselves into spheres We must therefore admit in this 
case both powers, and that where one power ends the other 
begins, agreeable to Sir Isaac Newton's* idea of what takes 
place not only in respect to the constituent particles of bodies, 
but to the bodies themselves. The incompressibility of li- 
quids (for I know no decisive experiments which have proved 
them to be compressible) seems most to favour the former sup- 
position, unless we admit, in the latter hypothesis, that the re- 
pulsive force is greater than any human power which can be 
applied. The expansion of water by heat, and the possibility 
of actually converting it into two permanently elastic fluids, 
according to some late experiments, seem to prove that a re- 
pulsive power exists between the particles ; for it is hard to 
conceive that heat can actually create any such new powers, 
or that it can of itself produce any such effects. All these un- 
certainties respecting the constitution of fluids must render the 
conclusions deduced from any theory subject to considerable 
* See his Optics, Que. 31. 
E 
MDCCXCV. 
