9 
implies that the density and pressure admit of variation, and 
consequently violates the second of the regulative principles 
stated in sec. 9, according to which a primary hypothesis must 
contain nothing expressive of variability. An explicit answer 
may be given to this objection. The two hypotheses which 
precede the third affirm conjointly that all matter consists of 
an aggregation of inert spherical atoms, of invariable magni- 
tude, susceptible of motion, and separated from each other by 
intervening spaces. Hence it is a direct inference from the 
antecedent hypotheses, and not a new hypothesis, to say that 
matter may be conceived of as composed of atoms in different 
degrees of aggregation, or that the number of atoms of a given 
substance in space of given dimensions may be different at 
different times and different positions. This inference may 
consequently be logically employed in the enunciation of the 
third hypothesis, which assumes not only that the ether presses, 
but that the pressure is always and everywhere in exact pro- 
portion to its atomic density. This last assertion is certainly a 
primary hypothesis, and as such is required to be conformable 
to the same regulative principles as the other primary hypo- 
theses. Now the mathematical expression of this hypothesis 
affirms that there exists, under all circumstances of the motion 
and density, an invariable numerical quantity by which, if the 
variable numerical quantity expressing the atomic density be 
multiplied, the product is the numerical quantity expressing 
the pressure. Hence as the quality of pressure, and the vari- 
ability of atomic density, may, from what is said above, be 
legitimately assumed in stating the third hypothesis, it follows 
that this hypothesis only postulates the existence of that 
invariable factor, and consequently, as being also expressed in 
intelligible terms, it may be pronounced to be conformable to 
rule. Mathematical investigation founded on the hypothesis 
has shown that the constant factor signifies that the fluid is 
endowed with a constant intrinsic elasticity, in virtue of which 
it has the property of pressing, and also of propagating the 
effect of any agitation produced in it at a constant rate 
through space. 
14. In addition to being conformable to the rules above 
indicated, the hypotheses are required to satisfy the condition 
of giving the means of instituting theoretical calculation, by 
the results of which, compared with observation and experi- 
ment, their truth may be tested. Their applicability and 
sufficiency for this purpose in all the different departments 
of physics will accordingly have to come under consideration 
in the sequel of our argument. 
15. Before proceeding farther in the general argument, it 
