510 
PHILOSOPHICAL SOCIETY OF WASHINGTON. 
The essential difference between a solid and a liquid is the relative ease 
of rearrangement of the molecules. In liquids the change is very easy; in 
solids, very difficult. Rigidity may briefly be defined as the difficulty of 
rearranging the molecules of the body in question. Can rigidity be re¬ 
duced by pressure ? A priori, it seems scarcely likely that forcing the 
molecules nearer together can give them greater freedom of motion. 
Generally rigidity is inversely as the intermodular distances. Ice is 
abnormal and cannot be taken as evidence pro or con. Lead, copper, and 
iron are all hardened by compression. All metals are harder, more rigid, 
in the rolled or hammered state than when cast or annealed. The rigidity 
of a steel pin was raised from 95,000 pounds to 110,000 pounds per square 
inch by that pressure. Two experiments bear directly upon the question 
and are convincing, although they gave unwelcome results. The first was 
made under the Ordnance Department, and will be found fully given in 
their report on “ Tests of Metals, etc., for 1884 -” A mixture of four parts 
wax and one part tallow was used as a “ straining liquid ” in “ tangential 
tests.” It was demonstrated that such a mixture would not transmit 
pressure through a hole inch in diameter and 2| inches long when the 
pressure at one end was 100,000 pounds per square inch and at the other 
30,000 pounds or less, whereas 2,000 pounds was sufficient to overcome 
all friction and force it through when there was no back pressure—that 
is, the wax and tallow were rigid enough under pressure to maintain a 
difference of 70,000 pounds per square inch (100,000 — 30,000) at the two 
ends of that hole. The second experiment was also made with the testing 
machine of the Ordnance Department at Watertown arsenal, Mass. (See 
Am. Jour. Sci. (3), XXXIY, 1887, p. 280.) In that experiment silver coins 
on top of beeswax and paraffine in the holder, instead of sinking through 
a liquid under 6,000 atmospheres, were pressed so hard against the top of 
the holder that their impression in the steel was easily seen and felt* 
The paraffine and wax were rigid enough to impress silver into steel. 
Such facts lead us to believe that pressure increases rigidity; and when 
we remember that the pressure at the center of the earth is millions of 
atmospheres, a demand for the rigidity of steel seems trifling. What is 
the rigidity of steel ? Simply a rigidity capable of resisting a deforming 
force of 80,000 to 100,000 pounds per square inch. But distinguished geol¬ 
ogists have made the fatal mistake of using “ rigidity of steel ” and “ ab¬ 
solute rigidity ” as synonymous and equivalent terms. Nothing is more 
misleading. 
Upheavals and depressions and other geological phenomena are most 
beautiful examples of the viscous flow of solids. The forces causing a 
glacier to flow are trifling as compared with those generated in the earth’s 
crust by shrinking, and undoubtedly to cause any body to flow we only 
need sufficient force and time. 
Can pressure impart to solids the ability to change crystallographically> 
mineralogically, or chemically ? Prismatic sulphur naturally changes to 
octahedral, and in many other cases changes take place under ordinary 
conditions of pressure and temperature. We would scarcely expect press- 
