‘ THE FLOW OF SOLIDS. 239 
vessel to a horizontal surface. As true liquids we should then have 
such as alcohol, water, glycerine, molasses, etc. Viscous liquids require 
several seconds to fill their containing vessel to a level surface; thick 
tar is a good example. When the substance requires hours or even 
weeks in which to yield to gravity and change its form, I would eall it 
a viscous solid; paraffin, shoemaker’s-wax, and even lead and some 
other metals are such. A true solid retains its original shape indefi- 
nitely under ordinary conditions of pressure and temperature, as steel, 
glass, etc. Of course such a thing as an absolutely or perfectly rigid 
substance is as unknown to us as is an absolute or perfect fluid. 
If the above ideas are correct, *‘ true liquefaction” is the diminishing 
of the rigidity or viscosity of a substance until its molecules change 
their relative positions as easily as in a true liquid. 
I give these definitions merely that I may be understood in the use 
of these terms, and not because I think them new or especially good. 
in order that a substance may undergo a change in its chemical or 
crystalline character, it is undoubtedly necessary that it should be in 
the condition, at least, of a viscous solid, so that the molecules can 
slowly re-arrange themselves, if there be any force urging them thereto. 
Our question is, Will pressure alone impart to the molecules such a 
freedom of motion? <A priori it is inconceivable to me how or why it 
should. For with the exception of a few isolated substances at particu- 
lar temperatures—as water between 4° C. and ice at zero—an increase 
of liquidity or a diminution of rigidity is simultaneous with an increase 
of volume—that is, with an increase of the inter-molecular distances, 
which is accomplished by ** heating” the substance. In general, for one 
and the same substance over considerable ranges of condition, the 
rigidity diminishes as the inter-molecular distances increase. How then 
can pressing the molecules nearer together be expected to give them a 
property which always accompanies their separation ? 
THE APPARATUS. 
The first requisite tor the experiments was pressure, and naturally 
desiring the best machine, we were able, through the kindness of Gen. 
Benét, Chief of Ordnance, to have the use in its spare moments of the 
testing machine built by A. H. Emery for that department, and situated 
at Watertown, Mass. This machine undoubtedly enables the operator 
toobtain—measure—and manage—high pressures better than any other. 
Personally, I am greatly indebted to Capt. J. Pitman, of the Ord- 
nance Corps, for suggestions as well on the construction of the holders 
as on the theoretical points; and also to Mr. J. E. Howard, the engi- 
neer in charge of the testing machine, for his knowledge of the capacity 
of materials, and their best shape and quality to obtain the results 
desired. The apparatus was constructed by the American Tool and 
Machine Company of Boston, Mass. 
