238 THE FLOW OF SOLIDS. 
tion of a little higher pressure to accomplish the result even with the 
most refractory. Further than this, Mr. Spring has investigated the 
second corollary, and finds that chemical reaction takes place during 
this fusion; * at least when the volume of the products is less than that 
of the original substances. 
Unfortunately however for the conclusive character of Mr. Spring’s 
works, they have been seriously called into question, especially by Ch. 
Friedel.t Eduard Jannetazt repeated many of Spring’s experiments, 
and his results confirm Friedel’s criticisms rather than Spring’s conelu- 
sions, which he (Jannetaz) contradicts in every essential point. 
Such was practically the condition of the question two years ago, 
when the Director of the U.S. Geological Survey, J. W. Powell, re- 
quested me to devote my time and thoughts to what we hoped would 
be its final settlement. § 
It would be unjust to leave unmentioned here the elaborate and ex- 
haustive series of experiments made by Henri Tresea || on “the Flow 
of Solids,” which are fundamental as regards the point investigated, 
which however is but a small part of the general question. 
In order that my meaning may be clear, I wish for myself and for 
this paper to impress certain meanings upon certain terms or words. 
Primarily, I wish strongly to distinguish between causing a body to 
“flow” and rendering it a true liquid. Any substance may “flow” 
when the force acting to cause the molecules to change their relative 
positions is greater than the force with which the molecules are held in 
their original positions; 7. ¢., is greater than the rigidity or viscosity of 
the substance. This can occur from two causes,—an increase of the 
force tending to disturb the molecules, or a diminution of the resisting 
power, the rigidity of the material. The first cause may take the form 
of pressure, strain, or such like force; the second cause is heat, and 
possibly other agencies. Whether rupture or flow takes place when 
the deforming overcomes the resisting force depends upon the nature 
of the substance, its limiting conditions, and the time allowed for the aec- 
complishment of the motion. 
It is impossible to draw a sharp line between “ liquids” and “ solids;” 
for convenience they may well be classed as true liquids, viscous 
liquids, viscous solids, true solids. In the first class would fall 
such substances as, in a small fraction of a second, fill their containing 
Bull. de la Soc. Chim. de Puris, 1883, vol. xxx1x, and 1886, vol. XLvt. 
+ Ch. Friedel, Bull. de la Soc. Chim. de Paris, 1883, vol. XXxXIx, p. 626. 
t{ Ed. Jannetaz, Bull. dela Soe. Chim. de Paris, 1884, vol. xi; Bull.de la Soc. Minéral. 
de France, 1885, vol. v111, p. 168. 
§ This paper is essentially taken trom a report made to Maj. Powell, dated at Wa- 
tertown, Mass., September, 1885. 
|| Henri Tresca, Wém. de U Inst. Savantes Etrangers, 1868,vol. XV1It. Comptes Rendus, 
1868, vol. LXvVI; 1869, vol. Lxvitt. See Tresca, in Bibliography, at end of article. 
