386 Scientific Proceedings, Royal Dublin Society. 
2) 
showed that his “ quartz fibres” retained any desired form (e.g. 
a coiled form when so held in a box) when raised above the 
melting point of copper (1050°), but refused to do so below that 
temperature. By increasing the distorting force Dr. Joly has 
lowered the temperature of permanent deformability to 800°. To 
my mind Professor Joly’s experiment simply proved that amor- 
phous silica is ductile at 800°. Ductile is, of course, only a relative 
term ; but I wish to imply that the phenomenon observed was one 
of ductility, and not one of melting in the usual sense. 
For practical purposes it is often convenient to observe the 
melting of a solid by means of optical tests, or by noticing the 
instant at which gravitational and surface tension forces belong- 
ing to the liquid state overcome the forces of rigidity which are 
characteristic of the solid state of aggregation. For rough prac- 
tical purposes, I have said, these are convenient and sufficiently 
accurate methods, but if carried to an extreme they may lead to 
serious error, and for accurate purposes they are wholly inade- 
quate. Dr. Carl Barus who has recently been awarded the Rum- 
ford medal by the American Academy of Arts and Sciences, for his 
researches on heat, and who has a good right to speak with 
authority,! says:—‘ My results have long since shown that in a 
comprehensive study of this question the crude optical and other 
methods hitherto used as criteria of fusion (criteria which have no 
inherent relation to the phenomenon to be observed) must be dis- 
earded. In their stead the striking volume changes which nearly 
always accompany change of physical state, in a definitely con- 
stituted simple substance, are to be employed.” Workers on the 
fusion of metals have, on the other hand, very generally employed 
the evolution of heat at the moment of consolidation as the cri- 
terion.2 This is exhibited on the cooling curves, so commonly 
drawn by metallurgists, by a horizontal line indicating a constant 
temperature fora certain interval of time. These two phenomena 
probably occur simultaneously ; as, for instance, was proved by 
1(. Barus, ‘‘ The Continuity of Solid and Liquid,’’ Amer. Jour. of Science 111.° 42 
(1891), p. 125. 
2 Gf. W. C. Roberts-Austen, Report to the Alloys Research Committee. Proc. 
Institution of Mechanical Engineers (1891), p. 569. 
