242 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
the glass and metal joint cooled, the glass would flow and follow 
the contraction of the metal and so the stress would, to some extent, 
be relieved. And this is an example of the need of asking the 
question: Is glass truly a solid, and how far can a glass be made 
which will flow, very much less, of course, than pitch, but in an 
analogous manner? The other bearing of the question is on the 
cracking of glass vessels with rapid changes of temperature. It is 
clear that if the strain set up by such changes can be quickly and 
readily released, the danger of cracking will be very small. The 
coefficient of expansion is, of course, a very important factor in this 
respect. It is well known that vitreous silica vessels can be heated 
to redness and plunged into cold water without cracking, and this is, 
no doubt, correctly considered to be due almost entirely to the low 
coefficient of expansion of silica. In the case of glasses also, the 
lower the coefficient of expansion of a glass the better will that glass 
stand rapid changes of temperature; but it is possible to make 
glasses approximately with the same coefficient of expansion and to 
find one — and that, perhaps, the one with a slightly higher coefficient 
of expansion — which will not crack under conditions in which the 
other cracks readily, and a study of the two glasses shows that the 
more stable one is the more plastic. On the whole, perhaps, it is not 
too much to say that glass may be looked upon as an extremely 
viscous liquid so slow in its movements in some tjqoes of glass that 
ages might elapse before any marked change in form could be 
observed under a strain just short of a breaking one for the glass, 
while in others it is possible to show that the glass does flow, even 
at the ordinary temperature, to a small extent in a relatively short 
time. 
Tlie next problem to be considered may be also put in the form of 
the question: Is glass truly amorphous or vitreous or has it any 
crystalline structure or tendency to crystalline structure? There 
are many substances which can be obtained in the vitreous state 
and also in the crystalline state, and which can be changed from one 
to the other. As one example, arsenic trioxide may be mentioned. 
It can be produced as a clear transparent glass which slowly changes 
at the ordinary temperature into an opaque white substance re- 
sembling porcelain in appearance. The opaque white substance is 
crystalline. It is the crystalline variety into which the vitreous will 
be slowly but completely changed. Again, if sulphur near its 
boiling point be poured in a thin stream into cold water it sets as 
plastic threads, and for our purposes we may speak of this as the 
vitreous form of sulphur ; left at the ordinary temperature, it slowly 
changes into the crystalline form. 
Two other examples may be given which, in a way, are perhaps more 
closely analogous with glass, since the^^ are solutions of substances, 
