240 ANI^UAL REPORT SMITHSONIAN INSTITUTION, 1919. 
glasses are silica, boron trioxide, arsenic trioxide, and pentoxide, 
phosphorus pentoxide, tin oxide, and antimony pentoxide. The chief 
bases are the oxides of potassium, sodium, lithium, barium, calcium, 
magnesium, zinc, and lead. Aluminium oxide, a constituent of sev- 
eral glasses may, in some, play an acid part and in otliers may act 
as a base. With certain reservations, this may be said also of anti- 
mony trioxide. The list is not exhaustive, but is sufficient to indicate 
that a number of glasses is possible from various combinations of 
these acids and bases. (The materials used for producing opals and 
colored glasses will be referred to later.) The number of glasses made 
is very large, and it would take at least all the time at my disposal 
to describe in any adequate manner how they differ from one another 
in composition and in those properties which malie each one suitable 
for the purpose for which it was devised. It may, however, be ap- 
propriate here to mention that in the great variety of optical glasses, 
there are many which do not differ materially in composition from 
glasses used for other purposes. For example, a good window glass 
could be made with pure materials and stirred in the process of its 
manufacture, so as to secure such a clear and homogeneous product 
as would serve as one type of optical glass. The chief general prop- 
erties which are desirable in all optical glasses are identity of com- 
position throughout the whole mass of the glass, great clearness and 
transparency for all the colors of the spectrum^ freedom from strain 
arising through imperfect annealing, and durability under ordinary 
exposure to the atmosphere. It will be seen, therefore, that apart 
from considerations of special optical properties, I refer to the re- 
fractive indices and dispersive powers of various optical glasses ; the 
main difference between them and other glasses is that the highest 
art of the glass manufacture is called for in their production, and 
great care is needed to insure purity of the materials used and 
accuracy of proportions, so as not only to be able to produce glass 
of the optical properties required, but to reprodvice it with the closest 
possible identity of composition. With these very brief remarks on 
glass generally, we may turn now to some of the problems which I 
thought might be interesting to consider, and the first one is how 
far can glass be called a solid? 
' A solid is defined in a dictionary as having a fixed form and being 
in a state in which the component parts do not tend to move freely 
among themselves. With regard to glass, we may ask for how long 
is it fixed in form, and what are the limitations of freedom of move- 
ment which we ought to consider? It is a common experience that 
long straight pieces of glass rod or tubing, left supported so that 
their own weight tends to bend them, will bend in the course of time, 
and in some years will become definitely bowed. Varieties of glasses 
