GLASS — JACKSON. 247 
phoresceiice is free from anythinii: crystalline, since there may be 
crystalline structure in it Avhich is not in the sensitive state to be 
revealed by phosphorescence. It would take up too much time to 
elaborate this further, but o;enerally it may be said that a number of 
experiments on glasses, borates, etc., go to show that in some non- 
phosphorescent glasses there is most likely some crystalline forma- 
tion, since the introduction of minute amounts of certain bodies not 
usually present in these glasses, as manufactured, will render them 
quite markedly phosphorescent ; and again, the surfaces of Rupert's 
drops made from these sensitive glasses show no phosphorescence. 
Before going on to deal shortly with some points about devitrifica- 
tion, I may point out that boric anhydride, which has a marked effect 
in preventing crystallization in glasses and in enhancing the stability 
of the vitreous form, is a fatal ingredient to add to a uranium 
glass if strong fluorescence in ultra-violet light be aimed at. 
It may be concluded from Avhat has been said or suggested that 
the question whether glass is crystalline or not has a bearing on the 
problem of devising, manufacturing, and annealing optical glasses. 
It has, perhaps, a more obvious bearing on the problem of producing 
glasses capable of being freely worked in a furnace or in the flame of 
a blowpipe. To the flame worker especially, a glass prone to devitri- 
fication is a source of trouble. It would take at least a wlxole lecture 
to deal adequately with all the changes noticed in tlie numerous tAq^es 
of glasses which have been studied for their behavior in the flame. 
I will confine myself merely to mentioning that the segregation of 
less fusible vitreous bodies giving a kind of crinkled sldn to the glass, 
separation of amorphous silicates, the formation of very minute bub- 
bles giving a gra}' look to the glass, as well as true crystallization, 
are all frequently referred to as devitrification. It is mainly about 
the last form of it that there is time in this lecture for a few remarks. 
There is great variety in the behavior of glasses in a flame. A 
soda-lime silicate can be made which is hardly workable at all in the 
flame, it devitrifies so soon; but the same glass may be worked, if 
heated by radiation — for instance, in a muffle furnace. At the same 
time, it must be understood that exposure in the muffle may bring 
about devitrification even quickly if the temperature is such as to 
bring the glass to the right state of fluidity for rearrangement of some 
of its particles in the crystalline form. It would appear, therefore, 
that the difference between the behavior of the glass heated in the 
flame, and heated by radiation, may be explained by the difference 
in temperature reached by the glass in each case, and no doubt this 
is a most important part of the explanation. Recalling what was said, 
however, about the conditions for crystallization of vitreous bodies, 
there is, apart from temperature, the question of purity to be taken 
