234 PROCEEDINGS OF SOCIETIES. 
supplementing the exhibition of Opercularia at the December 
meeting. 
Mr. Tichbourne exhibited a section of devitrified glass. The 
specimen of glass from which it was taken was procured from 
Messrs. Chance Bros. & Co., of Birmingham. Mr. Tichbourne 
remarked that it was a well-known fact that when glass is slowly 
cooled that portions become very infusible and opaque. This 
devitrification must be carefully distinguished from a crystalliza- 
tion that sometimes proceeds from over-heating the glass: in this 
latter case certain of the alkaline bases are volatilized, and 
crystals are found disseminated through the mass having a 
different composition to the vitreous matter. Dumas’ definition 
of this phenomenon is as follows :—* Devitrification is a erystalli- 
zation of glass due to the formation of definite compounds infus- 
ible at the temperature existent at the instant of devitrification.” 
This definition is more applicable, in Mr. Tichbourne’s opinion, to 
the specimens produced by over-heating, than to the one which 
was exhibited upon the table, which might be looked upon as 
proceeding from a physical change. In the specimen from which 
the section had been made it would be perceived that the devitri- 
fied portion had assumed the appearance of little masses about 
the size of small peppercorns, which were perfectly opaque. 
Each mass is detached from the vitreous portion, the fracture in 
most cases not entirely surrounding the crystalline sphere, and 
consist of vacuous spaces. As might be inferred from this fact, 
the density of the devitrified variety is greater than the vitrified. 
Although it was difficult to separate the two portions perfectly, 
the following were the results, in Mr. Tichbourne’s hand: 
Vitreous portion . . . 2°435 sp. gr. 
Devitreous portion. . . 2°460 
3) 
This increase in density is observed in all substances of a 
vitreous nature upon their assuming the crystalline form (arsen- 
ious acid, for instance). If the section is made so that it cuts 
through the axis of one of the warty masses, it will be seen that 
the crystals become more regular and larger towards the centre, 
and that therefore the interior becomes perfectly transparent, 
although it retained the crystalline character: the opacity of the 
external part being due to the smallest irregularity of the crystals. 
The crystallization was beautifully seen in the centres of some of 
these masses, forming fine microscopic objects. 
Mr. Crowe showed Raphides from Shallot. 
Mr. Archer showed a form of yolvocinaceous plant, which a 
priort might be looked upon as an undescribed species of Gonium. 
He had found this before, in May last, on the occasion of the 
excursion of the Club to the Vale of Ovoca, and, again, in a 
gathering made by Dr. Barker, and subsequently by himself in 
the Phenix Park. This little organism seemed to differ from 
