If a glass worker, in making an article of glass, desires to detach or cut apart certain sections, he applies a cold wet substance, such as an iron file wetted with water, to any portion of the hot glass, which causes it to fracture at the point of contact with the cold metal, and a slight jar is then sufficient to break the two portions apart. This method of chilling heated glassware to divide it is applied in the mechanical process of cutting up the long cylindrical tubes of glass into short sections for use as miners’ safety lamp chimneys. Wherever it is desired to cut them through, a narrow section or line round the cylinder is first heated by a sharp, hot pencil of flame projected from a burner against the rotating cylindrical tube of glass at equidistant short sections, and the divisions chilled by contact with a cold, steel point, or the heated area may be gently scratched with a diamond point, when a clean, sharp fracture results exactly where the chill or scratch has been applied and spreads round the whole circumference in a circle, giving neat, clean-cut divisions. In cutting narrow tube and cane, the fracture caused in the structure of the glass by scratching its surface with a steel file or diamond is sufficient to cause it to break apart without the application of heat. A piece of hot glass will weld on to another piece of hot glass of similar composition. The glass maker uses this method of welding for sticking handles on to jugs, etc., during the process of making table glassware. The density of glass varies according to its composition. Certain classes of lead and thallium glass for optical work are of very high density. The specific gravities of such glasses may vary from 3·0 to well over 4·0. In soda-lime glasses the density is less and approaches 2·4. Ordinary crystal glass approximates to a specific gravity of 3·1. The elasticity and thermal coefficient of expansion of glass can be regulated within normal limits. Glasses are now manufactured which can be perfectly sealed to copper, iron, nickel, and platinum wires. Glass, if kept heated for any length of time at a temperature just short of its softening or deformation point, becomes devitrified and loses its transparency, becoming opaque and crystalline. In this state it has much of the nature of vitreous porcelain and is totally different to manipulate, being tough and viscid on further heating. This devitrified state may occur during glassmaking, where the metal is allowed to remain in the pot or tank furnace for a considerable time under low temperature. Small stars or crystals first develop throughout the glass and continue to grow until it becomes a stony, white, opaque, vitreous mass. “Réaumur’s Porcelain” is a glass in a devitrified state, and is used for pestles and mortars, devitrified glass being less brittle than ordinary glass and similar to vitrified porcelain.