EARTHS. 283 



very beautiful. The glass, after being annealed, is cut up into squares 

 by a diamond ; the centre piece by which the wheel was supported, 

 is called the bull's eye, and is often seen in entry windows. Broad 

 glass is blown into a conical form ; cracked longitudinally while hot, 

 by touching it with a cold wet iron, and it is then spread out on a ta- 

 ble, whence its name ; it is afterwards annealed and cut. 



The annealing of glass, which means the cooling of it, very slow- 

 ly, in a peculiar kind of furnace, is important to prevent its crack- 

 ing by slight movements, or jars, or variations of temperature. 



Prince Rupert's drops are made by pouring melted green glass 

 into water, when the portions assume a tadpole shape ; they will bear 

 the moderate blow of a hammer, if lying on a smooth table, but if 

 the point is broken off, they explode into a thousand pieces. That 

 this peculiarity depends on an unequal contraction produced by sud- 

 den cooling, is evident, because if the drops are gradually heated red 

 hot, and gradually cooled, they will no longer fly on having the 

 point broken. 



The Bologna vial is blown with a thick bottom, but is cooled in 

 the air, without being annealed ; it will bear to be struck upon a table 

 with some force, but if a fragment of glass or sand be dropped into it, 

 it flies to pieces, and frequently it does so by slight changes of tem- 

 perature ; even, as I have observed, by the warmth of the hands. 

 Cups of green glass, unannealed, have been made three inches thick 

 at bottom, which were not broken by a musket ball falling from a 

 considerable height, but were shivered, by a piece of flint of two 

 grains weight falling into them. 



SEC. VI. ALUMINA. 



1. NAME. From alumen, the latin of alum, which has this earth 

 for its basis ; called also the argillaceous earth. Indicated by Geof- 

 froy, in 1727, established by MargrafF, of Berlin, 1756. Formerly 

 called argil, because it was the basis of clays. 



2. PREPARATION. 



(a.) To a solution of alum,* in 20 parts of water, add liquid ammo- 

 nia till precipitation ceases : or, precipitate by bicarbonate of potash ; 

 as a little sulphuric acid is apt to adhere, it may be re-dissolved in 

 nitric acid, and the solution tried for sulphuric acid, by nitrate of ba- 

 rytes ; when there is no farther milkiness, it may again be precipi- 

 tated by the above reagents, or the nitrate may be decomposed by 



heat.f 



(0.) Or, alum purified from iron, by repeated crystallizations, is 

 dissolved in 4 or 5 parts of water, at 212 ; add carbonate of potash 



* Alum is apt to contain iron, which will remain when the salt is decomposed, and 

 the earth dissolved by potassa ; or, if dissolved, it will, after a few hours, precipitate 

 in brown flocks. 



t Ann. de Chim. XXXII, p. 64. 



