Chemical-Apparatus. 105 



The effect of making them thin is, that their sur- 

 faces are more uniformly heated, that is, one 

 surface is not heated before the other, and con- 

 sequently they bear that disproportion of their 

 form which heat occasions. They are more 

 flexible too ; and a spherical form contributes to 

 this effect. White glass best resists sudden 

 changes of temperature, and common green glass 

 the higher degrees of heat in the sand furnace. 

 One circumstance in particular to be regarded 

 is the annealing of the glass. This is done by 

 putting the vessels, while red-hot, into a furnace 

 of nearly the same temperature, and letting them 

 cool gradually ; for if hastily cooled they are re- 

 markably brittle. This phenomenon is hard to 

 be accounted for ; but we have a strong instance 

 of it in what are called glass tears, or Prince 

 Rupert's drops, which is melted glass dropped 

 into cold water. The greatest number of these 

 drops falling into atoms, those that remain are in 

 the form of a pear, with a tail not thicker than a 

 thread. If we break a little bit of the tail, the 

 whole flies into powder with great explosion, and 

 they sometimes fly without any thing that we can 

 perceive disturbing them. A certain thickness 

 of the glass is crusted over : and this crust 

 being penetrated, the whole flies asunder ; yet 

 when the thick end is ground till a certain 

 point is worn off, the explosion does not take 

 place : this is a fact that cannot be explained till 

 we understand better the nature of cohesion. 



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