EFFECTS OF EXPANSION. 773 



other. When cold water is poured into a hot glass 

 vessel the rapidly cooled superficial portion contracts 

 more than that beneath the surface ; hence the former 

 cracks. Glass vessels with thick sides are more apt to 

 crack than those with very thin sides ; in the former 

 case it takes much longer before heat passes to or awny 

 from the inner portion than in the case of vessels with 

 thin sides, and these are besides more yielding than 

 thick vessels, and therefore less liable to break if both 

 kinds undergo like changes of form. 



Utensils of glass, especially those with thick sides, 

 after being formed from the molten material, require 

 to be cooled very slowly, otherwise they become ex- 

 tremely brittle and comparatively useless. The reason 

 of this is not yet quite exactly known, but it is probable 

 that during rapid cooling sufficient time is not given to 

 the particles to assume that uniform aggregation which 

 is a condition of their stable position; some neighbour- 

 ing molecules may possibly be farther apart from each 

 other than others, and hence a state of great constraint 

 may exist among the molecules, so that a comparatively 

 slight external impulse causes a sudden disruption of 

 the whole. 



Prince Rupert's Drops (or ' Dutch tears ') and 

 Bolognian Flasks are interesting examples of these facts. 

 The former (fig. 384 A) are large drops of fused glass 

 which have been let fall into a vessel of cold water. 

 Many of these drops break to pieces immediately when 

 they touch the water, but those which remain entire 

 exhibit considerable strength. When placed upon a 

 piece of wood they will bear pretty smart blows with a 



