106 Chemistry. [Lecture 28. 



It is, however, somewhat analogous to the 

 sudden crystallization of salts by agitation. The 

 contraction necessary for developing the matter 

 of heat is prevented in the first instance by the 

 sudden formation of a solid crust. On the latter 

 being split in several parts at once, the instanta- 

 neous mutual contact of the internal parts, and 

 consequent liberation of the latent heat, perhaps, 

 causes an explosion. 



%. Metal vessels want transparency, yet their 

 use is indispensable. Iron serves for sand heats, 

 distilling pots, and other purposes requiring 

 strength and durability; copper, for the more 

 bulky utensils employed in boiling and distilling. 

 Condensing worms and dyers' kettles are made 

 of tin a little hardened by copper ; lead, ham- 

 mered out from a thick plate, makes good boilers 

 for such acid and saline liquors as would corrode 

 iron. It is also much used for lining wooden 

 backs to perform crystallization in the great 

 way. 



But the experimental chemist requires cruci- 

 bles and evaporating vessels of pure silver and 

 platinum ; the former for the caustic alkalies, 

 which have no sensible action on silver, though 

 they attack almost every thing but that and gold. 

 Platinum makes almost an universal implement, 

 resisting the greatest heat of a charcoal fire, and 

 the action of all the simple acids, but not of ni- 

 tro-muriatic acid and the alkalies. Crucibles of 

 these metals, being fitted with an alembic head 



