of the glass becomes more or less attacked. The nature of 

 the attack, however, varies considerably in different glasses ; 

 some develop a general opacity of surface (a bad form of 

 attack), others show peculiar markings or exhibit tarnishing 

 or iridescence. The exact interpretation of the " autoclave " 

 test is, therefore, itself uncertain ; but treatment of glasses in 

 this way does undoubtedly give valuable comparative infor- 

 mation and will certainly indicate if a glass is very unsuitable 

 to withstand the attack of moisture. 



Some glasses, whilst very resistant in the " autoclave " 

 test, may be particularly susceptible to attack by special 

 re-agents. Thus specially dense flints are rapidly tarnished 

 by exposure to hydrogen sulphide, and extreme barium 

 crowns are appreciably soluble in strong acids and should 

 not be cleaned by such re-agents. 



