248 A TREATISE ON METAMORPHISM. 



In glass found in the lake at Walton Hall, near Wakefield, Bingley" found 

 that the alkalies had been wholly removed by decay. Another case of 

 devitrification largely clue to original state of strain is the glass of certain 

 old buildings, such as cathedrals. A well-known instance is that of St. 

 Andrew's Chapter House. 6 



Rate of devitrification. — The rate of devitrification of glass depends, among 

 other things, upon (1) composition, (2) strain or lack of strain, (3) pressure, 

 (4) mass-mechanical action, (5) temperature, (6) moisture. 



In any given case of devitrification several and sometimes all of these 

 factors enter, and hence it is impossible to discriminate the effect of each. 

 Very often devitrification has been described as hydro-metamorphism, but 

 by this no more can be meant than that water is usually an important 

 factor in the process. 



(1) The rate of devitrification of glass increases with its basicity. This 

 follows from the ready solubility of basic glasses. It has also been deter- 

 mined that glasses rich in soda devitrify faster than those rich in potash. 

 This corresponds with the fact emphasized in another place (see p. 516) 

 that minerals rich in soda are more readily decomposed than those rich 

 in potash. 



(2) It is shown in another place that a state of strain in minerals 

 promotes alteration. (See pp. 95-98.) The same is true of glass. It is 

 definitely known that unannealed glass, which therefore cooled irregularly 

 and is in a state of strain, independently of pressure or movement may 

 partly devitrify in a few years. For instance, drawn-glass tubing, such as 

 is used in the chemical laboratory, if kept for a few years may devitrify so 

 as to become useless. Another well-known case of devitrification probably 

 due to strain is the glass of certain cathedral windows. As large masses 

 of glass cool under natural conditions, they must often be almost at the 

 extreme of the unannealed condition, and therefore in a high state of 

 strain. So far as glass is in this condition, even without reference to any 

 extraneous pressure or movement, there is a marked tendency toward 

 devitrification. The stage of the process due to this cause is dependent 

 upon the amount of strain and the time. 



«Bingley, C. W., On the peculiar action of mud and water on glass, as more especially illustrated" 

 by some specimens of glass found in the lake at Walton Hall, near Wakefield: Kept. Twenty-eighth 

 Meeting British Assoc. Adv. Sci., London, 1859, pp. 45-46. 



b Brewster, Sir David, On the decomposition of glass: Rept. Tenth Meeting British Assoc. Adv. 

 Sci., London, 1841, pp. 5-7. 



