492 PROCEEDINGS OF THE AMERICAN ACADEMY 



If the glass contains different bases, a partial separation takes place ; 

 the silica divides itself between the bases, and thus forms compounds 

 of definite proportions which crystallize separately. In this case the 

 intimate mixture of the materials which constitute the true glass is 

 destroyed ; the glass becomes very hard, fibrous, opaque, much less 

 fusible, a better conductor of heat and electricity." 



" ' To sum up,' in the words of Dumas, to whom the editor is chiefly 

 indebted for what has been said on this interesting subject, ' devitrifica- 

 tion is a crystallization of glass, due to the formation of definite com- 

 pounds infusible at the temperature existing at the instant of devitrifica- 

 tion. Sometimes this infusibility is produced by the volatilization of 

 the alkaline base ; sometimes by a single division or separation, the 

 alkali then passing into that portion of the glass which preserves the 

 vitreous state. All glasses may therefore be devitrified, for all glasses 

 are capable of passing to the state of definite, and consequently crystal- 

 lizable, silicates. Glasses which contain at the same time indifferent 

 oxides and basic oxides will devitrify better than others, by the ten- 

 dency which the indifferent silicates and basic silicates have to combine 

 in definite proportions. Lastly, devitrified glasses will possess very 

 variable properties, for their composition itself will differ completely, 

 according to the nature of the glasses and the circumstances of devitri- 

 fication. Thus sometimes the devitrified glass will be crystallized in 

 needles of considerable size, as happens with window-glass ; sometimes 

 it will be crystallized in very fine needles, or even simply transformed 

 into an opaline mass, without appearance of crystals, as in the case of 



bottle-glass.' " 



Supplement. 



Since the preceding pages were prepared for publication, I have been 

 enabled, through the kindness of Dr. Gibbs and Prof. Cooke, to submit 

 Niobic Acid, Selenium, Tellurium, and Protoxide of Cerium to the 

 methods of blowpipe examination, of which I have just treated, and 

 with the following results. 



Niobic Acid. — Niobic Acid (Nbg O3), with borax, after fusion in 

 the outer flame, gives easily, either in the intermittent or fine-pointed 

 reducing flame, a whitish precipitate, tinged with blue, yellow, and vio- 

 let, and remarkably iridescent by transmitted as well as by reflected 

 light. I have observed no crystals in this precipitate ; but highly mag- 

 nified, it is resolved into minute spheroidal-shaped masses. The pre- 

 cipitate is the same in the reducing as in the intermittent flame, and 



