459 



cooled many degrees below their freezing-points and yet remain fluid. 

 Sulphur presents, in its viscid form, an approach to the glassy con- 

 dition ; but it may be obtained in the crystalline form on passing from 

 a state of fusion, and when cooled below freezing, instantaneously 

 crystallizes, like water, by mechanical disturbance. 



In phosphorus also there is the viscid state ; and when cooling after 

 fusion, it passes gradually, like glass, from the liquid to the solid con- 

 dition without crystallizing, though crystals are deposited from some 

 of its solutions. Selenium presents a state resembling the viscid 

 state of the preceding substances ; but when melted, and left to cool, 

 remains fluid below its melting-point, and solidifies very gradually in 

 its amorphous state (in which it has some of the characteristic pro- 

 perties of glass), and a thermometer immersed in it during the cool- 

 ing does not remain stationary at any point, or indicate any tempe- 

 rature at which heat is evolved by molecular change in the substance, 

 as if the selenium passed continuously from the liquid glassy state 

 to that of solid glass. At ordinary temperatures it retains this con- 

 dition for a long time as common glass does at higher, and as water 

 and sulphur will at lower temperatures; but when heated again, 

 between a certain temperature and its melting-point it becomes cry- 

 stalline and gives out great heat*. When glass is raised to a certain 

 temperature, and by its maintenence is preserved in a soft state, it 

 does the same. 



In sulphur, phosphorus, and selenium, therefore, the fluid state 

 below the temperature of solidification the intermediate condition 

 between fluidity and solidity the viscid state long retained the 



* These properties of selenium are here stated on the authority of Hittorff, 

 cited in Graham's ' Elements of Chemistry/ second edition, vol. ii. pp. 688, 689. 



The case of vanadic acid strongly resembles that of selenium, but extends this 

 series of concurrent phenomena to a range of temperatures nearly approaching 

 those which govern the molecular changes of glass. It fuses at a red heat, and 

 crystallizes on cooling, but femains fluid below its freezing-point. At the mo- 

 ment solidification commences, it again becomes red-hot, and remains so as long 

 as crystallization continues. 



The crystallization of glass, it has been seen, takes place at a high temperature, 

 from the ordinary state of solidity, heat being evolved. So the glassy variety of 

 gadolinite (like glass, a silicate with a compound base), when its temperature is 

 elevated above redness, remains solid, but evolves heat (becoming incandescent), 

 and crystallizes ; while the crystalline variety merely fuses and intumesces when 

 similarly treated. 



