564 



1400 C. Therefore if is neither possible to indicate an "inversion- 

 temperature", below whieh the «-form, and above which the /?-form 

 would represent the more stable phase: at all temperatures below 

 its meltingpoint, the /?-spodnniene is the only stable form of the 

 compound LiAlSl^Of^. Under what conditions the «-form was always 

 o-enerated in nature, while it was till now^ never obtained in the 

 laboratory from "dry" molten mixtures, may preliminarily be put 

 aside. 



The reason however, that the «-modification, once produced, has 

 remained so, notwithstanding its metastability with respect to the 

 /3-form, is to be ascribed to the enormous sloiimess, with which the 

 transformation a-^ (3 takes place. 



§ 17. To give an idea of this phenomenon, we will describe here 

 a series of experiments, made with the purpose to answer the 

 question, at what lowest temperature the transformation «— >/5- 

 form again will occur with a velocity just observable? Preliminary') 

 experiments had taught us that a long and little prominent heat-etfect 

 was observed between 900 and 1000° C, if a larger quantitj^ of 

 finely powdered «-spodumene was gradually heated; and the micros- 

 copical investigation also taught us soon, that within the mentioned 

 temperature-interval, a transformation is going on with observable 

 velocity. We therefore made the following series of experiments by 

 means of the already mentioned statical method. For it is evident, 

 that just, with reactions proceeding so enormously slowly, ^/m* method 

 can be used with great success, because // permits us to keep the 

 studied sid)stances at a constant temperature during an arbitrarily 

 lowj time; in this wa}^ one can be sure that the reaction is thus 

 completely finished, while the sudden chilling of the preparation in 

 cold mercurj' will fix the monienlaneous state of it in a most 

 effective way. 



The following data were obtained by observations with the thermo- 

 element IV; because the thermoelement was not placed m the mass, 

 but beside it, the whole furnace-system needed to be especially 

 calibrated for this series of experiments. 



The calibration of the used quenching-system was executed by 

 means of meltingpoint-delerminations after the statical method, with 

 substances, whose meltingpoints in terms of the gasthermometer were 



') Vide also : G. Tammann, KrystallisierenundSchmel/en, p, 114. Spodumene (f7=3,17^ 

 was tiansfonned ^i-adimlly into a much less dense substance ((7=2,94), by heating 

 on a Bunse}i\mvuGv during ten iiouts. The new product was attacked nmch more 

 rapidly by UF llian liie original substance. 



