18 



Day, etc. — Determination of Mineral 



values of apparent volume due to the fact that the gas escapes 

 from the quartz more rapidly than it can get out through the 

 graphite float-crucible. But above 1250°, when the evolution 

 of gas had practically ceased, there are a few points which indi- 

 cate that the volume either continues constant or goes on dimin- 

 ishing over the range from 950° to 1250°. About this time, 

 however, the formation of cristobalite becomes rapid and the 

 volume begins to increase greatly. 



The form of curve below 575° is not due to a time lag. No 

 effect due to the rate of heating was noticed, and the volumes 

 obtained on lowering the temperature were practically the same 

 as with rising temperature, except for a small increase (about 

 0*2 per cent) due to the formation of a few cracks at the inver- 

 sion temperature. Data with both rising and falling temper- 

 atures are included in fig. 6. 



Table III. 



Measurements made on Minas Geraes Quartz. 



Date 



Metal. 



Wt. of specimen 



Mark in Fig. 6 



9 January, 1911 

 18 

 21 



13 February " 

 21 •' *' 

 20 June 



Tin 



u 

 u 

 a 



Tin-lead eutectic 



43-98 

 same 

 34-12 

 35-02 

 39-96 

 85-18 



Circle 



Square 



Triangle 



Cross 



Inverted triangle 



Diamond 



The form of curve which we have found for the volume 

 increase up to 575° is somewhat different from that calculated 

 and published by LeChatelier.* His determinations below 575° 

 were at three temperatures only, and the volumes calculated 

 from his linear expansions at those temperatures fall almost 

 exactly on our curve. Above 575° his values for the volume 

 are too low, probably because his measurements were made 

 (photographically) on bars 100 mra long. With the ends exposed, 

 as they must have been, a considerable percentage of the length 

 may never have reached the transition temperature with its 

 accompanying large increase in length. 



The increasing dilatation of quartz as the temperature 

 approaches 575° is especially striking when compared with the 

 curves for various other properties. Among those properties 

 on which accurate measurements to 500° or higher have been 

 made are the following: linear expansion parallel to the axis, 



* LeChatelier, Compt. rend., cviii, 1046-1049, 1889. Bull. Soc. Fr. Min., 

 xiii, 112-118, 1890. 



