Day and Shepherd— Lime-Silica Series of Minerals. 277 



A similar charge obtained by crystallizing the glass at 

 1200° (three days) also showed no residual glass under the mi- 

 croscope. Its density is given under the heading " tridymite 

 from glass." A second table contains confirmatory measure- 

 ments upon a second preparation heated to a slightly different 

 temperature. 



Tridymite. 

 (H 2 at 25° = 1) 

 First Preparation, Second Preparation. 



(1200°). (6 days at 1160°). 



From quartz. From glass. From quartz.* From glass. 



2325 2-316 2-327 2*319 



2-330 2-318 2-325 2*318 



2-325 2-316 2316 



2-319 



Mean, 2326 (25°) Mean, 2*317 (25°) Mean, 2*326 (25°) Mean, 2-318 (25°). 



"Whether the quartz had not completely changed to tridymite, 

 or the glass was incompletely crystallized, or perhaps both, is 

 of little moment. It is a very slow change and the agreement 

 between the values obtained by the two methods is sufficiently 

 good, when considered in the light of their identical optical 

 properties, to establish the absolute identity of the tridymite 

 formed from the glass and from the quartz crystals. 



The Lime-Silica Series. — Having determined the properties 

 of our two component minerals, we are prepared to enter upon 

 the study of their relation to each other in mixtures of various 

 proportions. It will be borne in mind that inasmuch as we found 

 no proper melting-point for pure silica on account of the inert- 

 ness (jf we may so describe it) with which it resists molecular 

 deorientation when heated, so compositions which are immedi- 

 ately adjacent to the silica end of the series may be expected 

 to show the same property and to yield but little information 

 from a direct application of the usual pyrometric methods. 

 Similarly, pure calcium oxide and its immediate neighbors are 

 well out of reach of accurate measurement by any existing 

 pyrometers. But even without these important measurements, 

 we have been able to obtain sufficient information in the more 

 inaccessible portions of the curve to enable us to describe all 

 the reactions involved with little probability of error. Inasmuch 

 as lime is one of the most refractory minerals known, it will 

 require no apology if w r e simply leave its thermal constants 

 until greater perfection in pyrometric measurements shall have 

 been attained. 



Preliminary Orientation. — Given chemically pure and well 



mixed (by grinding and repeated melting) preparations, it is 



* This preparation was afterward found to contain some unchanged quartz. 



