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



the standard curve. Our temperatures were measured in 

 terms of this empirical curve. Since differences of less than 

 live degrees can hardly be distinguished by this apparatus at 

 such high temperatures, the assumption here made that the 

 radiation from iridium is of uniform quality throughout the 

 region between 1600° and 2100°, will hardly be called in ques- 

 tion. After the temperature scale was established in this way 

 the iridium fragment was laid on the top of each charge dur- 

 ing the measurement and all determinations were made in 

 terms of the radiation from it. 



With this pyrometric apparatus we undertook to examine all 

 the mixtures within reach from lime to silica which the. micro- 

 scopic study had shown to be important. The mixtures were 

 first examined in charges of 100 grams between 500° and 1600° 

 in a furnace of the type described by Day and Allen (loo. cit.) y 

 supplemented where necessary by observations at higher tem- 

 peratures in the iridium furnace with the optical pyrometer. 

 The relative sensitiveness of the two methods is roughly one 

 to ten, i. e., the smallest temperature changes which the opti- 

 cal pyrometer can detect are fully ten times as great as those 

 which can be readily measured with the thermoelement. Fur- 

 thermore, the optical pyrometer is merely a device for estima- 

 ting temperatures by observation from without the furnace. 

 It enables an observer to heat a charge to a certain tempera- 

 ture but not to tell whether anything takes place there except 

 by removing the charge from the furnace for examination. 

 It is not adapted to the determination of thermal constants by 

 direct observation except in the case of the melting tempera- 

 ture of pure compounds or eutectic mixtures which melt 

 sharply into a thin liquid. 



It has been our universal experience that changes of state 

 which are subject to lag are much more easily and certainly 

 determined from heating than from cooling curves. Under- 

 cooling in these ultra-viscous media cannot be prevented with 

 certainty by any of the usual methods. Superheating is also 

 unavoidable at times, as we have already noted in the case of 

 quartz and elsewhere. In these cases the change of state 

 required by conventional definitions simply cannot be deter- 

 mined as a point. Where the inertness is not so great as to be 

 prohibitive of all measurement, our uniform experience has 

 been that the melting temperature can often be established 

 with confidence, where solidifying temperatures cannot. 

 Similarly, inversions in the solid state can usually be fixed 

 upon a curve of rising temperatures with greater certainty 

 than upon a cooling curve. 



The temperature constants. — We have now definitely located 

 the compounds of lime and silica which can exist between the 



