GEOPHYSICAL LABORATORY. lOj 



x\s the matter now stands, therefore, we have succeeded in perfecting the 

 constant vokime gas thermometer until the aggregate error affecting the 

 measurements between 300° and 1,150° appears not to be greater then 0.5 , 

 but we are not yet able to offer adequate assurance that our scale can be 

 reproduced by another with this accuracy. This matter will receive further 

 attention in a later paper. 



(8) A telemeter with micrometer screw adjustment. Fred. Eugene Wright. Amer. 



Jour. Sci. (4), 26, p. 531, 1908. 



A description of a convenient instrument for the measurement of distances 

 in geological field work. 



(9) A device to aid in the explanation of interference phenomena. Fred. Eugene 



Wright. Amer. Jour. Sci. (4), 26, p. 536, 1908. 



A description of an instructive model for the use of students of optics. 



(10) On three contact minerals from Velardefia, Durango, Mexico (gehlenite, spurrite, 



and hillebrandite). Fred. Eugene Wright. Amer. Jour. Sci. (4), 26, p. 54S, 

 1908. 



A chemical and optical study of three contact minerals occurring near the 

 junction of altered limestone and intrusive basic diorite. The specimens ex- 

 amined were gathered by Messrs. J. E. Spurr and G. H. Garrey in Velar- 

 defia, Mexico. This appears to be the first recorded appearance of gehlenite 

 on this continent. The others are new minerals which have not been de- 

 scribed before. 



(11) Thermometric lag in calorimetry. Walter P. White. Phys. Rev., December, 



1908. 



In recent work in calorimetry by the method of mixtures, several attempts 

 have been made to avoid an error to which considerable importance has been 

 attached, due to the lag of the thermometer. It can be shown, however, that 

 this error does not exist at all : In a calorimetric run by the method of mix- 

 tures, all the temperature data lie upon a temperature time curve whose form 

 determines both the cooling correction and the main temperature interval. 

 The exact instant at which temperatures on this curve are read is unimportant 

 so long as the temperature intervals are preserved. If, now, all temperatures 

 are plotted the same number of seconds wrong, as they would be when the 

 same lagging thermometer is used throughout, no appreciable error can re- 

 sult. The importance of lag in calorimetric thermometers has therefore been 

 greatly overestimated. 



The above reasoning points to the existence of another error in calorimetry 

 hitherto generally overlooked, due to the unavoidable lack of uniform tem- 

 perature during the period of rapid heating, which produces all the effects 

 of a thermometric lag. A correction should be made for this effect. 



(12) Diopside and its relation to calcium and magnesium metasilicates. E. T. Allen 

 and Walter P. White. With optical study by Fred. Eugene Wright and E. S. 

 Larsen. Amer. Jour. Sci., January, 1909. 



The end members of the system CaSiOs-MgSiOg both exhibit enantio- 

 tropy. The inversion-point in the former is about 1,190°. The a-fonn, 

 pseudo-wollastonite, is unknown in nature. The j8-form is the mineral wol- 



