172 CARNEGIE INSTITUTION OF WASHINGTON. 



have called attention to the advantages of standard caHbration tables for 

 translating the electromotive force of a thermocouple into temperature, and 

 such standard tables have been given for copper-constantan and for platinum- 

 platinrhodium couples. It has now been found advisable to extend the range 

 of the copper-constantan table so that this couple may be available for meas- 

 uring temperatures up to 400° and down to —200°. A table for chromel- 

 alumel (the Hoskins couple) has also been worked out and the previous 

 platinum-platinrhodium table for temperature between 0° and 1755° is 

 reprinted without change. The question of fixed-junction corrections is 

 discussed and the best methods for making such corrections are described 

 in detail. 



(37) The relations between tridymite and cristobalite. Clarence N. Fenner. J. Soc. 



Glass Technology, 3, Trans., 116-125 (1919). 



Several articles have appeared recently in French and British journals, in 

 which some of the conclusions regarding the stability relations between the 

 various forms of siUca, pubHshed several years ago (C. N. Fenner, The Stabil- 

 ity Relations of the SiHca Minerals, Am. J. Sci., 36, 331-384, 1913), have been 

 questioned. The principal basis for doubt seems to the present writer to be 

 not that new evidence has been discovered, but that the observers have failed 

 to take fully into consideration the rather remarkable properties of sihca, 

 which tend to obscure stabihty relations and whose effect was discussed in 

 some detail in the article cited; moreover, that some of the evidence set forth 

 there has been overlooked or disregarded, and explanations have been ad- 

 vanced which are inconsistent with this evidence. For this reason it has 

 seemed well to take up the matter anew and present the evidence which bears 

 upon the specific points involved. The points at issue are especially those 

 which deal with the relations between tridymite and cristobahte. Certain 

 evidence previously given is repeated in somewhat different form, and to 

 support it further evidence is offered which either has not been given before 

 in detail or is entirely new. In addition to the writer's work, other directly 

 relevant information supphed by the work of Ferguson and Merwin (see abstract 

 23 above) on the system CaO-MgO-Si02 is cited in confirmation. All of the 

 results are in accord with the conclusions previously announced, and it is 

 beheved that the explanations suggested by the foreign observers are directly 

 at variance with the experimental evidence. The conclusion is reached, as 

 before, that the field of stability of tridymite is hmited by the temperature 

 of 1470° =t 10°, and that at higher temperatures up to the fusing-point cristo- 

 balite is the stable form. 



(38) A method for determination of the volatile matter in oxides of lead. Olaf Andersen. 



J. Am, Ceram. Soc, 2, 782-783 (1919). (Papers on Optical Glass, No. 18.) 



The amount of volatile constituents in a sample of litharge or other oxide 

 of lead can be accurately determined by conversion of the PbO into PbSiOs. 

 The sample is mixed with a weighed quantity of silica, equal to about one- 

 third the weight of the PbO; heated in a platinum crucible in an electric 

 furnace at 800° to form lead siHcate glass; quickly raised to 1000° for a few 

 minutes; cooled and weighed. 



(39) The volatilization of lead oxide from lead-silicate melts. Olaf Andersen. J. Am. 



Ceram. Soc, 784-789 (1919). (Papers on Optical Glass, No. 19.) 



Experiments were made on the amount of PbO volatiUzed from the surface 

 of lead siHcate glasses at temperatures from 900° to 1400° C. It was found 

 that the volatilization from an unstirred glass in 15 minutes took place at 

 practically the same rate as from a stirred glass heated for a longer period. 



