Measurements on the thermal dilatation of glass at high temperatures

previous work 452 II.Purpose 455 III.Experimental method 455 IV.Description of apparatus 458 1. Interferometer and specimens 458 2 .Furnace 458 V. Experimental procedure 460 1. Optical adjustments 460 2. Temperature regulation and observations 461 VI.Experimental results 462 1. Reduction of observations 462 2. Values for mean coefficient of expansion 462 3. Chemical composition 463 4. Final values of ---'.464 5. Dimensional change above the critical temperature 476 6. Comparisons of dimensional changes and heat absorption 483 7. Annealing temperature 484 VII.Summary 487 ^^^j'J Thermal Dilatation of Glass 451 temperature.This permanent contraction amomited to 0.052 per cent of the length of the tube on the first heating and 0.026 per cent on the second.Later in their work on the boiling point of sulphur, Callender and Griffiths^determined the thermal expan- sion of a specimen of glass tubing.A permanent contraction of 0.056 per cent of the sample's length was found after the first of ^ve heatings to 400° C. The amount of the contraction decreased upon successive heatings.This same effect was observed by Holbom and Griineisen^in their work on the thermal expansion of Jena 59™ glass.A rod (5.7 by 435.8 mm), when held (in a horizontal electric furnace) ^t 550° C for five hours, shortened 0.79 mm.With the tempera- ture held at 500° C, readings taken in one-half hour intervals revealed successive contractions of 0.084, 0.038, 0.036, and 0.018 mm.Thiesen, Scheel, and Sell,^and Henning,^used the micrometric method for the determination of the thermal expansion of ther- mometer glasses between -191 to +100° C.M. So® investigated the thermal expansion of glass rods, i by 300 mm, which were suspended within a vertical, electrically heated, iron-tube furnace.A 25 g weight was fastened to the lower end of the rod, and the expansion was measured with micrometer microscopes fastened to the same iron pillar which supported the specimen.The expansion of this pillar, which might have introduced an error in the determination, was not considered.With this apparatus sixteen different kinds of glass were examined, the data and curves for four of which are presented in his paper.These curves show a nearly linear expansion from room tempera- ture to 400° C. With the unannealed glass, a considerable contraction, not shown by the annealed glass, occurred during the next 50° rise in temperature.Beyond 450° a rapid elongation took place, due perhaps to a stretching of the plastic glass by the 25 g weight.