proceedings: philosophical society 351 



Littleton spoke on some of the characteristics of Pyrex glass. The 

 paper was also discussed by Messrs. White and Peters. 



The last paper of the evening was by Mr. L. H. Adams on The an- 

 nealing of glass. 



The paper was illustrated by lantern slides. 



.Abstract: The class of materials known as glasses is peculiar in 

 many ways. At ordinary temperatures glasses are as hard, rigid, and 

 elastic as ordinary solids, but when heated they gradually become 

 softer and change without discontinuity into viscous and finally into 

 thin liquids. During cooling the reverse process takes place and there 

 is therefore a long range of temperature in which so-called solidification 

 takes place. It is primarily due to this fact that cooled objects of glass 

 acquire internal strain. 



Optical glass must be free from strain, first because if the strain is ex- 

 cessive the glass may fracture when handled or when heated again, 

 and second because the surfaces of a badly annealed lens or prism may 

 gradually warp and thus impair the definition of the finished instrument. 



For the practical annealing of optical glass it is sufficient to know 

 for several temperatures the time required for the removal of strain. 

 The necessary measurements have been made for nine kinds of optical 

 glass and the application of the results to the annealing of optical glass 

 in the factories has met with entire success. 



Discussion: Mr. White inquired if the method of annealing glass 

 by holding it at a given temperature for a rather long time and then 

 subjecting it to sudden cooling had not been introduced by Williamson 

 and Wright. Mr. Williamson stated that his use of the method was 

 due to the fact that it could be easily carried out by ordinary glass- 

 workers and with the available furnace equipment. Major Wright 

 stated that the method had been rather unconsciously adopted. 



The 815th meeting was held at the Cosmos Club, March i, 1919; 

 President Humphreys in the chair; 30 persons present. The minutes 

 of the 814th meeting were read in abstract and approved. 



Mr. W. P. White presented the first paper on Change of state in solids 

 — facts and theories. The paper was illustrated by lantern slides. 



Abstract: Changes of state in the solid, that is, changes of crystalline 

 form, are from thermodynamic necessity like the change of state solid- 

 liquid, that is, melting, in that the production of the high temperature 

 form is always accompanied by an absorption of heat, but they show 

 great variety in many phenomena. Nearly every law or rule which 

 has been set up for such transformations has been shown to be abund- 

 antly violated. For example, in cases where of two forms both can be* 

 cooled to absolute zero, it is demonstrable that the high temperature 

 form must at the temperature of inversion have the greater entropy, 

 which involves that at some portion of the temperature scale below that 

 point its specific heat must be higher, and the supposition is natural 

 that the specific heat would be higher at all temperatures. In 40 per 



