THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 813th meeting was held at the Cosmos Club, February i, 191 9; 

 President Humphreys in the chair; 68 persons present. The minutes 

 of the 812th meeting were read in abstract and approved. 



Major Fred E. Wright presented the first paper on War time de- 

 velopment of the optical industry. (No abstract). 



The second paper was by Mr. E. D. WHvIvIamson on Temperature and 

 strain distribution in glass. The paper was illustrated by lantern 

 slides. 



Abstract: When a temperature gradient is set up in a solid, stresses 

 result, owing to the unequal expansions of the various parts. The 

 same is true of a viscous liquid, except that in the case of the latter there 

 is a slow yield to the forces and in time the stresses and strains would 

 vanish for that temperature distribution. At any time, then, there is a 

 specific temperature distribution (dependent on the previous heat 

 treatment) for any piece of glass, under which the glass is unstressed. 

 If this temperature distribution be known the stresses produced when 

 the temperature becomes equal throughout the mass can be calculated 

 from elasticity data. 



The paper dealt in detail (i) with the nature of the temperature 

 gradients produced by uniform heating or cooling, at a constant rate, 

 of the surface of solids of the following shapes, viz, slab, square rod, 

 cylindrical rod, brick, short cylinder, and sphere; and (2) with the 

 stresses set up in solids whose unstressed temperature distribution is 

 that considered in (i). The stress relations were treated in full for the 

 case of a sphere only. 



The next paper was by Messrs. A. Q. Tooi. and J. Valasek on Some 

 characteristics of optical glasses in the annealing range. This paper was 

 presented by Mr. Tool and was illustrated by lantern slides. 



Abstract: In order to carry out the process of annealing glass effi- 

 ciently a thorough knowledge of its various properties in the range of 

 temperatures employed is necessary. In this respect the viscosity of 

 the glass is very important, since any information as to its magnitude 

 and variation with the temperature enables one to calculate the time 

 required for the stresses to relax to any predetermined ratio of their 

 initial value and to draw some conclusions as to the best cooling pro- 

 cedure. The term annealing temperature is used here to designate that 

 constant and uniform temperature, consistent with the most efficient 

 annealing procedure, at which the glass should be held while the stresses 

 disappear. This temperature is determined by a study of the relaxation 

 of stresses in a sample of glass at various temperatures. The two types 

 of methods which have been employed for this purpose involve either 

 a determination of the rate of decrease of internal forces by means of 

 the double refraction exhibited, or the measurement of the rate of 

 deformation of a suitable piece of glas§ under externally applied forces. 



A form of the first type, often recommended for practical testing, was 

 tried after some modification. Briefly, it consisted of heating a glass 



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