594 WRIGHT: MEASUREMENT OF STRAIN IN GLASS 



two different kinds were kept saturated with silver nitrate they 

 produced identical results. 



The practice of nearlj^ all observers has been to continue the 

 washing of the deposit until the presence of silver nitrate can no 

 longer be detected in the wash waters by chemical tests, but 

 many have taken the further precaution of allowing distilled 

 water to stand on the deposit for a considerable period of time. 

 As a test on the completeness of the washing, we compared the 

 conductivity of the water before being put in the cup with its 

 conductivity after it has stood in the cup for various periods 

 of time. In every case the conductivity increased with time. 

 It was at first supposed that this increase was due to entrapped 

 silver nitrate gradually soaking out, as we could detect the silver 

 in the water after allowing to it stand overnight. All subse- 

 quent experiments, for which reference is made to the complete 

 paper, showed that this is not the case, but rather that an electro- 

 lytic process was taking place by which the silver was passing 

 into solution at the rate of about 0.006 mg. per hour from a 4 

 gram deposit of silver on platinum. To confirm this we were 

 able to show by a galvanometer that a current actually pass.ed 

 from the silver through the water to the platinum. In washing 

 the deposits overnight, this effect becomes appreciable. The 

 work was closed with a final comparison of the voltameters in 

 the light of our experiments and unusually good agreement found. 



PHYSICS. — A simple method for the accurate 7neasure7nent of 

 relative strain in glass. Fred. E. Wright, Geophysical 

 Laboratory. 



The phenomena attendant upon strain in glass have long 

 interested physicists and glass-makers, and much thne has been 

 spent in the investigation of the different phases of this subject. 

 The optical effects resulting from strain were first studied in 

 detail by Brewster^ at a time when only the simplest of optical 

 apparatus was available and but little was known of double 

 refraction. Notwithstanding this, Brewster deduced from a 



' Philos. Tnins. 1814, 1815, ISKi, 



