﻿Method for examining Optical Qualities of Glass Plates. G13 



rainy weather, was passed for 102 hours through a soda-lime 

 tube* (1*5 metres long), the last third of which was filled 

 with small pieces of potash, and then over a weighed 

 sulphuric-acid vessel, with the result that the latter gained 

 *0041 gramme. 



In this connexion it may be of interest to note that quan- 

 titative measurements show pure anhydrous CuS0 4 to be a 

 very efficient drying agent for air containing little more than 

 traces of moisture (it will take up about 0"05 per cent, of its 

 own weight). It has the advantage that it can be used over 

 again after heating to 210°-220° C. in an air-stream. The 

 dehydrating property of CuS0 4 seems difficult of explanation, 

 for, according to theory f , the substance should not absorb 

 water- vapour unless the partial pressure is greater than that 

 of the hydrate CuS0 4 1 Aq. 



LXIL Note on a Sensitive Method for Examining some Optical 

 Qualities of Glass Plates. By the Earl of Berkeley, 

 F.R.S., and D. E. Thomas, M.A., B.Sc.% 



IN the course of a research on the relation between the 

 concentration and the partial pressures of the vapours 

 of miscible liquid s, we have used a Rayleigh interferometer 

 for determining the refractive index of the vapours and their 

 mixtures. The new method for the examination of glass 

 plates arose out of certain troubles experienced in the 

 adjustments of this instrument. 



The interferometer is one of Zeiss's, designed for working 

 with columns of gas one metre in length, and was modified 

 so as to double the optical path as shown in fio-s. 1 

 and 2. 



Fig. 1 is a plan of the apparatus as set up for the exami- 

 nation of glass plate X — the arrangement differs from 

 that used in examining gases only in the removal of the 



* These soda-lime tubes have proved to be very convenient : they are 

 made in the shape of a very broad S (as seen in plan), the bends of the 

 S are turned up at an angle of 45° with the horizontal and are fitted with 

 ground-in glass stoppers for rapidity in tilling. The vertical inlet and 

 exit tubes are relatively narrow and carry mercury cups, and the whole 

 tube can staud on the bench without supports. 



t Cp. Lehfeldt's translation of van't HofFs Lectures on Theoretical 

 and Physical Chemistry, Part I. p. 60. 



X Communicated by the Authors. 



