TRAKSACTIOXS OF SECTION A. G37 



tioTi, but of intensity of magnetisation, since botli values liecome constant together, 

 and that tlie relation between them is not a logarithmic curve, but is a curve 

 showing one fiexure, anJ clearly indicating in its upper portion the condition of 

 the iron represented by Professor Swing's third stage of magnetisation. 



WEDNESDAY, SEPTEMBER 18. 



The following Papers and Report were read :— 



1. On the Change of Molecular Refraction in Salts or Acids dissolved in 

 Water. By Dr. J.H. Gladstone, F.R.S., and Walter Hibbert, F.I.C. 



The authors bad recently undertaken a research on the questions— Does the 

 specific refractive energy of a salt or acid when deduced from its solution in water 

 differ from that of the solid compound ? and Does the specific refractive energy 

 vary according to the amount of water ? The outcome of the experiments of the 

 authors and others is that the water does bring about in many cases a small altera- 

 tion, especially in passing from the solid or liquid to the dissolved condition ; 

 that this alteration is sometimes an increase, at other times a decrease ; and that 

 it depends upon the chemical nature of the compound. Some_ points of physical 

 interest were, however, noted, and were being more fully examined. One of these 

 is the analogy of this refraction change in several instances with the change in the 

 power to rotate the plane of polarised light as determined by Dr. Perkin. As this 

 small change of refraction evidently indicates some rearrangement of the consti- 

 tuents of the salt or acid in the water, it may throw light upon present theories 

 of solution. The experiments, even those of Kohlrausch and Hallwachs on ex- 

 tremely dilute solutions, do not support the view that the binary compound when 

 greatly diffused tends to exhibit the properties of a gas. There is an evident 

 relation between this change of refraction and the electric conductivity of the 

 solution. Thus, in the acids the order of the two phenomena is the same, the 

 hydrochloric acid showing the greatest effect, rapidly followed by hydrobromic 

 and hydriodic acids ; then nitric acid, afterwards sulphuric acid ; and at a great 

 distance acetic and other organic acids. In the case of nitric and sulphuric acids 

 the general form of the curves representing the change of electric conductivity and 

 of refraction is similar ; in the case of the latter there is a special depression during 

 the rise of the conductivity curve which makes its appearance as a slackening of 

 the rise in the curve representing change of refraction. This connection of the two 

 phenomena is being further carefully examined at present. 



2. Report on Electrical Standards. — See Reports, p. 195. 



3. On the Choice of Magnetic Units. 

 By Professor Silvanus P. Thompson, F.R.S. 



Professor Silvanus Thompson pointed out that the giving of names was a 

 ■detail, and that agreement was wanted upon the units themselves in which mag- 

 netic quantities were to be expressed. He agreed with the Standards Committee 

 that the two most important units to be defined were those of magnetic flux and of 

 magnetic potential, and urged that no other units should be defined until these 

 had been tried. But he differed from the suggestion to take the weber as 10^ 

 C.G.S. lines as being a unit of too great an order of magnitude to suit practical 

 needs. He preferred simply to take the line, with its natural multiples the kiloline, 

 and the megaline as the unit of flux. If the name wther were given to the line 

 itself the Committee's recommendation would then be identical, so far as this unit 



