June 30, 1904] 



NATURE 



The academic address to the University College of North 

 Wales was delivered on Friday last by Sir Arthur Riicker, 

 F.R.S., principal of the University of London, who chose 

 as his subject " University Organisation in Great Britain." 

 Sir Arthur Riicker traced the various phases through which 

 the university systems of our country had passed, starting 

 with the residential university, represented at the present 

 time by Oxford and Cambridge (and in former days by 

 Stamford). Next in order of development came the purely 

 examining University of London, which led to the form- 

 ation of provincial university colleges. The federal uni- 

 versity came next in the Victoria University and University 

 of Wales. The Universitv of Birmingham represented a 

 new phase, namely, the municipal university, of which at 

 the present time there were two representatives in Lanca- 

 shire and two in Yorkshire. The impossibility of raising 

 by private subscriptions sufficient funds for the endowment 

 of universities and universitv colleges resulted in the 

 necessity of Government subsidies, and a great deal more 

 ought to be expected in this direction in the near future. 

 In the further development of university organisation a 

 number of interesting questions would have to be answered. 

 These related to such points as how far Government assist- 

 ance was to be given to colleges and how far to universi- 

 ties, whether institutions partaking of the character both 

 of colleges and universities should be subsidised under both 

 headings, whether it was desirable to confer on new uni- 

 versities generally the powers of examining external students 

 as provided for in the charter of the Birmingham University, 

 and the extent to which universities subsidised at the same 

 time by municipalities and the Government should be under 

 the inspection of both bodies. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, June 2. — "Colours in Metal Glasses and in 

 Metallic Films." By J. C. Maxwell Garnett. 



The first part of the paper is devoted to coloured glasses. 

 The phenomena which it seeks to explain were observed 

 by Siedentopf and Zsigmondy. 



It is proved in this paper that every medium made up 

 of metal spheres embedded in a non-absorbing substance 

 in such manner that the average distance between two 

 adjacent spheres is much less than a wave-length of light 

 has a perfectly definite colour, depending only on the optical 

 constants of the metal of which the spheres are made, on 

 the refractive index of the substance in which they are 

 embedded, and on the quantity of metal present, but not 

 on the size or distance apart of the spheres. 



It is shown that the particles which Siedentopf and 

 Zsigmondy observed in gold glasses are spherical when 

 their diameters are less than lo-^ cm. The presence of 

 the metal spheres accounts for the red colour of gold and 

 copper ruby glass, and for the yellow colour of silver glass, 

 and would give a blue-violet colour to " potassium-sodium " 

 glass (potassium-sodium being an amalgam of which the 

 optical constants have been determined by Drude). 



Experiments are described proving that these character- 

 istic colours can be produced in a colourless metal glass 

 containing the metal in solution or in combination (the 

 state in the manufacture of gold or copper ruby glass before 

 the second heating) by the 3 radiation from radium. 



The calculated properties of media containing many 

 metal spheres to a wave-length of light account for the 

 changes of colour, for the initial increase in absorption, and 

 for the final change to almost complete transparency which 

 Mr. G. T. Beilby observed during the annealing of gold 

 and silver films. Explanations are given of the changes 

 of colour on heating observed by Prof. R. W. Wood in 

 potassium and sodium films deposited on the insides of 

 exhausted glass bulbs. The increase in strength of colour 

 which was generally observed in the light transmitted by 

 these films when the plane of polarisation of obliquely 

 incident light was changed from that of incidence to a 

 perpendicular position is also explained. 



Evidence is adduced to show that the allotrppic silvers 

 obtained by Carey Lea are further examples of this type 

 of medium. 



NO. 1809, VOL 70] 



" A Method of Measuring directly High Osmotic 

 Pressures." By the Earl of Berkeley and E. G. J. 

 Hartley. Communicated by W. C. D. Whetham, F.R.S. 



This is a preliminary paper describing the authors' 

 method of determining high osmotic pressures. It is as 

 follows : — 



A porous porcelain cylinder, glazed only at the ends, has 

 a copper ferrocyanide membrane deposited on its outer 

 surface. The solution surrounds the cylinder, and the 

 inside, which is connected to a graduated glass capillary, is 

 filled with water. By means of a plunger, which works in 

 a steel cylinder and is actuated by a lever and weights, 

 pressure is put upon the solution. So long as this pressure 

 is less than the osmotic pressure of the solution, water 

 from the inside of the cylinder passes through the membrane 

 into the solution, and consequently the water-level in the 

 capillary falls. When the pressure on the solution is 

 gradually increased, the rate at which the level falls 

 gradually decreases, and this continues until the osmotic 

 pressure of the solution is reached ; then the level in the 

 capillary is stationary. A further increase of pressure on 

 the solution will then cause the level to rise. The rate of 

 movement of the level in the capillary is a function of the 

 difference between the osmotic pressure and the pressure 

 on the solution, so that by observing the changes in this 

 rate consequent on the corresponding changes in the 

 pressure, the point at which the latter is equal to the osmotic 

 pressure can be deduced. The results of some experi- 

 ments with cane sugar, extending up to a solution having 

 an osmotic pressure of 45 atmospheres, are given. 



The semipermeable membranes are made partly by follow- 

 ing Pfeffer and partly by a modification of Morse's electro- 

 lytic method. By this means a membrane that withstood 

 120 atmospheres pressure was obtained. 



" On the Electric Effect of Rotating a Dielectric in a 

 Magnetic Field." By Dr. Harold A. Wilson. Communi- 

 cated by Prof. J. J. Thomson, F.R.S. 



It was shown by Faraday in 1831 that an electromotive 

 force is induced in a conductor when it moves in a magnetic 

 field so as to cut the lines of force. The object of the 

 experiments described in this paper was to see if a similar 

 electromotive force is induced in a dielectric when it moves 

 in a magnetic field. 



-According to Maxwell's electromagnetic theory as de- 

 veloped by H. A. Lorentz and Larmor, such an electro- 

 motive force should be induced in a dielectric, and should 

 be equal to that in a conductor multiplied by the factor 

 I — K-', where K is the specific inductive capacity of the 

 dielectric. 



The method employed was to rotate a hollow cylinder of 

 ebonite in a magnetic field parallel to the axis of the 

 cylinder. The inside and outside surfaces of the cylinder 

 were provided with metal coatings, with which electrical 

 contact was made by sliding brushes. The inside coating 

 was connected to earth, and the outside coating to one 

 pair of the quadrants of a sensitive quadrant electrometer, 

 the other pair of quadrants being connected to earth. The 

 magnetic field was then reversed, so reversing the induced 

 electromotive force in the ebonite. The resulting electric 

 displacement was measured by means of the electrometer, 

 the quantity of electricity required to produce a given 

 deflection of the electrometer needle being determined by 

 means of a small parallel plate guard ring condenser. 



The cylinder used was 10 cm. long and 2r^ = l^■l^ cm., 

 21-, = 201 cm. It was mounted in a solenoid having 95 

 turns per cm., by which a magnetic field of strength 1500 

 could be produced. The cylinder was driven by a h horse- 

 power motor, and could be run at 200 revolutions per 

 second. 



The mean result obtained for the quantity of electricity 

 set free on the outside coating of the cylinder, on reversing 

 the magnetic field, only differs from the amount calculated 

 theoretically by i per cent. The specific inductive capacity 

 of the ebonite, as determined by measuring the capacity of 

 the cylinder, was 3-54, while the value calculated from the 

 results obtained was 3-64. 



The results obtained are thus in complete agreement with 

 the theories of Lorentz and Larmor, and may be regarded 

 as a confirmation of these theories. 



