June 6, 1901] 



NA TURE 



141 



the University in London which imposes such an entrance test 

 for engineering, and unless and until the University is prepared 

 to adapt its matriculation to suit the requirements of particular 

 classes of students, which itis empowered to do under tne new 

 Statutes, and especially to engineering students, no very general 

 or substantial improvement can be expected.'' Appended to 

 the Report is an address given by Prof. Armstrong upon his 

 retirement from the office of Dean of the College, his term 

 having expired, and an address delivered by Sir Alexander K. 

 Binnie at the opening of the current session. Both at the 

 Central Technical College and at the Finsbury Technical College 

 there was an increase in the number of students in the electrical 

 departments, owing possibly to the development of electric 

 traction in this country. 



The president of the Massachusetts Institute of Technology, 

 in his annual report, records that there were in the Institute at 

 the end of last year no less than 1277 students — the largest 

 number yet reached. Of this number 193 were fourth-year 

 students. The average age on entrance is eighteen years and 

 ten months, which is a few months more than the average age at 

 which .students enter the Central Technical College, London. 

 An increasing number of students remain for a fifth year or 

 enter the Institute for post-graduate courses. There are thirteen 

 courses extending over four years, and including such subjects 

 as chemical engineering, sanitary engineering and electro- 

 chemistry. In looking through the " Annual Catalogue " con- 

 taining the outlines of the work done in these courses, we are 

 reminded of the statement made in connection with the recent 

 dismissals at the Royal Engineering College, Coopers Hill, that 

 Indian engineers only need to know chemistry "to the extent 

 required to enable the engineer to interpret results given by 

 professional chemists." This is not the way in which engineers 

 are trained at the best technical colleges in the United States, 

 and if Lord George Hamilton and the Board of Visitors of 

 Coopers Hill had seen the programmes of the engineering 

 studies at the Massachusetts Institute they might have decided 

 upon a more liberal action with regard to the subjects to be 

 taught and the provision for teaching in a college where engin- 

 eers are trained for the public service. The Faculty of the 

 Massachusetts Institute has decided to discontinue the announce- 

 ment of the degree of Doctor of Science, and to make the 

 requirements for the degree of Doctor of Philosophy include 

 " high attainments of a grade which qualifies the recipient as a 

 scientific investigator and teacher.'' During 1900 the Institute 

 received 100,000 dollars (less succession tax) under the will of 

 the late Mr. R. C. Billings. The gift of 50,000 dollars by the 

 late Mr. A. Lowell to constitute " The 'Teachers' Fund " has 

 been increased to 100,000 by the executors, in conformity with 

 his wishes. Other gifts received during the year amount to 

 about 45,000 dollars. The total amount of the Institute pro- 

 perty, both real and personal, was increased during the year by 

 a net amount of 219,853 dollars, after deducting the sum of 8593 

 dollars, which is the excess of expenses over income. 



SOCIETIES AND ACADEMIES. 



London. 



Royal Society, May 23.— "A Comparative Crystallo- 

 graphical Study of the Double Selenates of the Series 

 R.,M(Se04)2,6H.,0— Salts in which M is Magnesium." By 

 A. E. Tutton, B.Sc, F.R.S. 



This memoir on the magnesium group of double selenates, in 

 which R is represented by potassium, rubidium and CLiisium, is 

 analogous to that which was presented to the Society in March 

 1900 concerning the zinc group. 



The conclusions derived from the study of the morphological 

 and physical properties of the crystals of the three salts are 

 generally similar to those arrived at from the study of the zinc 

 group. There is observed an uniform progression with regard 

 to every property in accordance with the order of progression of 

 the atomic weights of the three alkali metals present. That is 

 to say, the constants of the rubidium salt are generally inter- 

 mediate between those of the potassium and cresium salts. 



The magnesium group has, however, proved particularly 

 interesting, inasmuch as the progressive diminution of double 

 refraction, according to the rule which has now been established 

 for this series of double sulphates and selenates, leads in the 

 case of Ciusium magnesium selenate to such close approximation 



NO. 1649, ''■QL. 64] 



of the three refractive indices that the crystals of this salt exhibit 

 exceptional optical phenomena. This includes dispersion of the 

 optic axes in crossed axial planes at the ordinary temperature, 

 the uniaxial figure being produced for wave-length 466 in the 

 blue ; and the formation of the uniaxial figure for every wave- 

 length of light in turn as the temperature is raised, the attain- 

 ment of uniaxiality for red lithium light occurring at the temper- 

 ature of 94°. As the life-history of the -salt terminates at 100°, 

 owing to the presence of water of crystallisation, this substance 

 exhibits the property of simulating uniaxial properties at some 

 temperature within its own life-range for every wave-length of 

 light, while still retaining the general characters of monoclinic 

 symmetry, including slight dispersion of the median lines. In 

 this respect it resembles to a truly remarkable extent the 

 analogous sulphate, which the author has shown to possess like 

 peculiarities, but it is even more striking than the sulphate, as 

 the dispersion is much larger. It is interesting to observe that 

 these optical properties of ca?sium magnesium selenate could 

 have been predicted, given the constants of the potassium salt 

 and the rules of progression established for the double sulphate 

 and for the zinc group of double selenates. For the double 

 selenates resemble the double sulphates so closely that in 

 general it may be said that their properties are precisely parallel, 

 the constants and curves being merely moved on to a slight 

 extent by the replacement of sulphur by selenium without 

 disturbing their relationships. 



Physical Society, May 31. — Prof. S. P. Thompson, presi- 

 dent, in the chair. — A paper on the resistance of dielectrics and. 

 the effect of an alternating electromotive force on the insulating 

 properties of india-rubber, by A. \V. Ashton, was read by Prof. 

 Fleming. The author has obtained from his experiments 

 formulae for the charging and discharging currents of a con- 

 denser with rubber dielectric. The currents are exponential 

 functions of the time. Curves for various potential differences 

 have been plotted and were exhibited. These curves show 

 that the insulating properties of rubber are increased by the appli- 

 cation of high alternating electromotive forces. — Prof. Fleming 

 then read a note by Sir. Ashton on the electrification of 

 dielectrics by mechanical means. A sheet of pure Para rubber 

 was placed in a condenser, the plates of which were connected 

 to a quadrant electrometer. A two-pound weight was then., 

 dropped upon the condenser from a height of 3 inches. The 

 electrometer received two impulses of opposite sign, one quickly 

 following the other. The rubber was then stretched while in. 

 position and a potential difference of seven volts was shown be- 

 tween the plates, the top plate being negative. The condenser 

 and electrometer were then discharged, the sheet reversed and) 

 the experiment repeated. The same effect was produced, the 

 top plate again being negative. It appears, therefore, that polar- 

 isation of a dielectric being thus produced by mechanical energy, 

 some part of the mechanical energy expended on the india- 

 rubber during manufacture would remain in the dielectric as 

 electric energy. — A model which imitates the behaviour of 

 dielectrics, by Prof. Fleming and Mr. Ashton, was exhibited by 

 Prof. Fleming. The behaviour of dielectrics with regard to their 

 residual charge is analogous to that of wires subjected ta 

 mechanical stress. A simple twisted wire is not, however, able 

 to imitate all dielectric effects, and the present paper describes 

 a model which represents things more completely. Six pistons^ 

 separated by springs, are placed inside a vertical cylinder. The 

 bottom piston fits fairly tightly in the cylinder. The secondi 

 piston fits slacker than the first. The third piston has a small, 

 hole in it, and each succeeding piston has a greater area cut 

 away, the top piston having just sufficient metal left to make the 

 spring come to rest without vibration after being compressed. 

 "The cylinder is filled with machine oil and vaseline. To the top. 

 piston is attached a rod by means of which pressures can be 

 exerted on the pistons for any length of time. This represents 

 the charging of the condenser. The motion of the rod after re- 

 leasing the weights represents the discharge of the condenser. 

 This is registered graphically by a revolving drum, and the curves 

 obtained are very similar to those froin condensers with dielec- 

 trics. Prof. Ayrton said he would like to know in what respect 

 the model shown was superior to a strained wire. He had no- 

 ticed, about ten years ago, that alternating E.M.F.'s appeared to 

 improve condensers. He was then working with comparatively 

 small voltages, and he was interested to know that Mr. Ashton, 

 working with high voltages, had established the improvement. 

 The deflection obtained by stretching the india-rubber sheet might 

 be due to changes in temperature, the dielectric having a high 



