238 



NATURE 



[January 7, 1892 



importance (an exaggerated importance according to Marignac's 

 criticism) to an interesting observation made by Prof. Nordens- 

 kiold, who had announced {Comptes rendus, ciii. (l886), 795) 

 the fact that the crude mixture of yttria earths, as precipitated 

 from different minerals, although really a compound body, has 

 nevertheless always a constant molecular weight, whatever may 

 be the mineral from which it is extracted. To this mixture of 

 earths, which thus behaves like an element, Prof. Nordenskiold 

 gave the name gadolinia, or oxide of gadolinium. In a 

 subsequent paper {Ofversigt af Vetenskaps-Akademiens For- 

 hattdlingar, 1887, No. 7) in answer to the criticisms of 

 Marignac and Rammelsberg, he showed that although the 

 molecular weights for the individual earths which make up 

 gadolinia vary between 136 and 394, yet the molecular weight 

 for the mixture as a whole, obtained from a number of different 

 minerals from various localities, only varied from 258 to 271. 



The work of which an account is given in the paper now 

 before us was undertaken in order to discover whether the 

 above slight differences were only dependent on variations in 

 the experimental methods. The author has not attempted to 

 make any separation of the individual earths, but has confined 

 himself to further determinations of the molecular weight of the 

 group as a whole. The result of fifty-four determinations made 

 upon such minerals as gadolinite, orthite, samarskite, monazite, 

 &c., from various localities was to show that gadolinia from 

 widely different sources has a molecular weight ranging from 

 247 '9 to 275 '8. The gadolinia used in these determinations 

 was obtained by first separating the cerium metals by precipita- 

 tion with potassium sulphate, precipitating the filtrate with 

 ammonia, dissolving this precipitate, and reprecipitating with 

 oxalic acid. Care was taken to avoid any fractionation which 

 might occur, but any fear on this account was discounted by the 

 unexpected discovery made during the course of the work that any 

 gadolinia fractioned out {e.g. thrown down with the cerium oxides 

 in the K2SO4 precipitate) had a molecular weight differing only 

 very slightly from that of the rest of the earth. 



The criticism which the author next applies to eighty-one 

 molecular weight determinations madeby Blomstrand, Engstrom, 

 Rammelsberg, and others, leads to the rejection of forty, either 

 owing to the small amount or to the impurity of the material 

 used. The numbers in the case of the forty-one determinations 

 retained fall within the limits obtained by the author. 



Now, the molecular weights of the twelve earths which are at 

 present stated to enter into the composition of gadolinia range 

 from that of scandia, 136, and yttria, 227, to that of ytterbia, 

 394 ; so that the fact that, in spite of this wide divergence in the 

 molecular weights of the constituents, the molecular weight of 

 gadolinia itself only varies by at most 5 '4 per cent, from the 

 mean value, 262, is sufficiently startling. Thus no gadolinia has 

 yet been found containing exclusively one only of these in- 

 dividual earths. The author shows, by a review of other minerals 

 containing isomorphous groups of elements, how distinct in this 

 respect is the behaviour of this group of yttria earths. 

 ■ We have seen what use Crookes has made of these curious 

 facts. We will conclude, therefore, by giving Marignac's con- 

 clusions on the same subject, as contained in his criticism of the 

 author's previous paper. He is of opinion that all that can be 

 affirmed is that yttria is always met with in nature mixed with a 

 variable number of analogous and isomorphous earths of ffiuch 

 higher molecular weight, and that it is always the dominating 

 element in this mixture. G. T. P. 



SOCIETIES AND ACADEMIES. 



London. 



Physical Society, December 18, 1891. — Prof. W, E. Ayrton, 

 F.R.S., President, in the chair. — The President announced 

 that it had been found necessary to alter the dates of the meet- 

 ings to be held after Christmas from those already published to 

 the following : January 22, February 12 and 26, March 11 and 

 25, April 8, May 13 and 27, June 10 and 24. — A note on inter- 

 ference with alternating currents was communicated by Mr. M. 

 H. Kilgour. Whilst studying Dr. Fleming's paper on some 

 effects of alternate current flow in circuits having capacity and 

 self-induction, the author constructed some additional curves. 

 He was thereby led to investigate whether the serious rises of 

 pressure produced by adding capacity would occur over con- 

 siderable ranges of capacity, or whether they would only take 

 place when the capacity was nearly equal to a particular value. 



NO. I I 58, VOL. 45] 



Taking the case of a condenser of capacity C farads, in series^ 

 with a circuit of resistance R ohms, and inductance L henrys, he 

 showed that the maximum value of A (the ratio of the pressure 

 across the condenser terminals to that across the condenser and 

 inductive resistance) is obtained when 



C = 



R--' + fU^' 



(I) 



(2) 



where/ = 27r times the frequency. The maximum value of \ 

 produced by this capacity being given by the expression 



A= ^(ElZZy 



R • • • • • 



Taking R = 10, and / = 2ir . 1000, curves plotted from equa- 

 tions (i) and (2) between C and L, and between A and L had 

 been drawn. The CL curve rises to a very sharp peak at 

 L = 0"00i5, and falls rapidly. That between A and L starts 

 horizontal and bends upwards, and approximates to an inclined 

 straight line for values of L greater than O'oo2 ; when L = o'l, 

 A = 63. Considering the question of the range of capacity' 

 with which a given rise of pressure can occur, it was pointed 

 out that when the values of L, R, and / are such as to make a 

 large rise possible, a rise exceeding a moderate value can only 

 be obtained for values of C differing little from that given by 

 equation (i). On the other hand, when the circuit is such that 

 the maximum rise possible is not large, then a rise exceeding a 

 given moderate value can be obtained over a much wider range 

 of capacity. Hence the author concludes that the larger the 

 possible rise the smaller is the probability of a serious rise 

 occurring. The effect of shunting the condenser by a circuit of 

 resistance r and inductance / is next dealt with in the paper, 

 and the values of C which make X a maximum determined, as 

 well as the maximum value L can have. Subsequently the 

 author examines whether the practical case of an alternator 

 feeding a transformer through a concentric cable may be simpli- 

 fied without introducing serious error by assuming the capacity 

 concentrated at either end of the cable, and concludes that in 

 ordinai-y cases little error will be thus made. In an experiment 

 made with a loo kilowatt alternator, f mile of ^t concentric 

 cable, and an 18 kilowatt transformer, a rise of i per cent, was 

 found to occur at theterminals of the alternator when the cable was 

 connected. Putting on the transformer unloaded or loaded with but 

 a dozen 16 candle-power lamps produced little change in the rise 

 of pressure, this in all cases being between 0"2 and o'3 per cent. 

 Dr. Sumpner asked whether the conclusions as to the range of 

 capacity with which a given rise of pressure was possible, were 

 true for small rises such as occur in practice. Cases where the 

 maximum possible rise was of the order 63 were not likely to 

 occur at ordinary frequencies. The highest rise he had ever 

 known was 11. He thought the time-constant of the inductive 

 coil chosen — viz, y^^^ of a second — was very large. In circuits 

 containing iron it was practically impossible to get such large 

 time-constants, for the power spent in the iron increased the 

 effective resistance. Referring to the narrow range of capacity 

 within which large rises were possible, he pointed out that such 

 cases were found in Hertz's resonators, where the rises were 

 immense, but to obtain them the adjustments had to be very 

 accurately made. Dr. S. P. Thompson said he regretted that 

 Prof. Fleming was not present, for he had recently investigated 

 Hertz's experiments, and had obtained curves very similar to 

 those got for the Deptford mains. The curve between A and L 

 was very interesting. It was, in fact, a curve between the secant 

 of the angle of lag and L, as could be seen from formula (2). 

 In practice one would be working on the lower portion, and 

 hence the rises would be small. Mr. Kilgour explained that in 

 the paper his first object was to show that the product of the 

 ange of capacity between which a rise greater than a given 

 value would occur and the maximum possible rise was approxi- 

 mately constant for different circuits. Secondly, he wished to 

 prove that the capacity of concentric cables could be assumed to 

 be localized at either end without introducing much error in the 

 rises of pressure calculated therefrom. Dr. I'hompson, speaking 

 of nomenclature, regretted that the word inductance should be 

 used sometimes for L and at other times for L/, and thought its 

 meaning should be restricted to the latter. Prof. Perry said a 

 name was needed for coefficient of self-induction. Resistance 

 was practically independent of frequency, and "inductance" 

 should have no reference to frequency. Dr. Sumpner thought it 

 important to have a name for L/, for that quantity comes into 

 calculations most frequently. He would have preferred that 



