October 4, 1S94] 



NA TURE 



54; 



has appeared on the effects, if any, which modifications 

 of the surroundings exert on this attraction is very re- 

 markable. Newton's hollow pendulum experiments, 

 repeated with greater accuracy by Bessel, seem to be 

 almost the only investigations which have been published 

 on what may be called the physical properties of gravita- 

 tional attraction. As far as we know, the attraction 

 between two given masses depends merely upon their 

 geometrical configuration ; it is independent of the 

 medium between them, of the physical state of the 

 masses, whether they are solid, liquid or gaseous, 

 amorphous or crystalline ; it does not depend upon the 

 temperature of the masses. Indirect evidence, often 

 derived from the Cavendish experiments, shows that the 

 preceding statements must at least be very approxi- 

 mately true. Again, chemical analysis is founded on the 

 hypothesis that the weight of an atom of a chemical 

 element is unaltered whatever chemical combinations it 

 may form, or whatever the temperature to which it may 

 be raised. It is, however, often difficult to tell the 

 degree of approximation to the truth of the preceding 

 statements which is indicated by such indirect evidence, 

 and a direct experimental investigation to determine an 

 inferior limit to the accuracy of some of the preceding 

 statements would not be superfluous. Take the case, for 

 example, of the statement that the weight of an atom 

 cannot be altered by chemical combination: it would be 

 for the advantage of science if this were proved with the 

 utmost possible accuracy attainable by present methods 

 for some definite chemical combinations. The question 

 is of interest in connection with the view that the atoms 

 of elements are aggregations of atoms of some primordial 

 substance not very much lighter than hydrogen. The 

 values of the atomic weights of the elements is incon- 

 sistent with this view if each atom of the primordial 

 element retains its weight unaltered in the complex 

 atom. If, however, it suffers a slight change of weight, 

 then we might expect to find traces, though perhaps 

 faint ones, of such a change in ordinary chemical com- 

 binations. 



Another question which has excited some interest is 

 a possible connection between the magnitude of gravi- 

 tational attraction and temperature. Prof. Hicks has 

 pointed out that Baily's results gave a value for the 

 mean density of the earth which uniformly diminished 

 as the temperature increased, indicating, if the effect is 

 a real one, that the attraction between two masses in- 

 creases with the temperature, and von Sterneck, in his 

 experiments at Freiberg, found a remarkable relation 

 Ijstween the temperature and the value of gravity. Prof. 

 I'oynting discusses these and other results, and comes to 

 the conclusion that they are to be explained by other 

 causes, and do not afford any evidence at all that the 

 attraction between bodies varies with the temperature. 

 The point is one which has a direct bearing on Prof. 

 Poynting's own experiments, as the final value for the 

 mean density is got by taking the mean of two sets of 

 experiments, and as the temperature in the two sets 

 differed by about y^ C, an uncorrected temperature 

 effect would affect the result. 



We are very glad to find from this essay that Prof. 

 Poynting is engaged on an investigation as to whether 



NO. I 30 1, VOL. 50] 



the attraction between two crystals depends on the re- 

 lative position of their axes. 



The second part of the essay consists of Prof. Poynt- 

 ing's paper " On a Determination of the Mean Density 

 of the Earth,' published in the /'////. Trans, for 1S91. 

 This, in addition to the actual determination of the mean 

 density, is almost a treatise on the method of using a 

 balance so as to get great sensitiveness. The work is a 

 model of patient care and skill, as well as of clearness 

 of exposition, and we feel as we read it that the utmost 

 has been made out of the apparatus and the method. 

 Prof Poynting is of opinion that it is only air-currents 

 which prevent the balance being used with an accuracy 

 far beyond anything hitherto approached ; the ordinary 

 balance is more sensitive than the torsion balance to 

 air-currents, since these produce the greatest disturbance 

 in the vertical direction, which is the direction of dis- 

 placement in the ordinary balance. Prof Boys has shown 

 that to minimise the effect of air-currents the size of the 

 apparatus ought to be reduced as much as possible. 

 Prof. Poynting says that if he were designing his ap- 

 paratus again, instead of using, as he did, an exceptionally 

 large balance, he would go to the opposite extreme and 

 use a very small one. 



Of all the methods hitherto used to determine the mean 

 density of the earth, the arrangement used by Prof Boys 

 seems to be the one capable of the greatest accuracy. 

 There are, however, certain points about the method 

 of the common balance, such as the simplicity of the 

 most important measurements, and the absence of the 

 necessity to determine a time of swing with great ac- 

 curacy, which make it worthy of such a work as Prof. 

 Poyr.ting has devoted to it. 



The mean density of the earth found by Prof Poynting 

 is 5'49. Prof. Boys' result is 553. 



MINING. 

 A Text-book of Ore and Stone Mining. By C. Le 

 Neve Foster, B.A., D.Sc, F.R.S. Svo. Pp. 744, 

 with Frontispiece and 716 Illustrations. (London; C. 

 Griffin and Co., Limited, 1894.) 



IN view of the paramount importance of the produc- 

 tion of minerals to Great Britain, and of the con- 

 stant enterprises for working gold ores and other minerals 

 in most of our colonies, it is certainly remarkable that 

 there has hitherto been no satisfactory systematic treatise 

 on metalliferous mining available. It is a matter of con- 

 gratulation, therefore, that so eminent an authority as 

 Dr. Le Neve Foster has found time, with his many duties 

 as H.M. Inspector of Metalliferous .Mines, and as Pro- 

 fessor of Mining at the Royal College of Science and 

 Royal .School of Mines, to fill up so important a gap in 

 technical literature. His compendious volume will un- 

 doubtedly be warmly welcomed as an invaluable work 

 of reference, not only by mining students, but by all 

 English-speaking mining engineers. The subject is so 

 extensive, that the author's task of keeping his text-book 

 within moderate limits, without erring- on the side of 

 omission, was one of considerable difficulty. He has, 

 however, been thoroughly successful ; and the extremely 

 methodical arrangement of the material obviates, as 1 



