May i8, 1893] 



NA TURE 



{Axiom 2). The acceleration of a given piece of matter is 

 proportional to the (eftective or resultant or unbalanced) force 

 aclini; on it, and is in the sain:; diit-ction. 



{Experience 4). Stresses in a boily do not accelerate it as a 

 whole. 



{Axiom 3). The two forces of a stress are always balanced. 



[Or otherwise (after Experience 3).] 



(Definition 2). The ratio of the force acting to the acceleration 

 produced in a given piece of matter is called its " inertia." 



{Axiom a). The inertia of a given piece of matter is uncon- 

 ditionally constant, and has no direction. 



(Jiemari). Inertia is therefore taken as the most fundamental 

 property of matter, and is used to measure its massiveness or 

 "nias=." . 



Definition^). The centre of mass of a system is a point such 

 that 2(oti ) = o ; or, it is a point moving with speed v, such that 

 2(ot)v = 2(otv). 



{Axiom b). The centre of mass of a system is not accelerated 

 by internal stresses, but only by one component of a stress whose 

 other component acts on a body foreign to the system. 



{Deduction). The two forces of a stress are always equal and 

 opposite. 



I 



(Remark). A brief and convenient statement of Axiom 2, by 

 help of Definitions I and 2, is Ftit = mdv. Note that F and 

 dv have necessarily the same sign ; they are parallel vectors. 



{Experience 5}. Every force is one component of a stress ; in 

 other words, bodies can only mechanically act on one another 

 {i.e. so as to affect each othei's moiion) by means of strtss ; or 

 stress is essential to mechanical action. 



(Remark). This might have been made part of Axiom 3, but it 

 is really a distinct statement. Perhaps it should be stated as an 

 axiom. 



(Axiom 4). A stress cannot exist in or acro.ss empty space. 



(Deduction). Therefore bodies (or any media) immediately 

 acting on each other are necessarily in contact, and stress exists 

 at the point of contact, where the normal components of their 

 velocities (v) are idenlical. 



(Experience 6). When stress and motion coexist, action 

 occurs or activity is manifested. 



(Definition 4). The scalar product of the two vectors Fv is 

 called " activity." 



{Deduction). The activities of two immediately acting bodies 

 are equal and opposite. 



(Remark). Elastic bodies under stress, and moving bodies with 

 inertia, are found to be able to manifest activity, and are said to 

 possess energy whereby they can do work on other bodies. Stress 

 energy is called potential ; motion energy is called kinetic. 



(Definition 5). Work done = i (activity)rf/ = energy gained 



or lost. 



(Remark). There are two ways of regarding this quantity : 



/ 



Yvdt ; namely either as ¥{vdt) = Fds = change of potential 



energy, or as v{Fdt) = v.mdv = change of kinetic energy. 



A l)ody for which Fv is positive is losing energy ; a body for 

 which Fv is negative is gaining energy. 



(Deduction). Since the activities of two immediately acling 

 bodies are equal and opposite it follows by Definition 5 that 

 energy lost by one is gained by the other ; i.e., that energy is 

 simply translerred without loss or gain across the point of con- 

 tact in the direction of the common velocity. 



(Axiom 5). Energy which is not being actively transferred 

 from one body to another remains unaltered in quantity and 

 form. 



(Remark). Energy which is being transferred from one body 

 to another changes its form. The kind of transformation de- 

 pends on the sign of dv with respect to the common velocity of 

 the acting bodies at their point of contact. 



If Wf is positive, energy is being transformed into kinetic ; 

 if vdv is negative, it is being transformed into potential ; if vdv is 

 zero, there is a mere flux or transmission of energy without 

 transformation. 



(Deduction). Since transference of energy is essential to activity 

 it follows that only bodies which are able automatically to part 

 with some of their energy, are able automatically to do work. In 

 other words, automatically transferable energy is alone avail- 

 able or potential. 



(Experience 7). The automatically transferable or potential 



NO. 1229, VOL. 48] 



energy of a body or system is liable to transfer and transform 

 it.sell into kinetic. Hence 



(.l.ciom 6). The potential energy of a system tends towards 

 zero. 



(Experience). Kinetic energy is only available when associated 

 with appreciable or relative momentum. 



The following statements may be made about the irregular 

 and aggregate motion oj particles called Heat. 



(Experience). Heat will not flow from low to high tempera- 

 ture liy mere conduction (as it could, for a time, if it possessed 

 inertia, like water, air, or electricity). It can only flow from 

 cold to hot by help of convection by matter or something else. 

 Such flow is therefore not a cyclical or perennial process. 



(Deduction). Energy of average temperature is useless for 

 continuous work. In other words, the only available or potential 

 portion of heat-energy, when dealt with in the aggregate, is that 

 which a body is able Ireely to emit to colder bodies. 



(Definition). The absolute temperature of a body is to its total 

 heai-energy as the available fall of temperature is to its poteniial 

 heat-=nergy. 



Oliver Lodge. 



THE ROYAL SOCIETY SOIREE. 



T^HE Royal Society Soiree on May 10 was in every way most 

 ■* successful. It was very numerously attended, and much 

 interest was excited by many of the exhibits and by the demon- 

 si rations. In the following account of the exhibits we give a 

 full account only of such objects as have not before been relerred 

 to in Nature :- 



Captain Abney, C.B., F.R.S., and General Fesling, F.R.S., 

 exltibited experiments on the extinction of light and colour. 



Sir J. B. Lawes, Bart., F.R..S., and Dr. J. H. Gilbert, 

 F. K.S , exhibited a series of photographs relating to the work- 

 ing of the Rothamsted Laboratory. In experiments on the 

 growth of root-crops year after year on the same land, it was 

 found ihat after a very few years of growth without manure, 

 the root no longer developed the swollen character of the 

 cultivated plant, but remain fusiform as in the uncultivated con- 

 dition. The photographs strikingly show the same characters 

 in roots grown in rotation without manure ; also that mineral 

 manures alone greatly favour the development of the swollen 

 root, but that mineral and nitrogenous manures together do so 

 in a much greater degree. The results further show how 

 artificial a product is the cultivated root-crop, and how 

 dependent it is on an abundant supply of food within the soil — 

 nitrogenous as well as mineral. Indeed, details of the experi- 

 ments afford conclusive evidence that it is quite fallacious to 

 suppose that root-crops derive a large amount of their nitrogen 

 from atmospheric souices by means of their extended leaf 

 surface. 



Three instruments for the study of Crystals were shown by 

 Mr. H. A. Miers. (i) A goniometer by which crystals can be 

 measured under liquids, or during their growth from solution. 

 (2) A stage goniometer by which small cr)'stals or fragments 

 can he adjusted and rotated under the microscope. It is here 

 fitted to the stage of the petrological microscope designed by 

 Mr. A. Dick. (3) An improved form of polariscope on the 

 plan ilevised by Prof. W. G. Adams, F. R.S. The hemispheres 

 which enclose the crystal section can in this instrument be 

 accurately centred so that exact measurement of the optic axial 

 angle is possible. 



Irof. Riicker, F.R.S., and Prof. Thorpe, F.R.S , exhibited 

 maps showing the forms of the true lines of equal declination, 

 equal horizontui force, and equal dip in the United Kingdora 

 for the epoch, [an. I, 1891. 



Mr. A. A. C. Swinton exhibited high frequency electric 

 experiments. (l) The filament of an ordinary 5 c.p. loovolt 

 lamp is caused to incandesce with current conveyed through the 

 human body. (2) Sparks, evidencing a difference of potential 

 of some thousands of volts, produced between the two hands 

 of the same operator. (3) Luminous spiral produced in ex- 

 hausted glass tube by molecular bombardment fr mi wire spiral 

 wound outside tube. (4) Some effects produced by high fre- 

 quency discharges passing through semi-conducting sub^tances, 

 and striking liquids. 



Sodium potassium high temperature thermometers, and speci- 

 mens of thealloy, were exhibited by Mr. E.G. C. BiilyandMr. J. 

 C. Chorley. These thermometers are filled with an alloy of sodium 

 and potassium which is liquid at ordinary temperatures. Their 



