KNOWLEDGE & SCIENTIFIC NEWS. 



[July, 1905. 



second and succeeding- years after their application. 

 Other experiments deal with calcium cyanamide, the 

 new manure containingf nitrog-en derived from the 

 atmosphere, and with the various cuhivations of bac- 

 teria which have been recently introduced for the inno- 

 culation of leg^uminous crops, with the view of makinj^ 

 them more elKcient collectors of atmospheric nitrogen. 



During the year in question seven papers have been 

 issued from the Station in the " Transactions of tlic 

 Chemical Society," the "Journal of .Agricultural 

 Science," &c., all of which deal with investigations on 

 the soil, methods of soil analysis, &c. The annual losses 

 of carbonate of lime in the Rothamsted soil has been 

 determined, both that due to natural agencies and that 

 caused by the use of manures. Certain restorative 

 actions have been investigated which account for the 

 maintenance of the fertility of many soils which are 

 almost devoid of lime. .Another of the papers de:Js with 

 the remarkable accumulations of fertility in certain 

 plots of land which have been allowed to run wild for 

 the last twenty years, and have in that time gained 

 nitrogen to an extent not readily explicable by the 

 accepted theories. 



A considerable list of investigations in progress is 

 indicated, in which respect the Station receives con- 

 siderable help from several voluntary workers, e.g., two 

 Carnegie Research scholars from the University of 

 Hdinburgh, and other post graduate students from 

 Oxford and Cambridge are accommodated and provided 

 with material for investigation, so that the Station, with 

 its unrivalled opportunities for research, is becoming .i 

 training ground for experts in agricultural science. 



The Lawes Trust Committee continue to find their 

 income very inadequate to the proper development of 

 the Station, only donations and subscriptions from 

 various sources, including £300 from the Goldsmiths' 

 Company, £50 from the Clothworkcrs' Company, £.^0 

 from Lord Rothschild. &c., have prevented a serious 

 deficit on the year's working. Mr. J. F. Mason has also 

 promised to erect and equip a new laboratory for agri- 

 cultural bacteriology, which will be the first of its kind 

 in this country, as a continuance of the experiments 

 carried on for many years by his father, the late Mr. 

 James Mason, at Eynsham Hall, Oxon. 



RoLdiation. Pressure. 



Pkof. J. H. i'owiiNG, I'.R.S., the newly-clectcd 

 President of the Physical Society, delivered an interest- 

 ing address before that Society on " Radiation Pres- 

 sure," of which the ff)llowing is an abstract : — 



" A hundred years ago, when the corpuscular theory 

 held almost universal sway, it would have been easier 

 to explain the pressure of light than it is to-day, when 

 it is certain that light is a form of wave-motion. The 

 means at the disposal of early experimenters were in- 

 adequate to detect so small a quantity; but if the 

 eighteenth century philosophers had been able to carry 

 out the experiments of Lcbedew and of Nichols and 

 Hull, and had they further known of the emission of 

 corpuscles revealed to us by the kathode stream and 

 by radio-active bodies, there can be little doubt that 

 Young and Fresnel would have had much greater diffi- 

 culty in dethroning the corpuscular theory and setting 

 up the wave theory in its place. The existence of 

 pressure due to waves, though held by Kuler, seems to 

 have dropped out of sight until Maxwell, in 1872, pre- 



dicted its existence as a consequence of his electro- 

 magnetic theory of light. The first suggestion that 

 it is a general property of waves is probably due to 

 Mr. S. T. Preston, who, in 1S76, pointed out the 

 analogy of the energy-carrying power of a beam of 

 light with the mechanical carriage by belting, and 

 calculated the pressure exerted on the surface of the 

 sun by the issuing radiation. It seems possible that in 

 all cases of energy transfer, momentum, in the direc- 

 tion of transfer, is also passed on, and that there is, 

 therefore, a back pressure on the source. Though there 

 is as yet no general and direct dynamical theorem ac- 

 counting for radiation pressure. Prof. Larmor has given 

 a simple indirect mode of proving the existence of the 

 pressure which applies to all w.ives in which the average 

 energy density for a given amplitude is inversely as the 

 square of the wave-length. He has shown that when a 

 train of waves is incident normally on a perfectly reflect- 

 ing surface, the pressure on the surface is equal to 

 E (i + 2«/L'), where E/2 is the energy density just out- 

 side the reflector in the incident train, U is the wave- 

 velocity, and u the velocity of the reflector, supposed 

 small in comparison with U. In a similar manner it 

 can be shown that there is a pressure on the source, 

 increased when the source is moving forward, decreased 

 when it is receding. It is essential, however, that we 

 should be able to move the reflecting surface without 

 disturbing the medium except by reflecting the waves. 

 Though Larmor's proof is quite convincing, it is inter- 

 esting to realise the way in which the pressure is 

 produced in the different types of wave-motion. In the 

 case of electro-magnetic waves. Maxwell's original 

 mode of treatment is the simplest. A train of waves is 

 regarded as a system of electric and magnetic tubes 

 transverse to the direction of propagation, each kind 

 pressing out sideways; that is, in the direction of 

 propagation. They press against the source from 

 which they issue, agairjst each other as they travel, and 

 against any surface on which they fall. In sound- 

 waves there is a node at the reflecting surface. If the 

 variation of pressure from the undisturbed value were 

 exactly proportional to the displacement of a parallel 

 layer near the surface, and if the displacement were 

 exactly harmonic, then the average pressure would be 

 equal to the normal undisturbed value. But consider 

 a layer of air quite close to the surface. If it moves up 

 a distance, y, towards the surface, the pressure is in- 

 creased. If it moves an equal distance, y, away from 

 the surface, the pressure is decreased, but to a slightly 

 smaller extent. The excess of pressure during the 

 compression half is greater than its defect during the 

 extension half, and the net result is an average excess 

 of pressure on the reflecting surface. Lord Rayleigh, 

 using Uoyle's Law, has shown that this average excess 

 should be equal to the average density of the energy 

 just outside the reflecting surface. In the case of 

 transverse waves in an elastic solid, it can be shown 

 that there is a small pressure perpendicular to the planes 

 of shear, that is, in the direction of propagation, and 

 that this small pressure is just equal to the energy 

 density of the waves. The experimental verification 

 of the pressure of elastic solid waves has not yet been 

 accomplished, but the pressure due to sound-waves 

 has been demonstrated by Altberg, working in Lebe- 

 dew's laboratory at Moscow, the pressure obtained 

 sometimes rising to as much as 0.24 dynes per sq. cm. 

 By me.'ms of a telephone manometer it was found that 

 through a large range the pressure exerted on a surface 

 was proportional to the intensity of the sound. 



" Both theory and experiment justify the conclusion 



