When a piece of metal prepared by a commercial process is heated for the first time in vacuo , a considerable amount of gas is usually evolved. With the more refractory metals like nickel or molybdenum, evacuation for a few minutes at 1000° C is sufficient to remove part only of gas present, and prolonged heating for many hours is necessary before the evolution ceases to be measurable. It may be assumed that some part at least of this gas is derived from the body of the metal and must presumably reach the surface by diffusion. The total gas is extracted much more rapidly if the metal is actually melted, and this method of estimating the gases in steel and other metals has been very generally adopted. The fact that the evolution of gases from metals on heating depends upon a process of diffusion from the interior to the surface of the metal, and subsequent evaporation, has been established for the simple gases. The desorption of nitrogen from molybdenum, and of hydrogen from hydrogen-charged nickel has been shown to follow theoretical equations derived from Fick's diffusion law. In practice, the gases obtained from commercial metals often contain a large proportion of compound gases, particularly the oxides of carbon, but very little information is available on the diffusion of such gases through metals.
A COMPARISON of the products obtained by chlorinating the isomeric chlorotoluenes, and by brominating the isomeric bromotoluenes, shows that the two halogens possess a different orienting effect. Cohen and Dakin (T., 1901, 79, 1111) found that o-chlorotoluene, on chlorination in presence of the aluminium-mercury couple, gives mainly 2:4together with some 2:3-and 2:6-dichlorotoluene; whilst Cohen and Dutt (T., 1914, 105, 501) found that on brornination o-bromotoluene gives mainly 2 : 5-, with a smaller quantity of 2:4-dibromotoluene. I n the case of the meta-and para-substituted toluenes the same products were obtained in both cases. It was thought that some further light might be thrown 6~2
This fits in well with the fact that s2 takes only the discrete eigenvalues / (/ + 1) /*2, where 21 is an integer. 5-S u m m a r yBorn and Infeld derive the co-ordinates of the energy centre and the inner angular momentum of a closed electrodynamical system, but these do not have simple commutation laws as in non-relativistic quantum mechanics. It is here shown that a co-ordinate vector and an inner angular momentum can be defined which do obey these simple com mutation rules. They are rather artificial from a relativistic point of view; nevertheless, the absolute magnitude of the inner angular momentum vector is a relativistic invariant. The Diffusion of Gases through MetalsBy C. J. S m it h e l l s , D.Sc., and C. E. R a n s l e y , B.Sc. {Communicated by R. H. F o w l e r , F.R.S.-ReceivedSince the original discovery by Graham in 1866 that hydrogen could diffuse through platinum, the phenomena of diffusion of gases through metals have been the subject of many investigations. Various empirical equations have been proposed to represent the effect of temperature and pressure on the rate of diffusion, and in 1904 Richardson, Nicol, and Parnell* arrived at an equation from theoretical considerations which was in good agreement with their own measurements of the diffusion of hydrogen through platinum. This equation, generally known as Richard son's equation, may be writtenwhere D is the rate of diffusion per unit area of surface, P is the gas pressure, T the temperature, d the thickness of the metal, and b a constant for the gas-metal system. The equation applies to conditions in which gas is maintained at a pressure P on one side of the metal, whilst a vacuum is maintained on the other side. If instead of a vacuum a pressure Pi, * ' Phil. M ag
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