While regular use is being made in industry of the thermal insulating properties of powders and other fibrous or cellular materials, the physical principles underlying the processes of heat transfer through such bodies have not received much attention. The problem, in regard to powders, has been studied by Smoluchowski in 1911, and later by Aberdeen and Laby. Smoluchowski’s investigation included the effect of such factors as the grain size, the kind, and density of the gas on the conductivity of powders of different materials. The experimental procedure followed in his work is unfortunately open to serious objections, and in consequence the data obtained by Smoluchowski are probably not reliable. Further the method, a cooling one, could not be applied to powders having high conductivities. In this laboratory, Aberdeen and Laby have studied the conductivity of the insulating powder silox filled with several gases, as a function of the gas pressure. The powder was contained between two concentric cylinders and the heat transfer was measured across a central portion where the flow was shown to be strictly radial. Platinum thermometry was used so that reliable data are to be expected. Their experiments, however, were limited to a single powder, and, at the suggestion of Professor Laby, a more extensive investigation using powders of different materials and of various grain sizes has been undertaken.

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of the result with the figures given by Smith and Giebe (1-00052 and 1-00051 1 respectively) shows th a t with a little more elaboration very high accuracy j should be attainable. The use of the method for accurate measurement U f frequency will be described elsewhere.The Absorption of X-Rays. Introduction.An account is given here of the measurement, by a balance method, of the mass-absorption coefficients of a num ber of elements, primarily relative to aluminium, over a range of wave-lengths from 0-3 to 0-7 A.U., and of the absolute coefficient of aluminium itself for three wave-lengths.The main objection to the direct method of measuring absorption coefficients is the difficulty, with ordinary facilities, of obtaining an even approximately constant source of X-rays. This necessitates the use of some form of com pensation or comparison method.Siegbahn has described such a method. Two similar monochromatic ■ beams from the same X -ray tube are obtained with the aid of two " half spectrometers.The currents through the two ionisation chambers are opposed, and a null method is used, with an electrometer as detector. With I the absorbing screen in the path of one beam, the intensity of the second is 1 equally reduced by means of a rotating disc, the transm itting sector of which can be varied.The present method embodies essentially the same idea, but its much greater ? ! mplicity gives it an added advantage. A single spectrometer is used> the beams, one above the other, being reflected by a single crystal into the ionisation chambers, which are fixed together and replace the usual single chamber of the Bragg spectrometer. After the beams are equalised in intensity Phy8' Zeit" ' To1' **' P' 83 <1920,; WingSrdh' ,Zei*' '• on May 9, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from Absorption o f X-R ays. 313absorbing screen is placed in the p ath of one and the second is cut down al a balance is again obtained by moving across it a wedge of aluminium. tL the determ ination of the absorption coefficient of aluminium the wedge mused in conjunction with a rotating sectored disc.Apparatus. mnerah-A Coolidge medium-focus tungsten radiator-tube, the radiator ■red with a water-cooling jacket, and a medium-focus water-cooled molybffiim tube were used. These were supplied w ith unrectified current from » a t son 100-K.V. oil-immersed transform er, working from the town mains, he general arrangem ent of apparatus is shown in fig. 1. The anticathodes j£e about 40 cm. from the crystal, the angles of incidence of the X-rays igag measured from a 24-cm. circle. >ifhe X-rays from the anticathode, A.C., pass through an aperture in the fed box, B, and are divided into two beams, slightly divergent, by the pairs cmlits, Sl5 S2 ; these are reflected from the single calcite crystal, C, through H further pair of slits, S3, into the ionisation chambers, V x, V2. The chambers at raised to opposite potentials of 200 volts. E ither or both of the insulated fiictrodes, B^, R 2, m ay be connected to the electrometer by means of the &s...