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1. The experiments described in the following paper form part of an investigation with which one of the present authors has been engaged for many years past. The object of this investigation is the direct determination, in absolute measure, of the loss of energy, from a heated body to cooler surroundings, under differing conditions as to (1) the dimensions of the cooling body; (2) the nature of the surfaces of the cooling body and of the enclosure; (3) the mean absolute temperature of the cooling body and the envelope; (4) the nature of the atmosphere surrounding the cooling body. Several papers have already been published on this subject in the ‘Philosophical Transactions of the Royal Society’ and in the ‘Proceedings,’ and have been communicated to the British Association and elsewhere. In all of these the lowest temperature available was the atmospheric temperature, or rather the temperature of the water supply of the laboratory; and the body from which radiation was to take place was heated to a temperature as much above the atmospheric temperature as the circumstances connected with the experiment would, from time to time, permit. Thus the upper and lower limits of these experiments have been something below 1000° C. as the highest temperature of the cooling body, and, say, 10° C. as the lowest temperature of the enclosure. 2. Some time ago, through the kindness of Lord Blythswood, F. R. S., an unlimited supply of liquid air was placed at our disposal, to be followed later by a supply of liquid hydrogen. It seemed therefore most desirable to extend the research in the direction indicated by the new facilities given to us, which both greatly extended the limits governing the enquiry, and also made it possible to obtain determinations, in absolute measure, of thermal radiation between bodies at extremely low temperatures. It seemed specially interesting, for example, to determine, in absolute measure, the radiation under given circumstances from a body at, say, ordinary atmospheric temperature, to an enclosure at a temperature which may perhaps approximate to that of space. With the exception of some preliminary results, which were communicated 18 months ago to the British Association, nothing of the kind has been published, so far as we are aware.

In a series of papers the author, partly in collaboration with the late Prof. H. Kamerlingh Onnes, has recently published the results of some investigations on paramagnetism at low temperatures. Included in this work were the measurements of the principal susceptibilities of two crystals (cobalt ammonium sulphate and nickel sulphate) at temperatures ranging from about 300° K. (atmospheric temperature) down to the lowest temperature obtainable with liquid hydrogen, 14° K. These data, with the exception of those of Foëx for siderose, are the only ones yet obtained for the principal susceptibilities of paramagnetic crystals at low temperatures over any extended range of temperature. The results showed that, at not too low temperatures, the principal susceptibilities Z 1 , Z 2 , Z 3 follow the law Z n (T + Δ n = C, ( n = I, 2, 3), in which T = absolute temperature, Δ n is a constant, and C is the Curie constant which has the same value for each of the principal magnetic axes of the crystal. The constants Δ n are intimately connected with the structure of the crystal, being a function of the “spacing” of the paramagnetic atoms in the corresponding directions in the crystal. The precise connection between these quantities could not, however, be deduced, firstly, because of the present scantiness of the data, and, secondly, because the accuracy with which the Δ’s could be determined was small. The susceptibilities themselves were determined with an accuracy of about 1 per cent., but, since Δ is only an additive constant, the error in its determination is greater than that of the determination of the susceptibility. Other interesting points were raised by the results, and a continuation of the research seemed likely to give results of considerable theoretical importance and interest. The present work was undertaken in continuation of that just mentioned. It was decided to carry out the measurements over a range of temperature of from atmospheric temperature down to the lowest obtainable with the aid of liquid air, and to aim at reaching an accuracy of one part in a thousand in the measurements. For this purpose the apparatus described in the following pages was designed and constructed. I he desired accuracy was not attained in the first measurements given later in the present paper, the accuracy of these being about 1 per cent. As experience was gained with the apparatus, it was seen that the desired accuracy was probably attainable when special attention was paid to the working conditions, in particular the constancy of the temperature.

The observations recorded in this paper may be regarded as an Appendix to a former paper, elucidating a point which remained unsettled. It has been shown if stalloy in laminated ring form is subjected to a magnetising force due to a direct current, and ringing from 3 to 14 C. G. S. units, whilst it is cooling through the temperature at which it regains its magnetic properties, and is at the same time shielded from the influence of the earth's magnetism, the permeability recovered at atmospheric temperature has a maximum value of over 10,000 when the magnetic induction is of the order 6000-7000 C. G. S. units. It had been shown previously that high values of the permeability can be obtained without the use of a special magnetic shield, when the iron has impressed upon it a magnetising force due to an alternating current, during the time that it cools through the temperature at which it regains magnetic properties. The authors of this last mentioned paper, however, used an iron tube to inclose the specimen, and the tube itself was heated with the specimen. It seemed, therefore, desirable to discover whether these high values of the permeability can be obtained when there is no question of magnetic shielding.