REPORT OP THE SECRETARY 11 



LOW TEMPERATURE RESEARCHES 



The Smithsoniun has made two small grants during the year 

 to Prof. Dr. W. H. Keesom, director of the Cyrogenic Laboratory 

 at Leiden, in aid of his important researches on the properties 

 of matter at very low temperatures. Two investigations were in 

 progress by Doctor Keesom, with the aid of his collaborators, namely, 

 the measurement of the specific heat of gases at very low tempera- 

 tures, and the measurement of the thermomolecular pressure differ- 

 ences at very low temperatures. 



In connection with the lirst, it seemed desirable to obtain measure- 

 ments on the specific heat of helium gas at the temperatures obtain- 

 able with liquid helium. Such a measurement had already been 

 made by Meissner in Berlin, who found the specific heat of helium 

 gas at a temperature of 5.5° K. (approximately —450° F.) and a 

 pressure of 0.75 atmosphere to be about 65 per cent of the normal 

 value, and ascribed this result to quantum effects. There is reason 

 to doubt, however, whether quantum effects can be demonstrated in 

 such a way because of the influence of intermolecular forces on 

 specific heat. To investigate this matter. Doctor Keesom and his 

 assistants elaborated a method of measuring the velocity of propaga- 

 tion of sound at these very low temperatures and at pressures smaller 

 than 1 atmosphere. From this velocity, the specific heat may be 

 derived. 



Measurements of this velocity have already been made with great 

 accuracy at the temperatures of liquid oxygen and of liquid h3'dro- 

 gen, and preliminary measurements have been made of the velocity 

 of sound in helium gas at the temperature of liquid helium, but 

 further developments in the method must be made for this last 

 investigation. 



Doctor Keesom's second research relates to the investigation of 

 the thermomolecular pressure differences between the bulb of the 

 helium thermometer and the manometer used in the measurement of 

 the lowest temperatures obtainable. In his latest measurements the 

 temperature recorded was 0.89° K. (approximately, =458° F.). Ex- 

 act measurements of these pressure differences have now been made 

 at the temperatures of boiling oxygen and of boiling hydrogen; 

 measurements at the temperatures obtainable with liquid helium will 

 follow. 



DIVISION OF RADIATION AND ORGANISMS 



The report of the director on the first year's work of this new 

 and impoitant branch of the Smithsonian's investigations in physical 

 science shows remarkably rapid progress. The construction of labo- 

 ratories and their equipment has been particularly difficult because 



