Vol. 8, 1922 
PHYSICS: FLETCHER AND WEGEL 
5 
up by this process. A sample of the material was therefore ground in an 
agate mortar and the finest particles were separated by stirrring up the 
material with water and allowing it to settle for 48 hours. The milky 
supernatant liquid was then removed and evaporated to dryness at low 
temperature. When sufficient material had been obtained in this way, 
the average size of the particles was determined with a micrometer mi- 
croscope and they were found to be 0 . 0023 mm. in diameter. The density 
of this material was then measured and found to be 2.224, an increase 
of 2% over the previous value, this showing definitely the presence of 
extremely small pore spaces. 
The facts, that (a) the index of the calcined flint agrees with that of 
cristobalite and ih) its density is much higher than that of the calcined 
chalcedony are readily explained by the presence of the impurities which 
are of such a nature as to act as a flux at high temperatures and thus to 
promote the growth of the cristobalite crystals. 
^ Amer. J. Sci. New Haven, 36, 331(1913). See also Ferguson and Merwin, Ihid.,A:6y 
417 (1917). 
THE FREQUENCY-SENSITIVITY OF NORMAL EARS 
By H. FivKTchkr and R. L. Wnonh 
Research Laboratories of the American Telephone and Telegraph Company 
AND Western Electric Company, Inc. 
Communicated by J. J. Carty and F. B. Jewett, November 14, 1921 
A large amount of work has been done during the last fifty years in an 
endeavor to determine in absolute terms the minimum amount of sound 
that the human ear can perceive. The results obtained by different 
investigators have varied throughout a very wide range. Two causes 
contributed to this; namely, that adequate apparatus was not available, 
and it was not appreciated that so-called normal ears vary so widely in 
their ability to hear. 
The development of the vacuum tube, condenser transmitter, and ther- 
mal receiver has given us precision apparatus for work of this kind. In 
this investigation an air damped receiver was held tightly against the 
ear by means of a head band. It was actuated by an alternating current 
which was sent from a vacuum tube oscillator having a range of frequencies 
from 60 cycles to 6000 cycles per second. By means of a specially con- 
structed attenuator the current entering the receiver could be varied 
approximately three millionfold. This was accomplished by moving 
a single dial switch. 
By means of condenser transmitters and thermal receivers, this system 
was calibrated so that from the reading of the attenuator dial switch and 
the electric current entering it, the alternating pressure impressed upon 
