350 
ANATOMY: H. H. DONALDSON 
be detected by the other observers because of their low frequencies of 
alternation. The result was that the curves of these observers gave them 
apparent abnormal increases in the mobility of the negative ion, which 
increased in value with decreasing pressures. In my experiments these 
electrons were of course absent, and so no apparent abnormal increase 
was obtained. 
The conclusion to be drawn from these results seems to be that the 
^cluster' theory, which has until now been most generally accepted, is 
not correct. This forces us to accept the ^ small ion' theory in some 
form or other. 
Summary. — 1. The mobilities of positive ions have been determined 
in electric fields very nearly strong enough to cause ionisation by col- 
lision at atmospheric pressures and have been found to be perfectly 
normal within the limits of error of the measurement. 
2. The mobilities of the negative ions have also been determined, 
under the same circumstances, with the result that they not only showed 
no relative abnormal increase in value over those of the positive ion, 
but also showed a perfectly normal absolute value of the mobiHty. 
3. These results, though at variance with those of most observers at 
low pressures for the negative ions, are in good agreement with recent 
results of Wellisch,^ and likewise lead to the conclusion that the ^ cluster' 
theory is no longer tenable. 
^ Langevin, Thesis, Ann. Chim. Phys., Paris, 1902. 
2 Wellisch, London, Phil. Trans. R. Soc, A, 209 (1909). 
3 Rutherford, Cambridge, Proc. Phil. Soc, 1898. 
4 Todd, Phil. Mag., London, June, 1913. 
^ Wellisch, Amer. J. Sci., New Haven, May, 1915. 
^ Townsend, Electricity in Gases, 1914. ' 
' Lattey, London, Proc. R. Soc, A, 84, 1910. 
^Kovarick, Physic. Rev., Ithaca, 1911. 
9 Chaffee, Boston, Proc Amer. Acad., 47, No. 9, 1911. 
THE RELATION OF MYELIN TO THE LOSS OF WATER IN 
THE MAMMALIAN NERVOUS SYSTEM WITH 
ADVANCING AGE 
By Henry H. Donaldson 
WISTER INSTITUTE OF ANATOMY AND BIOLOGY. PHILADELPHIA 
Read before the Academy, April 17, 1916. Received, June 6. 1916 
Starting from birth, the water-content of the mammalian body 
diminishes with age, and the same statement holds for the several 
anatomical systems which compose the body (LowreyO • My own studies 
have been made on the albino rat in which the changes in the water- 
