Vol. 6, 1920 BOTANY: F. V. COVILLE 435 
The fact that our northern trees and shrubs after they become dormant 
in the fall require a period of chilling before warm weather will start them 
into growth again, is a protective adaptation of the highest importance to 
these plants, for if warmth alone would start them into growth they would 
begin growing in Indian summer and the stored food that the plant re- 
quires for its normal vigorous growth in the following spring would be 
wasted in a burst of new autumn growth, which would be killed by the 
first heavy freezes, and would be followed by a winter of weakness and 
probable death. 
Further investigations on the effects of chilling are urged upon those 
engaged in experimentation bearing on the improvement of horticultural 
and agricultural practices. It is desirable especially to determine the 
proper temperatures for the storage of different kinds of seeds, bulbs, cut- 
tings, and grafting wood; proper temperatures for the treatment of plants 
which are to be forced from dormancy to growth at unusual seasons; 
and proper temperatures for the storage of nursery stock, so that the 
nurseryman may have plants in proper condition for shipment on any 
date he desires. 
ON THE NATURE OF THE NEGATIVE CARRIERS PRODUCED 
IN PURE HYDROGEN AND NITROGEN BY PHOTO- 
ELECTRONS 
By Leonard B. Lokb* 
Ryerson Physical Laboratory, UNivERSirY of Chicago 
Communicated by R. A. Millikan, June 5, 1920 
As the result of an investigation of the cause of the abnormalities of the 
negative ions in air at low pressures (which is to appear shortly), it was 
found that the results obtained could be explained quite satisfactorily 
on the basis of a theory proposed by Sir J. J. Thomson.^ This theory 
assumes that the electron does not attach to a neutral molecule to generate 
a negative ion on its first impact, but that on the average it will have a 
chance of uniting in one out of n impacts with a given type of molecule, 
where n is a constant which is a characteristic of the type of molecule 
considered. For air this constant was found to be in the neighborhood 
of 2.5 X 10^. If the oxygen molecule is the molecule to which the nega- 
tive electron attaches in air — a point of view for which there is consider- 
able evidence — this means that in only one out of 5 X 10"^ encounters 
with oxygen molecules does the electron have a chance of attaching itself 
to a molecule to form a negative ion. It accordingly seemed of interest 
to see how nitrogen and hydrogen molecules behaved in respect to this 
theory, and to determine n for them if possible. 
