131 
by counting the collections of organisms that are caught and devel- 
oped on appropriate glass plates prepared according to the methods 
of Miquel at Montsouris. Their observations show that in 39 cases 
out of 40 the catch of germs within the forest is less than the catch 
outside the forest, the average ratio being as 3 to 1, so that the forests 
act as a strainer upon the organisms carried by the wind. Wollny 
suggests that the result would be even still more decided if the wind 
were stronger and the forests more extensive. (Wollny, Forschun- 
gen, 1891, XIV, p. 176.) 
ATMOSPHERIC ELECTRICITY. 
IN GENERAL. 
The relations of atmospheric electricity to vegetation and crops 
are too little understood to justify any attempt to present this sub- 
ject. In fact, it does not seem clear that any appreciable influence 
is exerted by this atmospheric or geophysical element upon the 
development of plants. In natural conditions evaporation is un- 
doubtedly facilitated by the dissipation of an electric charge, but 
we do not know that transpiration is at all affected by it, and have 
no reason to think that assimilation is affected by it. The passage 
of an electric current through the earth in proximity to the roots 
may affect the decomposition of the soil and setting free of nutritious 
substances or may affect the temperature of the soil. A few experi- 
ments have been made to show that artificial earth currents stimulate 
the growth of the plant, but nothing has yet been found to show that 
under natural conditions electric currents. have any appreciable 
influence. Nevertheless, observations are made regularly at some 
stations, such as Kew, Montsouris, Potsdam, and at a few agricul- 
tural experiment stations. 
An excellent series was maintained for many years by Wisliczenus 
at St. Louis, Mo., a summary of which is published in the transac- 
tions of the Academy of Science at St. Louis and also at page 65, 
Report of the Chief Signal Officer for 1871. The following table 
gives the monthly means for Montsouris and for St. Louis. The 
record for Montsouris expresses the potential in units of 1 Daniell 
cell, which is approximately 1 volt at a point 2 meters above the 
soil and 1 meter from a wall, for the calm days of the years 1880 to 
1887. The record for St. Louis gives the electric intensity on a scale 
