164 
indeed, modern ones, should have mistaken them for animal - 
cuke. The particles which exhibit the greatest activity are 
exceedingly minute, ranging from 10,000th to 30,000th of 
an inch in diameter ; they remain active a considerable time 
if they are nearly of the same specific gravity as the solu- 
tion in which they are immersed. One simple mode of 
producing them is to rub a little gamboge in water, on 
a glass slide, and place a thin glass cover on it, using a 
power of from 800 to 1,200 diameters. They can easily be 
distinguished with less magnifying power, but are not so 
effectually shown. If they are required for prolonged 
examination, Dr. Brown recommends that the solution of 
gamboge be mixed with a little almond oil. The minute 
globules of water are thus surrounded by oil, and rapid 
evaporation is prevented. 
The cause of the phenomenon is not yet satisfactorily 
accounted for. Some have imagined that it is the physical 
repulsion of the particles when uninfluenced by gravitation. 
The author has tried many experiments with electricity 
and magnetism without success. He thinks that the 
movement may possibly be connected with the absorption 
and radiation of heat. 
Those interested in Dr. Brown’s experiments and observa- 
tions on active molecules, can refer to his republished 
papers in Yol. I. of the Ray Society’s publications for 1866. 
After the meeting, slides containing active molecules 
were exhibited to the members. 
A conversation took place on the preservation of dried 
plants, in which Mr. Bailey stated that he uses with suc- 
cess glue with carbolic acid for attaching them to the paper, 
as a preservation against mites, placing also a few crystals 
of the dry acid in his cabinet. 
