8 
Transactions of the Royal Microscopical Society. 
II. — An Explanation of the “ Brownian ” Movement. 
By W. N. Hartley, F.B.S.E., King’s College, London. 
(. Read before the Royal Microscopical Society, June 6, 1877.) 
If tlie view be correct that the attraction of bubbles by heat and 
the constant vibration of minute bubbles, described in my two 
papers presented by Professor Stokes and lately read before the 
Eoyal Society, are due to the alteration in surface tension of the 
liquid on one side of the bubble consequent on an alteration in 
temperature, it follows that light solid particles beneath the surface 
of water should be attracted by a source of heat. In order to deter- 
mine this point, I made the following experiments : 
Gamboge was rubbed up in water, and observed under the 
microscope to be in irregular motion. 
Application of a hot wire caused the particles to be attracted 
and to become stationary. 
Water contained in four shallow glass dishes was dusted over 
with gamboge, carmine, and plumbago. 
The liquid was stirred up to prevent the particles floating. 
When a heated wire was plunged below the surface of the 
water, the immersed particles of those substances were energetically 
attracted. 
When a particle immersed in water is attracted by heat it is 
because the surface of liquid surrounding and in contact with 
the solid has its tension diminished on that side to which the warm 
wire is presented, hence the tension of the liquid on the opposite 
side urges it forward. When the “ Brownian ” movement occurs 
in a simple liquid like water, and not in a mixture of two liquids, 
and when the motion is a constant and steady vibration or irregular 
shuffling of very minute particles, I find that, as in all other cases, 
the movements cease, and attraction is caused by the approach of 
a source of heat. I conclude that, as in the case of minute bubbles 
in constant motion, the cause is the continual passage of heat 
through the liquid rendering alternate sides of the particles the 
warmer, and consequently diminishing the tension of the opposite 
liquid-surface in contact with the particle. 
Very energetic movements are noticed in water when alcohol 
is mixed with it. Solid particles seem to be swept along in currents, 
and are hurried hither and thither. A number of causes are at 
work in such a case. The alcohol and water are not liquids of the 
same density, nor having the same surface tension ; moreover, their 
mingling causes an evolution of heat, hence currents may be easily 
set up. And again, evaporation from the surface of the liquid or 
from the edges of the thin covering glass may cause movements 
similar to those which cause the tears of wine which Professor 
