On the Motions and Collisions of Perfectly Elastic Spheres. 19 
remarked that every time I touched it the fluid im the electro- 
meter rose, indicating an increase of temperature, and implying 
also an increase of conducting power in the metal thus touched. 
I found that this was owing to a reduction of its temperature ; 
for on subsequently moistening it with ether, water, &c., or by 
blowing upon it, the fluid rose in the electrometer as the tem- 
perature was reduced, whilst the application of a spirit-lamp to 
increase the temperature of the wire produced a corresponding 
fall in the thermometer. Two electrometers were subsequently 
employed in circuit, the same current passing consecutively 
through them. To one of the electrometers a second battery 
was applied. The result was an increase of temperature of the 
included wire ; and I discovered that, by raising or lowering the 
second battery so as to gradually increase or diminish the tem- 
perature of one of the wires, the fluid as it rose and fell in that 
electrometer gave rise to a reverse motion of the fluid in the 
other, so that as one rose the other fell, and vice versd. 
Although these experiments were made more than thirty 
years since, I am induced to believe that they may still appear 
novel to some, since, in a conversation a short time since with 
one of the first electricians of the day, he would scarcely credit 
them, alleging that they were contrary to all our experience ; 
they must, however, be taken as indicating only the results due 
to the peculiar arrangements and conditions herein described. 
V. Illustrations of the Dynamical Theory of Gases.—Part 1. 
On the Motions and Collisions of Perfectly Elastic Spheres. 
By J. C. Maxweti, M.A., Professor of Natural Philosophy 
in Marischal College and University of Aberdeen*. 
So many of the properties of matter, especially when in the 
gaseous form, can be deduced from the hypothesis that 
their minute parts are in rapid motion, the velocity increasing 
with the temperature, that the precise nature of this motion 
becomes a subject of rational curiosity. Daniel Bernoulli, Hera- 
path, Joule, Krénig, Clausius, &c. have shown that the relations 
between pressure, temperature, and density in a perfect gas can 
be explained by supposing the particles to move with uniform 
velocity in straight lines, striking against the sides of the con- 
taining vessel and thus producing pressure. It is not necessary 
to suppose each particle to travel to any great distance in the 
same straight line ; for the effect in producing pressure will be 
the same if the particles strike against each other; so that the 
straight line described may be very short. M. Clausius has de- 
termined the mean length of path in terms of the average distance 
* Communicated by the Author, having been read at the Meeting of the 
British Association at Aberdeen, September 21, 1859, 
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