186 Atomic Matter and Luminiferous Ether. (April, 
The metal potassium, like other metals, is mechanically 
compressible only to a very limited extent : probably no means 
are known by which so great a weight or pressure could be ap- 
plied as to compress, say, 450 units of it into the bulk originally 
filled by 400 units. Faraday points out that a space originally 
filled by 430 units of the pure metal potassium, if filled by the 
same potassium when by chemical action made into carbonate 
of potash, would then contain 686 instead of 430 units of 
the same metal, and, in addition, 2744 units of oxygen and 
carbon. He shows that this power of compression is not 
restricted to carbonate of potash, nor to potassium, but is 
even more strikingly exemplified when some other sub- 
stances enter into chemical combination. No other force 
measured by gravity gives parallel manifestations. 
Comparing the mechanical equivalent of gravity with the 
same equivalent of other forces, gravity is thus proved to 
be feeble, and, requiring little effort to originate it; it yields 
small effects, and is the more likely to be very slowly dis- 
sipated. 
Perhaps an objection may be made that, although astro- 
nomically we see no effects of the loss of heat in the sun 
and planets, yet we see a hot body loses heat on the earth, 
but do not see it loses any weight whatever. 
The hot body is noticeable because it is hotter than its sur- 
roundings. Itonlyloses the excessof heat it had,and becomes 
cooler than the earth only when its surroundings are excep- 
tional, and are, so to say, means through which its heat energy 
escapes, and it remains of the same heat as the earth, when 
subjected to the same conditions. Heat is a mode of motion; 
communicated heat is communicated motion; by substi- 
tuting gravity for heat we can say gravity is a mode of 
motion, and communicated gravity is communicated motion. 
All substances are influenced by the motion of heat; 
analogically, all substances are influenced by the motion of 
gravity. The earth’s heat maintains the heat in its parts, 
and the action is reciprocal; so, too, the gravity of the 
earth and its parts are reciprocal, and similarly of all related 
gravitating bodies, and hence no loss of gravity is apparent. 
Mr. Crookes’s recent experiments on the weight of bodies 
in vacuo prove that heat and gravity have some connection, 
and motions at right angles to each other do influence one 
another, although each passes on distinctly. One of the 
cannon-balls before alluded to moves in a path which is 
neither wholly vertical nor horizontal, although the vertical 
and horizontal planes it passes through are those through 
which each force alone would have sent it in the same order 
