368 
PHARMACEUTICAL SOCIETY, EDINBURGH. 
motion of the particles is developed ; by the passage of ice into water and subsequently 
into steam, when the molecular motion gives rise to much dilation, and by the expansion 
of the liquid in the thermometer, which is a measurer of the intensity of heat, by the 
motion of the particles of the fluid. Heat becomes electricity, as observed in the thermo¬ 
electric batteries, where the heating of two metals in contact with each other developes 
electricity. Heat becomes light when it impinges upon a solid body, as by the introduction 
of a piece of platinum or infusible fire-clay in the flame of hydrogen, when a brilliant light 
is developed from the heat of the burning gas. Heat becomes chemical action, as in our 
fires and gas-jets, the combustibles in which will not burn till a lighted taper is applied, 
and the heat so brought in contact with them determines chemical affinity between the 
combustible and the oxygen of the air; and in the explosion of gunpowder by the 
application of a heated rod, when the heat determines a new chemical arrangement of 
the particles of the gunpowder. 
Electricity becomes motion when the lightning flash strikes a house and dislodges 
the stones. Electricity becomes heat when a powerful current is arrested in its course 
by its passage along a thin wire, or when the lightning flash sets fire to combustible sub¬ 
stances. Electricity becomes light when it bridges over a short space between two char¬ 
coal points, and gives rise to the electric light. Electricity becomes magnetism when 
the force conveying the current is wound round a piece of soft iron, which is immediately 
converted into a powerful magnet. 
Light becomes chemical action, as in the photographic processes, where it developes a 
change in the chemical nature of the substances on the photographic plate ; and in the 
formation and fading of natural and artificial colours. The sun’s heat and light, lying 
dormant in the coal and wood, become chemical action during the combustion, which 
then developes itself as heat and light once again. 
Magnetism becomes motion when a magnet draws or moves towards itself the iron 
keeper or nails which are placed in its vicinity, and the development of motion from 
magnetism is most clearly evidenced in the electro-magnetic machines which revolve 
with greater or less rapidity. Magnetism is ccnverted into electricity in the magneto¬ 
electric machines which are now so largely employed for electro-plating and electro¬ 
gilding, and for telegraphic communication. 
Chemical affinity becomes motion during the combination and decomposition of sub¬ 
stances when there is a molecular rearrangement of the atoms. Chemical affinity 
developes itself as heat in the operations which occur in our ordinary fireplaces, where 
the chemical union of the oxygen of the air with the coal gives rise to heat. Chemical 
affinity passing into electricity occurs in every kind of galvanic battery where the che¬ 
mical action of the acid liquid upon the zinc plates developes a sensible amount of elec¬ 
tricity. Chemical affinity becomes light, as evidenced in the burning of every candle 
and gas jet, where the oxygen of the air enters into chemical union with the oil or gas, 
and yields light. 
The foregoing illustration of the correlation of the physical forces will suffice to show 
that one form of force can pass into and become another form of force ; and that as there 
is no loss of matter on the surface of the earth, there is likewise no loss or annihilation 
of force. The sun, which is not only the centre and mainspring of our planetary sphere, 
but is also the great reservoir of force for the planetary orbs, is unceasingly throwing 
out force, and as unceasingly absorbing force from planetary space. The heat thrown 
off from the surface of the sun is equal to what would be evolved by the combustion of 
three-fifths of a ton of coal on each square foot of surface during each hour, and the 
temperature is as much higher than our artificial sources as the light of the sun is above 
all artificial sources of illumination. The heat evolved by the sun is so great, that a 
cylinder of ice forty miles in diameter driven with the velocity of light, viz. 192,000 
miles in a second, would be liquefied before it reached the sun; and during every minute 
the sun radiates forth heat sufficient to boil 12,000,000,000 of cubic miles of ice-cold 
water. The proportion of the heat of the sun which falls the upon earth is less than 
a 2,000,000,000th of the whole heat-force evolved from that great calorific luminary, 
but this moiety is sufficient for the mighty physical changes which occur in this com¬ 
paratively pigmy world, but to us a great globe. The sun’s heat and light are absorbed 
by every plant, and become stored up in the wood and food supplied by the vegetable 
kingdom, so that when the wood is burned in our fireplaces, the heat and light of 
the sun are evolved for economic purposes ; and when the food is partaken of by the 
