208 
MAGAZINE OF SCIENCE AND ART. 
The difference in the experimental results is ascribed 
by Prof. Stokes o the difference in the nature of the 
gratings employed by himself and by the German 
experimentalist, the substance of the diffracting body 
being supposed to exert an effect upon the polarization 
of the light, which is diffracted by it under a great 
obliquity; I learn from Prof. Stokes that he proposes 
to resume the experimental inquiry, and to test this 
supposition by employing gratings of various substance's. 
If the conjecture should prove to be well founded, it 
will greatly complicate the dynamical theory of light. 
In the mean time the hypothesis is one of importance in 
itself, and deserves to be verified or disproved by inde¬ 
pendent means. I would venture to suggest that it 
may be effectively tested by means of the beautiful 
Interference-refractor of i\I. Jamin, which the inventor 
has already applied to study the effects upon light 
produced by grazing a plate of any soluble substauce 
inclosed in a fluid. It is well known that the refractive 
index of bodies increases with their density; and the 
theory of emission has even expressed the law of their 
mutual dependence. That theory, it is true, is now 
completely overthrown by the decisive erperimentum 
crude of .Mil. Fizean and Foucault. It was there¬ 
fore, probable, a priori, that this law—the only one 
peculiar to the theory—should bo found wanting. Its 
truth has recently been put to an experimental test by 
M. Jamin. Water, it is known, has its maximum of 
density at about 40° of Fahrenheit; and accordingly, if 
Newton's law were true, its refractive index should also 
have a maximum value at the same temperature. This 
has been disproved by M. Jamin, by observing the inter¬ 
ference of two raj-s, one of which has passed through 
air, and the other through water; and thus the last 
conclusion of the emission-theory has been set aside. It 
would occupy too much of your time were'I to touch, 
e ven lightly, upon the subject of the chemical action of 
light and the many beautiful and important discoveries 
of the art to which it has given rise. I may, however, 
mention, as one of the latest of the marvels of photo¬ 
graphy, that JI. Poitevin has succeeded in producing 
plates in relief, for the purposes of engraving, by the 
action of light alone. The process depends upon 
the chanco in the affinity for water, produced by the 
action of light upon a tlrin plate of gelatine, which is 
impregnated with bichromate of potash. 
In the whole range of experimental science there 
is no fact more familiar, or longer known, than the 
development of Heat by friction. The most ignorant 
savage is acquainted with it; it was probably known 
to the first, generation of mankind. Yet, familiar os 
it is, the science of which it is the genn, dates back bnt 
a very few years. It was known from the time oflllack, 
that heat disappeared in producing certain changes of 
state in bodies, and reappeared when the order of those 
changes was reversed; and that the amount of heat, 
thns converted, had a given relation to the effect pro¬ 
duced. In one of these changes—namely, evaporation, 
a definite mechanical force is developed, which is 
again absorbed when the vapour is restored by pressure 
to the liquid state. It was therefore not unnatural to 
conjecture, that in all cases in which beat is developed 
by mechanical action, or vice Versa, a different rela¬ 
tion would be found to subsist between the amount e 
the action and that of the beat developed or absorbed. 
This conjecture was put to the test of experiment by 
Mayer and Joule, in 1S42, and was verified by the 
result. It was found that heat and mechanical poiocr 
were mutually convertible: and that the relation be¬ 
tween them was definite, 772foot-pounds of motive power 
being equivalent to a unit of heat- that is, to the 
amount of heat requisite to raise a pound of water 
through one degree of Fahrenheit. The science of 
Thermo-dynamics, based upon this fact, and upon a 
few other obvious farts or self-evident principles, has 
grown up in the bands of Clausius, Thomson, and Rau- 
Irine, into large proportions, and is each day making 
fresh conquests in the region of the unknown. Thus 
far the srience of beat is made to rest wholly npon the 
facte of experiment, and is independent of any hypo¬ 
thesis respecting the molecular constitution of bodies. 
The dynamical theory of heat, however, has materially 
aided in establishing true physical conceptions of the 
nature of heat : The old hypothesis of caloric, as a 
separate substance, was indeed rendered improbable by 
the experiments of Ilnmford and Davy, and by the 
reasonings of Young; but it continued to bold its 
ground, and is interwoven into the language of science. 
It is now clearly shewn to bo self-contradictory; and 
to lead to the result that the amount of heat in the 
universe may be indefinitely augmented. On the other 
hand, the identification of radiant heat with light, and 
the establishment of the wave-tbeorj r , left little doubt 
that beat consisted in a vibratory movement, either of 
the molecules of bodies or of the ether within them : 
still, the relation of heat to bodies, and the phenomena 
of conduction, indicate a mechanism of a more compli¬ 
cated kind than that of light, and leave ample room 
for farther speculation. The ooly mechanical hypo¬ 
thesis (so far as I am aware) which is consistent ruth 
the present state of our knowledge of the phenomena 
of heat, is the theory of molecular vortices of Mr. Ran- 
kine. In this theory all bodies are«npposed to consist 
of atoms, composed of nuclei surrounded with elastic 
atmospheres. The radiation of light and heat is as¬ 
cribed to the transmission of oscillations of the nuclei; 
while thermometric heat is supposed to consist in circu¬ 
lating currents or vortices amongst the particles of 
their atmospheres, whereby they tend to recede from 
the nuclei, and to occupy a greater space. From this 
hypothesis Mr. Rankine has deduced all the lavs of 
tliermo-dj-namics, by the application of known mecha¬ 
nical principles. He has also, from the same princi¬ 
ples, deduced relations (which have been confirmed by 
experiment) between the pressure, density, and absolute 
temperature of elastic fluids, and between the pressure 
and temperature of ebullition of fluids. The dynami¬ 
cal theory of heat enables ns to frame some conjec¬ 
tures to account for the continuance of its supply, 
and even to speculate as to its source. The heat of the 
sun is dissipated and lost by radiation,- and must be 
progressively diminished unless its thermal eaergy he 
supplied. According to the measurements of lions. 
Ponillet, the quantity- of heat given ont by the sun in 
a year is equal to that winch would he produced by the 
combustion of a stratum of coal seventeen miles in 
thickness; and if the sun’s capacity for heat be as¬ 
sumed equal to that of water, and the heat he supposed 
to be drawn uniformly from its entire mass, its tempe¬ 
rature would thereby undergo a diminution of 20-4 
Fahr. annually. On the other hand, there is a vast 
store of force in our system capable of conversion into 
heat. If, as is indicated by the small density of the 
sun, and by other circumstances, that body has not yet 
reached the condition of incompressibility, we have, jn 
the future approximation of its parts, a fond of heat 
probably quite largo enough to supply- the wants of tho 
tinman family to the end of its sojourn here- It has 
been calculated that an amount of condensation which 
would diminish the diameter of the snn by only tho 
ten-thousandth part, would suffice to restore the heat 
emitted in 2000 years. Again, on our own earth, vis 
viva is destroyed byfrictionin the ebb and flow of every 
tide, and must therefore reappear as heat. The amount 
of this must be considerable, and should not be over¬ 
looked in any estimation of the physical changes of our 
globe. According to the computation of Bessel, 25,000 
cubie miles of water flow, in every six hoars, from ona 
quarter of the earth to another. The store of meeba- 
nirnl force is thus diminished and the temperature of 
our globe augmented by every tide. We do not possess 
the data which would enable ns to calculate the mag¬ 
nitude of these effects. All that we know with cer¬ 
tainty is, that the resultant effect of all the thermal 
agencies to which the earth is exposed has undergone 
no perceptible eliango within the historic period. IVc 
owe this fine deducriou to Arago. In older that the 
date palm should ripen its fruit, the mean tempera- 
