440. Proceedings. 
ments can be extended it may be assumed that the heat expended in radia- 
tion is simultaneously re-developed in the photosphere of the sun. Before, 
then, we can say what heat-energy at the surface of the sun, the work its 
rays do at this distance, implies we have a great deal to learn, and a totally 
new series of experiments to make. 
Other considerations lead us to the same doubt of the reliability of the | 
estimates that have hitherto been made of the solar temperature. What is 
it that we call aray of heat from the sun? It is at the earth’s surface 
a vibration of the ether, a series of waves whose lengths may vary some- 
what, but is never very different from 55005 part of an inch, travelling ata 
speed somewhat less than 200,000 miles per second. We have only to 
draw together as many of them as will pass through an aperture measuring 
a few square feet, and at the point of their intersection they cause such a 
commotion as will dissipate into vapour any terrestrial substance whatso- 
ever. But if we let the focal point form in mid air, or in any gas, nothing 
happens to give us evidence of this storm of molecular motion. Yet, if it 
were what we call an enormous temperature that existed there and 
vapourized the granite which we subjected to ‘its action, it must be still 
present when nothing is presented to its action. Since then these rays 
emanated from masses of incandescent gas and vapour, the molecular 
motion, which we call the temperature of the sun, may differ very little 
from that induced in the molecules of a gas in which a bundle of these rays 
is concentrated by our lens. The sun’s rays, which at once drive asunder 
the molecules of a solid body until they have assumed vibrations and 
motions amongst themselves, which define the condition we call vapour, 
will pass through that vapour itself with scarcely any effect upon it, so far 
as we have been able to observe their action. Is it necessary to suppose 
that there is no limit to that molecular agitation we call heat? or that the 
molecules of the incandescent gases of the photosphere must vibrate @ 
million times more energetically than those which exist at ordinary tem- 
peratures on the surface of the earth? Again, we may well ask what 
meaning do we attach to “increase of temperature ?’’ What is the change 
in the nature of the vibration we call heat which corresponds to our words 
hotter and colder? As we at present conceive it, increased temperature 
means a greater amplitude of vibration in the heated molecule. Must there 
not be a limit to the excursions of the swinging atom? In that train of — 
vibrations, which we call a ray of heat, we can approximately measure the 
os e from one wave crest to another, and, so far as we know, the 
bh aaa in this distance for different temperatures is proportionately very 
ae cate if then the amplitude of vibration, the height of the wave, 
continually increased, is it not certain that a point must be reached at 
