Otago Institute. 439 
scientific men, armed with all the knowledge of the laws and character of 
heat which has already been accumulated; It appears to mo that this esti- 
mate is the absolutely necessary basis of any such calculations as Secchi, 
Ericsson, and others have been attempting. 
Again, as the basis appears to me yet wanting, so also is it with the 
methods of working back from it to the desired object—the temperature of 
the sun, and on this point, also, I ask leave to submit one or two considera- 
tions. 
We have discarded from our minds the old idea. that heat is a distinct 
unponderable substance, which so many considerations force upon us, that 
it is only a vibratory motion in what we call ether and in the ultimate 
molecules of those bodies which display it. Nevertheless, we are a long way 
from being able to conceive distinctly the character of this motion which we 
call heat. The discussions which have taken place on this subject of solar 
temperature and radiation appear to me to be clouded by the old notion of 
heat as a communicable substance. Not that any such idea was present in. 
the minds.of eminent men who have busied themselves with the question. 
But the want of a terminology in which to express definitely our modern 
ideas of heat makes itself felt the moment that any discussion of this sort 
is enterprised. This willremain very much the same until we can cease to 
speak of heat and define its manifestations as this or that species of molecular © 
action, and we are a. very long way yet from this desirable position. When 
we speak of the effects of solar radiation, we are, after all, only relating the 
effect this. will produce in the body we use as a heat measurer—say in the 
mercury of a thermometer bulb. We know very well that the sun’s rays 
never, under identical circumstances, communicate such a temperature as 
we speak of to the atmosphere. We talk of the different powers of absorb- 
ing heat possessed. by different bodies. The phrase is borrowed from our 
discarded theories of caloric. We want to know not how hot the mercury 
becomes, but how many thermal units per second radiated to it are neces- 
sary to maintain it at thattemperature in spite of the influence of all the sur- 
rounding circumstances. Time must be an element in the definition of solar 
heat-energy, and before we claim that. a certain temperature must exist at the 
surface of the sun, we must learn the relations between what. we call radia- 
tion of heat and its constant re-development at the radiating surface. All 
our experiments on this subject have hitherto (with few exceptions) been 
- conducted with bodies which were actually cooling rapidly. If the sun 
must be put in the category of bodies which are in process of cooling down 
—and we have as yet no evidence to prove this, or even to negative a con- 
trary supposition—its rate of cooling, as compared with human eras of 
time, must be infinitesimally slow. For any period over which our experi- 
