10 Scientific Proceedings, Royal Dublin Society. 
comes may have been originally unconnected bodies whose proper 
motions brought them near enough to each other so that they 
became, as it were, entangled into one system. He believes that 
Sirius may be younger, that is, in a less advanced stage of forma- 
tion, than our sun. Is it not quite possible that this may be due 
to collision with a companion of his now subject comes? But we 
must not indulge further in speculation, though, from the nature 
of the present case, it is perfectly legitimate for us to do so. 
In concluding this part of our subject, let us note the data 
that we must have in order to be able to calculate the amount of 
heat that there would be to be radiated by the solar system, 
and the power of radiating that heat. We may neglect the 
masses of the planets and the chemical energy of separation, 
both of which, though telling in our favour, are proportionally 
unimportant. 
We know, then, the mass of the original nebula pretty nearly ; 
it is conceded on all hands that the separation of its parts was 
very great—how great it is not necessary to know, because beyond 
the magnitude usually contemplated for it, any additional exten- 
sion, however great, would make very little difference in its 
potential energy. We know the unit of gravitation attraction. 
We know the mechanical equivalent of heat. Butthat is all. We 
are not in possession of the other essential elements of the ques- 
tion. We do not know the mode of distribution of mass in the 
original nebula. We do not know what critical points may 
intervene between the original and the present states of agerega- 
tion of the materials. We do not know what were, and are, the 
thermal capacities of the materials under the varying conditions 
of aggregation, temperature, and pressure. We do not know 
what were their radiating powers under those changing con- 
ditions. We do not know what operations may have come into 
play to interfere with their radiation ; one has been mentioned 
already. We do not know that the mutual gravitation of the 
parts of the nebula was the sole bond of union between them, and 
was the sole force that was to cause, and to regulate the rate of, 
the evolution of that part of the heat that was originally 
potential. Prof. Tait, himself, alludes to this, op. cit. p. 153. 
Under what are, relatively to our very limited knowledge, 
extreme physical conditions, such as those of the original nebula 
