8 Scientific Proceedings, Royal Dublin Society. 
fit state for the habitation of animals and vegetables. Professor 
Tait says that this cannot have been more than fifteen or 
twenty millions of years. But this question is one that cannot be 
settled by calculation, owing to our ignorance of some important 
elements of it. Granting the nebular hypothesis of the origin 
of the solar system, it is highly probable that the sun must 
have been formerly much hotter than it is at present; and if 
the earth (though sufficiently cool of itself) were fully exposed 
to the strength of his heat, no organic life, such as we know 
of, could have existed on the earth until after the sun had 
been cooling for a very long time; so that in that way a very 
considerable part of the sun’s duration would be lost for geological 
time. But, as Professor Tait himself points out, “we can imagine 
that one effect of his heat was to throw off from his surface such 
enormous clouds of absorbing vapour, which cooled as they left the 
surface, that the effective amount of radiation reaching the earth 
might not have been greater than at present ;” and, besides, the 
greater amount of vapour in the earth’s atmosphere may have 
helped to produce this effect. It is utterly impossible to calculate 
what the effect of this cloud-sereen may have been on the radiation 
of the sun; but Professor Tait states,in p. 175 of the book from 
which we are quoting, that if it made the sun to cool even at a 
uniform rate, it could not give us more than fifteen or twenty 
millions of years for the time of organic life ontheearth. But he 
seems to have forgotten another estimate given before, in p. 154 of 
the same book. Hesays: “We find by calculations in which there 
is no possibility of large error that this [nebular] hypothesis is 
thoroughly competent to explain 100 millions of years’ solar 
radiation at the present rate, perhaps more.” On comparing p. 152, 
it will be seen that this refers to past radiation only. It does not 
not apply to the whole, including future radiation. If thesun 
should have been losing heat uniformly, and at the present rate, it 
would make no difference how that uniformity had been attained, 
whether by a cloud-screen or otherwise; the period of cooling 
must be the same; and yet we find two such different estimates 
given by thesame writer as fifteen or twenty millions onone handand 
a hundred millions of years on the other; the latter number being 
five or six times greater than the former one, and being, moreover, 
the one proposed with the greater appearance of confidence, Of 
