74 REVIEWS—DONATI'S COMET. 
by him, and this theory has been revived in our days with elaborate 
circumstance by Prof. W. Thompson. Not only the suu but other 
fixed stars may share this benefit, and thus might be accounted for the 
sudden and irregular apparition of brilliant stars in the heavens, not 
known before. Nay, even the planets may be benefited in this way, 
for, says Newton :— 
«The tails therefore that rise in the perihelion positions of the 
comets will go along with their heads into far remote parts, and to- 
gether with the heads will either return again from thence to us, after 
a long course of years ; or rather, will be there rarefied, and by degrees 
quite vanish away. . . For all vapour in those free spaces is in a 
perpetual state of rarefaction and dilatation. . . And it is not un- 
likely, but that the vapour, thus perpetually rarefied and dilated, may 
be at last dissipated, and scattered through the whole heavens, and by 
little and little be attracted towards the planets by its gravity, and 
mixed with their atmosphere. For as the seas are absolutely neces- 
sary to the constitution of our earth, that from them, the Sun, by its 
heat, may exhale a sufficient quantity of vapours, which being gathered 
together into clouds, may drop down in rain for watering of the earth, 
and for the production and nourishment of vegetables ; or being con- 
densed with cold on the tops of mountains, (as some philosophers with 
reason judge) may run down in springs and rivers, so for the conser- 
vation of the seas and fluids of the planets, comets seem to be requir- 
ed, that from their exhalations and vapours condensed, the wastes of 
the planetary fluids spent upon vegetation and putrefaction, and con- 
verted into dry earth, may be continually supplied and made up. For 
all vegetables entirely derive their growths from fluids, and afterwards 
in great measure are turned into dry earth by putrefaction ; and a sort 
of slime is always found to settle at the bottom of putrified fluids- 
And hence it is, that the bulk of the solid earth is continually increas- 
ed, and the fluids, if they are not supplied from without, must be in a 
continual decrease, and quite fail at last 1 suspect moreover, that it 
is chiefly from comets that spirit comes, which is indeed the smallest, 
but most subtle and useful part of our air, and so much required to 
sustain the life of all things with us.’ And in another place, “‘ The 
vapours which arise from the sun, the fixed stars, and the tails of the 
comets, may meet at last with, and fall into the atmospheres of the 
planets by their gravity; and there be condensed and turned into 
water and humid spirits, and from thence by a slow heat pass gradual. 
ly into the form of salts, and sulphurs, and tincture, and mud, and 
clay, and sand, and stones, and coral, and other terrestial substances.” 
