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FOPULAR SCIENCE REVIEW. 
ing paper on the temperature and physical condition of the sun. Assuming 
that those prominences which present the appearance of eruptions are really 
produced "by the action of explosive forces, projecting vast quantities of 
glowing hydrogen into the chromosphere, he applies the principles of thermo- 
dynamics to determine the heat and pressure at different parts of the sun’s 
mass and atmosphere. He obtains as a probable minimum value for the 
temperature of the chromosphere 27,700° Centigrade, and for the tempera- 
ture in the interior region whence the hydrogen is erupted 08,400° Centi- 
grade. Assuming the atmospheric pressure at the base of the chromosphere 
to be 0180 mm. (about 7 inches of the mercurial barometer), he finds the 
pressure at the level of the nuclei of the spots to be about 184,000 atmo- 
spheres, and the pressure in the inner region before named no less than 
4,070,000 atmospheres. He further deduces the following general results : — 
1. We cannot conclude from the want of certain lines in the spectrum of 
a self-luminous star that the corresponding elements are necessarily absent 
from its substance. 
2. The layer in which the reversal of the spectrum takes place is different 
for each substance, and lies nearer to the centre of the star according as the 
vapour-density is greater and as the emissive power is less. 
3. Under otherwise equal circumstances this stratum lies nearer to the 
centre as the intensity of gravity is greater. 
4. The distances separating the reversal strata of given substances from 
each other, as well as from the centre, are greater as the temperature is 
greater. 
5. Under otherwise equal circumstances the spectra of different stars are 
richer in lines the lower the temperature, and the greater the mass of the 
star. 
0. The difference in the intensity of different dark lines in the spectrum of 
the sun and other stars does not depend only on the difference between the 
absorptive powers of the corresponding elements, but also on the different 
depths at which the reversal of the spectra in question takes place. 
Is the Resolvability of Star-groups a test of Distance ? — It has hitherto been 
regarded as tolerably certain that the power necessary to effect the resolu- 
tion of star-groups (including stellar nebulae) affords a satisfactory general 
means of estimating the relative distance of such groups. Mr. Proctor 
considers that he has been able to prove that this test is altogether untrust- 
worthy. It would be sufficient, if we had evidence of a general uniformity 
of texture, so to speak, in star-systems ; but the evidence we have is 
opposed altogether, he considers, to such a view. He quotes Sir John 
Herschel’s evidence respecting the Magellanic clouds, as tending to prove 
that portions of a star-system which lie at nearly equal distances may 
present wholly different characteristics as respects rcsolvability. Thus, Sir 
John Ilerschel says in one place, in recording his observations of the Nubecula 
Miner, 11 We are now in the cloud, the field begins to bo full of a faint light, 
perfectly irresolvable.” In another place he notes , 11 Upper limit, but here it 
is starry, at the other limit nebulous .” Elsewhere again, " The main body is 
resolved, but barely. . . . The borders fade away insensibly, and are less, or 
not at all, resolved.” Yet the relative distances of these portions cannot be 
very unequal. 
