Bicrerton.—0n Temporary and Variable Stars. 119 
radiated from each square yard of our sun’s surface is estimated to be equal 
to the combustion of ten cubic yards of coal in every hour, while the sun’s 
disc has four times the area enclosed by the orbit of the moon. The star of 
1866 when first seen was of the second magnitude, and its spectrum shows 
that it consisted of a nucleus of compressed gas, or of liquid or of solid matter. 
This was surrounded by an atmosphere of heated gas, having a greater 
monochromatic light than the nucleus; or it might have been simply a 
small permanent star in the same line of vision as the gaseous temporary 
star. I cannot say if this suggestion agrees with the present condition of 
the star. This star diminished from a star of the second magnitude to the 
tenth in about a fortnight. The spectroscope showed the star of 1877 to be 
ignited gas only, and from the number of the lines diminishing the tempera- 
ture and pressure probably did so likewise. The intensity diminished in 
four months from the third magnitude to the ninth. 
Many hypotheses have been formed to account for the nature of these 
stars, of which the following appear to be the most noteworthy :— 
1. Zoolner imagines a sun in which spots have covered the whole 
surface, the temporary stars being produced by the breaking of such a 
surface. 
2. Vogel assumes a volcanic bursting-out on a dead sun. 
In both of these hypotheses a decomposition and combustion of hydrogen 
and other elements is also assumed to account for the great intensity. 
9. Meyer and Klein suppose that a similar dark body is suddenly raised 
to incandescence by the projection of a planet or other body xd its 
surface. 
4. Proctor supposes that the atmosphere of a dead sun is suddenly 
brought to a high degree of luminosity by the passage of a meteoric train 
through it. 
In examining these hypotheses, we find that there is one thing in 
common, namely, the assumption of the existence of large dark bodies in 
space. The first two of them also depend on the existence of internal com- 
motion, attended with combustion. The last two depend upon the energy 
developed by gravitation. 
A little consideration will be sufficient to show that, on grounds of 
intensity alone, Zoolner’s and Vogel’s—in fact, any hypothesis not 
dependent upon gravitation—is improbable. Is it conceivable that a 
dark body should suddenly change its surface by voleanic or other internal 
action in such a manner as to heat gases to a pitch of luminosity as high 
as our sun’s, especially when it is considered that if a gas and solid be at 
the same temperature, the solid is much the more luminous of the two; nor 
would combustion or decomposition help it; generally the latter would take 
