92 POPULAR SCIENCE REVIEW. 
tides at a certain fixed temperature, this substance will give out nearly all 
the rays of heat belonging to that temperature ? Now the sun, even when 
we look into a spot, is certainly a substance of indefinite thickness ; and 
since a spot appears much less luminous than the ordinary surface, ought we 
not to conclude either that we there view matter of a lower temperature than 
the ordinary surface, or that the matter which appears within a spot has a 
very high reflecting power compared to the ordinary matter of the photo- 
sphere ? This last supposition is an unlikely one, and the probability is that 
in a spot we view matter of a lower temperature than the photosphere. 
Presuming this to be the case, it appears to imply one of three things. (1.) 
Either the general body of the sun at the level of the bottom of a spot is of 
a lower temperature than the photosphere ; (2) or the lower temperature is 
produced by some chemical or molecular process which takes place when a 
spot is formed ; (3) or it is produced by matter coming from a colder region. 
“ The first of these suppositions will not be generally received, unless we 
are fairly driven to accept it. 
“ The second hypothesis has already been started to account for the lower 
temperature of a spot ; but we think that, according to the laws by which 
we should be guided in receiving or rejecting an explanation in a case of this 
nature, this idea ought to be rejected. 
“No doubt, if we knew of a case of the production of low temperature, and 
had at the same time an independent proof of some chemical or molecular 
process, such as evaporation, it would be quite allowable for us to associate 
the chemical or molecular process with the production of cold, as at any rate 
the most likely hypothesis ; but we do not advance in our explanation of 
the low temperature by attributing it to an imaginary process, of the existence 
of which we have no proof, and which is equally mysterious with the pheno- 
menon for which it is supposed to account. leather let us see if this reduc- 
tion of temperature can be explained by any other phenomenon of the 
existence of which we have independent evidence. This leads us to consider 
the third hypothesis, which supposes that the reduction is produced by 
matter coming from a colder region. Now, in the first place, we have such 
a region in the atmosphere above the photosphere, which we have shown to 
be of a lower temperature than the photosphere itself. Again, the obser- 
vations of Chacornac and Lockyer on the behaviour of the matter surround- 
ing a spot appear to suggest the existence of a downward current, which is 
therefore a current from the colder regions above. On the other hand, the 
proper motion of spots observed by Carrington is in favour of this hypo- 
thesis, since a current coming from a region of greater to a region of less 
absolute velocity of rotation would be carried on forward, and most so 
nearest the equator ; and this is precisely the motion of spots observed by 
Carrington. Again, we have seen that the faculse fall behind ; so that we 
may imagine two currents to be engaged in the formation of a spot — tlifi one 
an ascending current carrying the hot matter behind, the other a descending 
current carrying the cold matter forward. One advantage of this explanation 
is that all the gradations of darkness, from the faculce to the central umbra, ape 
thus supposed to be due to the same cause — namely, the presence, to a greater 
or less extent, of a comparatively cold absorbing atmosphere 
“In conclusion, we would venture to suggest that if the- photosphere of the 
