732 
The Temperature of the Sun . [November, 
IV. THE TEMPERATURE OF THE SUN. 
By W. Mattieu Williams. 
t N the last number of the “ Monthly Journal of Science” 
Prof. Langley compares the radiations from the sun 
with those from a surface of molten metal which pre- 
sents to the thermopile a disk of equal angular diameter, 
and bases his estimate of an inconceivably high solar tem- 
perature upon this experiment. 
If we had any reasons for concluding that the surface of 
the sun is a heated solid, or liquid as athermanous as molten 
iron, his method might justify his conclusions ; but all we 
know of the sun leads to the conclusion that the photosphere 
is composed of flaming or incandescent gaseous matter, the 
outer or superficial strata of which are permeable by the 
radiations from the incandescent matter below or within. 
If so, it is evident that the effective radiations from such a 
body must be increased, not merely by the extent of its 
surface, but also by the thickness or depth of the diatherma- 
nous incandescent matter; and a comparison between a disk 
of this and another, similar disk of an athermanous body 
that radiates only from its surface must be quite delusive 
when applied for comparing their temperatures. This may 
he illustrated by simply presenting a plate of heated iron to 
a distant thermometer or thermopile, and then carefully 
placing behind this another plate of equal or less diameter, 
and equally heated. If direct radiation from the second is 
prevented, the thermometer will not rise when it is placed 
behind the first. Now try a similar experiment with two 
gas flames, and the effect of the radiation of the second 
flame through the first will be seen at once, though the sur- 
face of flame presented and the thermometer remain the 
same, and the temperature of the first or front flame is not 
sensibly raised. 
In Chapters 7 and 8 of “The Fuel of the Sun ” I have 
described a series of experiments made for the purpose of 
determining the transparency and diathermancy of flame to 
its own special radiations. The experiments on transparency 
were conducted with the advantages of the best attainable 
apparatus, — the photometer and dark chamber of the 
Official Gas Examiner of the Sheffield Corporation, — and 
the results showed that with proper precautions, there 
described in detail, the light received upon the screen of the 
photometer from a given number of gas-jets, each burning 
