THE RADIATION OF THE SUN—ABBOT. 151 
of the sun, accompanying the change of the intensity of the sun’s 
radiation. Changes of contrast along the sun’s diameter have already 
been found, but it is not yet decided whether they agree in point of 
time with the changes in the intensity of the solar radiation.1 
NATURE OF THE SUN. 
In view of what has been said, what is the nature of the sun? It 
appears, in consideration of its high temperature and low density, 
to be a great ball of incandescent gases. Of course, the pressure is 
so enormous that the gases approach the density of liquids. These 
gases are so hot as to exceed in temperature anything that we have 
upon the earth’s surface. It must not be supposed, however, that 
they are burning gases like the burning of illuminating gas in air. 
The temperature on the sun’s surface is so high that in general no 
compounds of elements are occurring there. If the ordinary com- 
pounds, for instance, products of combustion like carbonic-acid gas, 
should be present on the sun, the elements of which they are com- 
posed would separate, one from another, owing to the enormous tem- 
perature. As the sun gives off radiation, it tends to cool, and it may 
well be asked why, in the course of the millions of years which geolo- 
gists tell us have elapsed since the earth reached.substantially its 
present temperature, the sun should not have cooled off entirely. A 
partial source for this immense quantity of energy was suggested by 
Kant and discussed at length by Helmholtz, who showed that the 
enormous gravitation of the sun, tending to condense the gases and 
bring them toward its center, must, for every decrease of tempera- 
ture and consequent shrinking up of the volume of the sun, produce 
a certain quantity of energy. This source of the sun’s energy, how- 
ever, seems insufficient to account for that which geologists demand 
us to concede. It may be that the secret of the matter is in the 
breaking up of the atoms, such as is now found to occur with the 
element radium. 
If the sun is gaseous, the question naturally arises why it presents 
so sharp and round a boundary. The roundness of the sun is only 
~ what would be expected in view of its gravitation. The sharpness 
of its boundary seems explainable as follows: Gases, although very 
transparent, are not perfectly so, so that in the case of the earth the 
atmosphere above Mount Wilson transmits only about 95 per cent 
of the yellow light. If, then, 5 per cent of the sun’s radiation in the 
yellow is cut off by the earth’s atmosphere, it follows that a layer of 
the sun’s gases only a few thousand miles thick would be sufficient 
to prevent us from seeing any deeper. This 3,000 or 4,000 miles of 
thickness, as we look at the center of the sun, will extend vertically 
1 Experiments of 45 days in 1913 indicated that there is such agreement in point of time. Thus the sun’s 
variability from day to day is again independently confirmed. 
