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crust and the whole sun hke an immense bubble. This view, however, seems no longer tenable. It does not seem likely that there is any solid matter on the sun.

Attempts have sometimes been made to learn the temperature of the photosphere. It probably exceeds any that we can produce on earth, even that of the electric furnace, else how could calcium, the metallic base of lime, one of the most refractory of substances, exist there in a state of vapour? We all know that the air around us becomes cooler and rarer as we ascend above the surface of the earth, owing to the action of gravity and the consequent weight of the atmosphere, which gives rise to a constantly increasing pressure as we descend. Now, gravity at the sun is twenty-seven times as powerful as on the earth. Hence, going downward, temperature and pressure increase at a far more rapid rate on the sun than on the earth. Even in the photosphere the temperature is such that "the elements melt with fervent heat." And, as we go below the surface, the heat must increase by hundreds of degrees for every mile that we descend. The result is that in the interior the gases of the sun are subjected to two opposing forces which grow more and more intense. These are the expansive force of the heat and the compressing force of the gases above, produced by the enormous force of gravity of the sun.

The forces thus set in play merely in the outer portions of the sun's globe are simply inconceivable. Perhaps the explosion of the powder when a thirteen-inch cannon is fired is as striking an example of the force of ignited gases as we are familiar with. Now suppose every foot