or else that the latent heat of volatilisation of carbon is very considerably greater than that calculated from Trouton’s law. Even though this latent heat were as great as the heat of combustion of C to C 02, i.e., 7770, there would be an increase of about 70 percent, in the radiation for an increased pressure of 6 atmos. Such an enormous latent heat is unprecedented, and yet our experiments would, almost certainly, have shown such an increased radiation as this. So far, therefore, the experiments throw considerable doubt on the probability that it is- the boiling point of carbon that determines the temperature of the crater. It might be questioned whether there is energy enough in the current to do all this work, but upon an extravagant estimate of the amount of carbon volatilised in the crater, it appears that there is more than a hundred times as much energy supplied by the current as would be required for volatilising the carbon, even though its latent heat were as great as the heat of combustion of C into C02.
There is another considerable difficulty in the theory of the temperature of the crater being that of boiling carbon arising from tbe slowness of evaporation. The crater on mercury is dark, but then it volatilises with immense rapidity and the supply of energy by the current being more than 100 times that required merely for evaporation, there seems very little reason why even a considerable difference in latent heat should make any sensible difference in the rate of evaporation of mercury and carbon, especially as, at the same temperature, the diffusion of oarbon vapour is nearly three times as fast as that of mercury vapour and the temperature immensely higher.
We would, in conclusion, call attention to a cause of opacity in the solar atmosphere that is illustrated by the effect of convection currents in the long tube we were observing at high pressures; these convection currents behaved just like snow, or any other finely divided transparent body immersed in another of different refractive index. Light trying to get through is reflected backwards and forwards in every direction, until most of it gets back by the way it came. The consequence was that even the electric arc light was unable to penetrate the tube at high pressure, when these convection currents were active. The only light that came out of the tube was the feeble light outside, which was returned to us by reflection at the surfaces of these convection currents. In a similar manner we conceive that any part of the solar atmosphere which is at a high pressure, and where convection currents, or currents of different kinds of materials, are active, would reflect back to the sun any radiations coming from below, and reflect to us only the feeble radiations coming from interplanetary space. In his paper on “ The Physical Constitution of the Sun and Stars ” (‘ Roy. Soc. Proc.,’ No. 105, 1868), Dr. Stoney called attention to an action of this kind that might be due to clouds of transparent
