382 Effect of Pressure on temperature of Crater of Electric Arc. 



or else that the latent heat of volatilisation of carbon is very con- 

 siderably greater than that calculated from Trouton's law. Even 

 though this latent heat were as great as the heat of combustion of C to 

 C0 2 , i.e., 7770, there would be an increase of about 70 per cent, 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 tempera- 

 ture 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 CO 2 . 



There is another considerable difficulty in the theory of the tem- 

 perature of the crater being that of boiling carbon arising from the 

 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 evapora- 

 tion, 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 tem- 

 perature, the diffusion of carbon 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 con- 

 sequence 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 con- 

 vection currents. In a similar manner we conceive that any part of 

 the solar atmosphere which is at a high pressure, and where convec- 

 tion 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 



