Physical Constitution of the Sun. 345 



I formulated my view of the cause of the eruptive protuberances, 

 and their connexion with the spots and faculse, as follows* : — 



" The eruptive protuberances are produced by differences between 

 t fie pressure of a quantity of gas enclosed in or absorbed by the 

 liquid, and the external pressure of the atmosphere increased by 

 the cohesion and weight of the upper layers of the liquid. 



"Accordingly eruptive protuberances will most readily arise 

 in those places where the pressure to be overcome is the least, 

 most rarely or not at all where this pressure is the greatest. 

 Every ascending current in the atmosphere diminishes the pres- 

 sure at the place, just as every descending current augments it. 

 Now, as in the environs of the spots there are strong ascending 

 currents and in like manner the faculse are occasioned by currents 

 of the same kind, these places must be peculiarly favourable for 

 the development of eruptive protuberances "f. 



Respighi, in 1870, expressed these relations as general results 

 of his observations, thus { : — 



" On the contour of the spots there ordinarily arise gaseous 

 jets of extraordinary intensity and violence and of well-defined 

 form." 



" Usually the protuberances and eruptions are very frequent 

 and much developed in the localities of the faculse," &c. 



Similar results were also arrived at by Father Secchi in the 

 course of his numerous observations, which he sums up, in 



* Berichte d. K. Sachs. Ges. Feb. 11, 1871. 



t M. Tacchini thinks that only those protuberances should be regarded 

 as eruptions, which have the form of a tree or a fan — that is, are narrower 

 at their base than at their upper end. Without discussing more in detail 

 the claims of such a view, it is clear that, for the explanation of those erup- 

 tions, the physical conditions must necessarily be assumed to exist on the 

 sun's surface without which an eruption (the violent and sudden overcoming 

 of a resistance by compressed gas) is not conceivable. Such a resistance 

 can only be caused by a substance which is in a more coherent state of ag- 

 gregation than the gaseous; the existence of eruptive protuberances 

 requires therefore necessarily the existence of a liquid mass, out of which 

 the gases escape on the lessening of the pressure of the atmosphere resting 

 upon them, like the bubbles from water containing carbonic acid. 



Hence, when M. Tacchini, from the circumstance that among the nu- 

 merous protuberances only some 8 per cent, exhibit the eruptive character 

 mentioned, believes himself justified in concluding that the general form 

 of the protuberances is irreconcilable with the assumption of a solid or 

 liquid constitution of the surface of the sun, it is clear from the remarks 

 above made that the principle of majorities is logically inadmissible in the 

 present case ; for even if only one single protuberance out of the great 

 number observed were recognized as an eruption, we should be compelled 

 to attribute at least to the place in question of the sun's surface those pro- 

 perties by which alone the physical possibility of an eruption becomes con- 

 ceivable. 



\ Atti delta Reale Accadernia dei Lincei, Dec. 4, 1870. 



