"But this small gravitative power of the moon would prevent its retaining on its surface any of the gases forming our atmosphere, which would all escape from it and probably be recaptured by the earth. By no process of external aggregation of solid matter to such a relatively small amount as that forming the moon, even if the aggregation was so violent as to produce heat enough to cause liquefaction, could any such long-continued volcanic action arise by gradual cooling, in the absence of internal gases. There might be fissures, and even some outflows of molten rock; but without imprisoned gases, and especially without water and water-vapour producing explosive outbursts, could any such amount of scoriae and ashes be produced as were necessary for the building up of the vast volcanic cones, craters, and craterlets we see upon the moon's surface." From Is Mars Habitable? by ALFRED RUSSEL WALLACE F.R.S., ETC.
I am not a science expert at all, so I don't think myself fully understand the content of the excerpt above. But I cannot bring myself out of doubt that the highlighted part is positive sentence, considering the preceding sentence which is in negation. So the cluase in the highlighted part should be also negative, I guess. What I mean is not based on syntax, but semantics. What I undertand as follows: Volcanic activity cannot occur without internal gases(imprisoned gases), and no matter how powerful external aggregation is, it cannot make volcanoes on the moon. Please help me.