TWO:A learner will no doubt wonder why sand is used for moulding, instead of some more adhesive material like clay. If he is not too fastidious for the experiment, and will apply a lump of damp moulding sand to his mouth and blow his breath through the mass, the query will be solved. If it were not for the porous nature of sand-moulds they would be blown to pieces as soon as the hot metal entered them; not only because of the mechanical expansion of the gas, but often from explosion by combustion. Gas jets from moulds, as may be seen at any time when castings are poured, will take fire and burn the same as illuminating gas.Whether Plotinus was or was not the disciple of Ammonius, it is beyond all doubt that he considered himself the disciple of Plato. There are more than a hundred references to that philosopher in the Enneads, against less than thirty references to all the other ancient thinkers put together;428 and, what is more remarkable, in only about half of them is he mentioned by name. The reader is expected to know that he always means Plato. And it is an article of faith with Plotinus that his master cannot be mistaken; when the words of oracular wisdom seem to contradict one another, there must be some way of harmonising them. When they contradict what he teaches himself, the difficulty must be removed by skilful interpretation; or, better still, it must be discreetly ignored.429 On the other hand, when a principle is palpably borrowed from Aristotle, not only is its derivation unacknowledged, but we are given to understand by implication that it belongs to the system which Aristotle was at most pains to controvert.430
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