Before showing that such transitions are actually known in nature, it may be well to discuss here an objection which the customary way of thinking might plausibly advance against our whole method. It could be said that to assume a continuation of the sequence of the three ponderable conditions in the manner suggested is justified only if, as solids can be turned into liquids and these into gases, so gases could be transformed into a fourth condition and, conversely, be produced from the latter.