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462 Old Prices Remains. absurd than to suppose there is such a thing as positive cold, the driving off or " keeping out" of which makes things warmer; which yet I suspect a good number of adults believe at least as firmly as any article of their creed, especially during the prevalence of N.E. winds about Christmas. The old philosophers, (for no other class could ever have hit upon so thoughtful a blunder,) were probably led into that strange idea by observing that cer¬ tain substances, as iron, after being burnt (a process noto¬ riously apt to dissipate a portion of combustible matters,) were found< to have increased, contrary to ordinary expe- perience, in weight; and their hasty way of accounting for this, however erroneous, must have served to call atten*- tion to the fact, and to keep it before the scattered sons of science in those pre-Davyite days, till the controversies respecting the nature (and the "natura naturans,") of Phlo¬ giston (so the supposed substance of lightness was called,) led, in process of time, to the discovery of Oxygen, which combines with burning bodies (if, indeed, "burning" means anything more than combining with Oxygen,) and there¬ fore, of course, renders things heavier in the ordinary way, i^.,,not by subtraction but addition. I have a very parti¬ cular reason for introducing this apparently incongruous story in this part of my book. A certain class of Mathe¬ maticians have introduced into our present subject a theory, fully as much at variance with common sense as the Phlogiston notion, viz,,,that there" exist two different Quantities—positive and negative, the latter of which have the inherent property' of diminishing any Quantity to which they are joined; whereas the truth is that there is only one kind of Quantities, viz., positive, i.€>, actual, or real Quantities, the addition of which can only produce increase of any other Quantities. And it is just because they are