fixa quam aëris; atque eo magis non sequitur rationes flammæ, quod flamma etiam ipsa extinguitur per accidens, nempe a contrariis et destruentibus circumfusis, quam causam et necessitatem non habet pariter spiritus. Reparatur autem spiritus ex sanguine vivido et florido arteriarum exilium, quæ insinuantur in cerebrum; sed fit reparatio ista suo modo, de quo nunc non est sermo. PREFACE TO THE HISTORIA DENSI ET RARI. BY ROBERT LESLIE ELLIS. THE following treatise, which is one of the five histories mentioned in the Historia Naturalis, was published in 1658 by Dr. Rawley. A good deal of its contents occur in an imperfect and fragmentary state in the Phænomena Universi.' It has somewhat the appearance of having been left unfinished, and excepting a table of specific gravities and an account of the way in which this table was constructed, contains little that is now of interest. The table occurs also in the Phænomena Universi: in the Historia Densi et Rari one substance is omitted and six added, so that the whole number of substances mentioned, which is seventy-three in the former, is seventy-eight in the latter work. This table of specific gravities is the only collection of quantitative experimental results that we find in Bacon's works. Few experiments of the same kind had previously been made. The method which Bacon employed enables us to form some opinion as to the amount of his acquaintance with mathematical physics. The first table of specific gravities was constructed by Marinus Ghetaldus', whose Archimedes Promotus was published One of the fragments published by Gruter in 1651, which will be printed in Part III. of this edition. My own impression is that much of the first portion of the present treatise-from the first tabula down to the monitum, p. 259.— is of earlier date than Gruter's copy, and less perfect; and that the remainder only-extending from the first connexio to the end-is to be regarded as the Historia Densi et Rari which Rawley mentions as having been composed by Bacon during his last quinquennium; the previous part being made up of notes and loose papers written at various times, many of them long before, and never digested into order. See my note at the end of this preface.-J. S. 2 It appears from Harriot's papers, now in the British Museum, that before the in 1603. It contains only twelve substances, and is therefore, so far as the number of experiments is concerned, much inferior to Bacon's. But on the other hand Ghetaldus is the author of the method of finding specific gravities which, with certain modifications and corrections, has remained in use to the present day, whereas no one, probably, has attempted to find specific gravities by Bacon's process. The principle of Ghetaldus's method consists in weighing the substance which is to be examined in air and in water, and thus ascertaining the weight of the water which it displaces. By this method the comparison of the densities of different substances is made to depend on the first principles of hydrostatics. The often-told story of Archimedes and Hiero's crown contains the germ of the same method; and it is probably from this that Ghetaldus took the title of his book. It contains however, beside the tables of specific gravities, certain corollaries from propositions in Archimedes's treatise on the equilibrium of floating bodies, enough to show that Ghetaldus was entitled to profess himself a follower of Archimedes. Towards the end of his treatise he tells the story of Archimedes and Hiero, and remarks on the practical defects of the method which Archimedes employed. The chief inaccuracy arises from the effect of capillary attraction on the surface of the water, which makes it difficult to know when the vessel, into which the crown or other substance to be examined is introduced, is only just full. Ghetaldus's remark, that the water which overflows cannot be collected and measured without loss, is no doubt correct; but it does not seem that this way of trying the experiment was employed by Archimedes. After putting the crown into a vessel full of water and thus making a part of the water overflow, he filled the vessel again, measuring the quantity of water poured in. Repeating this experiment with a mass of gold equal in weight to the crown, and then again with a mass of silver also of equal weight, he found that the crown displaced more water than the gold and less than the silver, and thereby showed that the crown was not of pure gold. It does not seem, from what Vitruvius says, that Archimedes calculated the amount publication of the Archimedes Promotus, he knew how to determine specific gravities by weighing in air and water. We are not however entitled to assert, as Baron Zach has done, that his experiments preceded those of Ghetaldi. See the supplement to Dr. Bradley's Miscellaneous Works, by Prof. Rigaud, pp. 43 and 51. "Isti vero opusculo nomen ab Archimede, quem ducem sequor, imposui." |