Page:Supplement to the fourth, fifth, and sixth editions of the Encyclopaedia Britannica - with preliminary dissertations on the history of the sciences - illustrated by engravings (IA gri 33125011196181).pdf/755

 poor, and even on the patients in the Hotel Dieu, under pretence of charitably supplying them with biscuits. But Voltaire positively denies this horrible imputation, and says, that she never attempted the life of her husband, who overlooked a connection, of which he was the cause.

The information concerning the nature of the Eau de Brinvilliers, derived from the examination of Sainte Croix’s famous casket, is not satisfactory. It contained poisons enough to have killed a whole community; besides opium, lunar caustic, antimony and vitriol, more than 75 lbs. of corrosive sublimate, and two bottles of a liquid, like water, with a sediment in one. The clear liquid was probably his real poison; as none of the other substances could have been given so as to produce death, without instantly being detected, by their abominable taste; but what this liquid was, we can now only conjecture, for its examination, as reported by Pitaval, shows that the physicians, at that time, had not the slightest notion of the mode of detecting arsenic even in substance, much less in solution; and accordingly, although both the liquor and powder killed the animals to which they were given, it is candidly admitted, that the poison of Sainte Croix surpassed the art and capacity of the physicians, and that it baffled all their experiments to discover its composition. We have, however, no doubt, that arsenic was the only active ingredient of all these pretended secret poisons; as it is the only substance capable of explaining all the credible circumstances related of them. From the mode of administering them in small, but repeated and perhaps increased doses, there was some foundation for the belief that they could be given so as to kill in any determinate time, while their failing in any stance to produce death, was easily accounted for by supposing antidotes to have been administered. But although the progress of knowledge has proved, that there is no such thing as such antidotes, it has on the other hand, by rendering the detection of poison easy and certain, put a stop for ever to the trade of poisoner; and what is perhaps of equal importance, to the general alarm and cruel punishment of individuals which have often resulted from natural deaths being ascribed to poison. It is not because we know less, but because we know a great deal more than our forefathers, that the art of secret poisoning seems to be lost.—See in this Supplement.

ARACHNIDES. A class of animals of the type Annulosa. See Index to.  ARÆOMETER (composed of 🇬🇷, levis, tenuis, and 🇬🇷 mensura), a measure of the comparative density and rarity of bodies. The name does not occur in ancient authors; hydroscopium and baryllium being the ancient names of the instrument. This instrument was known in the civilized part of the Roman empire, about the year 400, as appears from the fifteenth epistle of Synesius, addressed to Hypatia, daughter of Theon; and to Hypatia some modern writers have erroneously ascribed its invention. The instrument is also described in some verses annexed to Priscian; and the principles on which its operation is founded, are to be seen in the treatise of Archimedes on floating bodies (De Humido Insidentibus). The term, as used by writers on natural philosophy, is chiefly applied to instruments which are made to float, so as to indicate the specific gravity of the liquids in which they are placed: the pêse liqueur, and hydrometer, in common use for measuring the specific gravity of vinous spirits, are instruments of this kind.

A floating body displaces a portion of the liquid, the weight of which is equal to its own weight, the liquid acting upwards with a force equal to this weight, and the weight of the body acting downwards with the same force, equilibrium takes place. If the body be afterwards placed in a liquid of less density, the part of the body immersed, will be greater than when the body was in the more dense liquid, because it requires a greater volume of this less dense liquid to equal the weight of the floating body. The absolute weights of two bodies being the same, their specific gravities are in the inverse ratio of their volumes $$\frac{G}{g}=\frac{v}{V}$$, when $$G$$ is put for the specific gravity of the first body, $$g$$ for that of the second; $$V$$ for the volume of the first, and $$v$$ for the volume of the second. On this principle the common hydrometer is constructed; the instrument described by Synesius, is also of this kind. In order that a small difference in the volume immersed may be sensible, the part which is intersected by the surface of the fluid is in the form of a very slender cylinder, the great bulk of the instrument being always immersed in the liquid. At the inferior part is a small ball, containing mercury or small load shot, which serves as ballast, bringing the centre of gravity low, so that the instrument may float erect, and without much lateral oscillation. The common hydrometers are made of glass, and sometimes of brass, or tin or pewter, and some have been made of amber as objects of curiosity: when made of glass, a scale, inscribed upon paper, is inserted in the cylindrical stalk; the division of the scale at which the surface of any liquid intersects the stalk, denotes the specific gravity of that liquid. The divisions of the scale should be formed by immersing the instrument in liquids of known specific gravity, and marking a number corresponding to that specific gravity opposite to each division. The specific gravities of water and alcohol mixed in various proportions, have been accurately ascertained by Mr Gilpin (see his Tables, and Dr Blagden’s paper in the Philosophical Transactions); on immersing the instrument in a mixture of known proportions of these two liquids, the point at which the surface intersects the stalk, is to be marked with the number expressing the specific gravity of the mixture taken from the table. Some hydrometers, such as that constructed by the French chemist Beaumè, and which is much used in France under the name of Aréometre de Beaumé, have the 