and reducing them into three classes, viz.: those which address themselves to the understanding; those which serve to correct or to inform the senses; and those which conduce to practice, i. e. to the invention of arts. To each of these twenty-seven species he assigns a characteristic, but somewhat fanciful name. To give the reader an idea of this part of the Novum Organum, we shall select a few of the principal Prerogative Instances, subjoining either Bacon's own examples, or such illustrations, disclosed by modern science, as will best show the author's design.

1. Instantia Solitariæ, or solitary instances, are either those which have nothing in common, except the quality whose form or cause is sought for; or those which have not that quality, but are in every other respect alike. Thus, if the cause or form of colour be the subject of inquiry, solitary instances of the first kind are to be found, says Bacon, in prisms, crystals, and globules of

dew, which under certain circumstances exhibit colour; but they have nothing in common with flowers, metals, and wood, except the colour itself. Of the second kind of solitary instances examples may be found in the variegated veins which are to be seen in some of our beautiful marbles; for these, principally, at least, differ only in colour, and not so much in substance or structure. The various colours of flowers offer another instance.

2. Instantiæ Migrantes, or migrating instances, are those in which the nature or property under consideration, travels or passes from one state to another, from less to greater, or from greater to less; approaching to perfection in the one case, and in the other, verging towards decay. In illustration of this class, Professor Playfair refers to the fossil shells which we see so perfect in figure and structure in limestone, and gradually losing themselves in the finer marbles, till they are no longer distinguish

able.* A more striking illustration, perhaps, may be drawn from the phenomenon of what is commonly called the attraction of cohesion. In hard bodies, such as the diamond or steel, we find this force extremely powerful; in caoutchouc it is very weak; liquids exhibit it in a still less degree; and if we apply the irresistible power of the voltaic battery to water, for instance, we may altogether destroy the cohesion of its particles, and convert the whole into its constituent gases of hydrogen and oxygen.

The atmospheric pressure affords another illustration: If we take a barometer in our

* Playfair's Prelim. Diss. p. 462. Even the colours of fossil shells belonging to species that are extinct, (as well as their figure and structure,) are sometimes preserved in so perfect a state that a common observer would suppose that they had just been taken from living testacea. Some fossil fish likewise retain their gaudy dress: one in the author's possession, found in the coal formation of Eisleben, is almost as bright in its hues as the brilliant gold-fish which enliven our ornamental

tanks.

hand, and ascend some high mountain, such as the Puy de Dôme, where Pascal made his ever-memorable experiment, we shall find, as did that illustrious philosopher, that the mercury will sink; and that the loftier our station is, the lower will be the barometrical column; because the weight of the incumbent air is less. The same principle is shown in its effects upon the human frame. When M. GayLussac, in his celebrated aeronautic voyage, ascended to the enormous height of twentytwo thousand nine hundred and twelve feet above Paris, (or sixteen hundred feet above the summit of the Andes,) he had great difficulty in breathing, and his pulse and respiration were much quickened. Even when M. de Humboldt ascended the Andes, the blood burst from his lips and ears.

3. Instantia Ostensivæ, or glaring instances, are those which show some particular nature or quality in its highest state of energy and power; either freed from its usual impediments, or counteracting them

Bacon adduces the

by its own strength. thermometer in illustration of this class, as it strikingly displays the expansive power of heat. The beautiful and well-known experiment of the Magdeburgh hemispheres is a glaring instance of the atmospheric pressure on solids. Pascal's experiment, before noticed, might also be adduced in illustration of this class. Hutton's discovery in Glen Tilt of veins of granite ramifying from the principal mass, and traversing the schist and primary limestone, is another example; and afforded so glaring and conclusive a verification of his theory of the igneous origin of granite, that it called forth such marks of exultation as convinced the guides that the geologist must have discovered a vein of silver or gold. The magnet, too, may be mentioned as offering a beautiful glaring instance of that quality in nature which is known by the name of polarity.

4. Instantiæ Conformes, i. e., analogous or similar instances, are those between which