things according to the lines most conspicuous to vulgar appre hensions. Hence, when words are examined, few instances are found in which, if at all abstract, they convey ideas tolerably precise and defined.

4. The Idols of the Theatre are the deceptions which have arisen from the dogmas of different schools.

As many systems as existed, so many representations of imaginary worlds had been brought upon the stage.. Hence the name of Idola Theatri. They do not enter the mind imperceptibly like the other three; a man must labor to acquire them, and they are often the result of great learning and study.

After these preliminary discussions, Bacon proceeds, in the Second Book of his Organum, to describe and exemplify the nature of induction.

The first object must be to prepare a history of the phenomena to be explained, in all their modifications and varieties. This history is to comprehend not only all such facts as spontaneously offer themselves, but all the experiments instituted for the sake of discovery, or for any of the purposes of the useful arts. It ought to be composed with great care; the facts accurately related and distinctly arranged; their authenticity diligently examined; those that rest on doubtful evidence, though not rejected, yet noted as uncertain, with the grounds of the judgment so formed. This last is very necessary, for facts often appear incredible only because we are ill-informed, and cease to appear marvellous when our knowledge is further extended. This record of facts is Natural History.

The Natural History being prepared of any class of phenomena, the next object is to discover, by a comparison of the different facts, the cause of these phenomena, or, as Bacon calls it, the form. The form of any quality in a body is something convertible with that quality; that is, where it exists the quality exists: thus, if transparency in bodies be the thing inquired after, the form of it is something found wherever there is transparency. Thus form differs from cause in this only: we call it form or es

sence, when the effect is a permanent quality; we call it cause, when the effect is a change or an event.

Two other subjects, subordinate to forms, but often essential to the knowledge of them, are also occasionally subjects of investigation. These are the latent process, latens processus; and the latent schematism, latens schematismus. The former is the secret and invisible progress by which sensible changes are brought about, and seems, in Bacon's acceptation, to involve the principle since called the law of continuity, according to which no change, however small, can be effected but in time. To know the relation between the time and the change effected in it, would be to have a perfect knowledge of the latent process. In the firing of a cannon, for example, the succession of events during the short interval between the application of the match and the expulsion of the ball, constitutes a latent process of a very remarkable and complicated nature, which, however, we can now trace with some degree of accuracy.

The latent schematism is that invisible structure of bodies on which so many of their properties depend. When we inquire into the constitution of crystals, or into the internal structure of plants, etc., we are examining into the latent schematism.

In order to inquire into the form of any thing by induction, having brought together all the facts, we are to begin with considering what things are thereby excluded from the number of possible forms. This conclusion is the first part of the process of induction. Thus, if we are inquiring into the quality which is the cause of transparency in bodies; from the fact that the diamond is transparent, we immediately exclude rarity or porosity as well as fluidity from these causes, the diamond being a very solid and dense body.

Negative instances, or those where the form is wanting, to be also collected.

That glass when pounded is not transparent, is a negative fact, when the form of transparency is inquired into; also, that collections of vapors have not transparency. The facts thus col

lected, both negative and affirmative, should, for the sake of reference, be reduced to tables.

Bacon exemplifies his Method on the subject of Heat; and though his collection of facts is imperfect, his method of treating them is extremely judicious,* and the whole disquisition highly interesting.

After a great many exclusions have been made, and left but few principles common to every case, one of these is to be assumed as the cause; and by reasoning from it synthetically, we are to try if it will account for the phenomena. So necessary did this exclusive process appear to Bacon, that he says, "It may, perhaps, be competent to angels or superior intelligences to determine the form or essence directly, by affirmations from the first consideration of the subject; but it is certainly beyond the power of man, to whom it is only given to proceed at first by negatives, and in the last place to end in affirmatives, after the exclusion of every thing else."

There is, however, great difference in the value of facts. Some of them show the thing sought for in the highest degree, some in the lowest; some exhibit it simple and uncombined, in others it appears confused with a variety of circumstances. Some facts are easily interpreted, others are very obscure, and are understood only in consequence of the light thrown on them by the former. This led Bacon to his consideration of Prerogative Instances, or the comparative value of facts as means of discovery. He enumerates twenty-seven different species: but we must content ourselves with giving only the most important.

I. Instantiæ solitaria: which are either examples of the same quality existing in two bodies otherwise different, or of a quality differing in two bodies otherwise the same. In the first instance the bodies differ in all things but one; in the second they agree in all but one. Thus, if the cause or form of color be inquired

*This is Playfair's judgment; a different opinion will presently be quoted from John Mill.

into, instantiæ solitaria are found in crystals, prisms, drops of dew, which occasionally exhibit color, and yet have nothing in common with the stones, flowers, and metals which possess color permanently, except the color itself. Hence Bacon concludes that color is nothing else than a modification of the rays of light, produced in the first case by the different degrees of incidence"; and second, by the texture or constitution of the surface of bodies. He may be considered as very fortunate in fixing on these examples, for it was by means of them that Newton afterwards found out the composition of light.

II. The instantiæ migrantes exhibit some property of the body passing from one condition to another, either from less to greater or from greater to less; arriving nearer perfection in the first case, or verging towards extinction in the second.

Suppose the thing inquired into were the cause of whiteness in bodies; an instantia migrans is found in glass, which entire is colorless, but pulverized becomes white. The same is the case with water unbroken or dashed into foam.

III. The instantia ostensivæ are the facts which show some particular property in its highest state of power and energy, when it is either freed from impediments which usually counteract it, or is itself of such force as entirely to repress those impediments.

If the weight of air were inquired into, the Torricellian experiment, or the barometer, affords an ostensive instance, where the circumstance which conceals the weight of the atmosphere in common cases, namely the pressure of it in all directions, being entirely removed, that weight produces its full effect, and sustains the whole column of mercury in the tube.

IV. The instances called analogous or parallel consist of facts between which a resemblance or analogy is visible in some particulars, notwithstanding great diversity in all the rest. Such are the telescope and microscope compared to the eye. It was the experiment of the camera obscura which led to the discovery of the formation of images of external objects in the bottom of

the eye by the action of the crystalline lens, and other humors. of which the eye is formed.

V. Instantiæ comitatus: examples of certain qualities which always accompany one another. Such are flame and heat: flame being always accompanied by heat, and the same degree of heat in a given substance being always accompanied with flame.

Hostile instances, or those of perpetual separation, are the reverse of the former. Thus transparency and malleability in solids are never combined.

VI. The instantia crucis. When in any investigation the understanding is placed in æquilibrio, as it were, between two or more causes, each of which accounts equally well for the appearances as far as they are known, nothing remains to be done, but to look out for a fact which can be explained by one of these causes and not by the other. Such facts perform the office of a cross, erected at the separation of two roads, to direct the traveller which to take: hence called crucial instances.

The experimentum crucis is of such weight in matters of induction, that in all those branches of science where it cannot be resorted to (an experiment being out of our power and incapable of being varied at pleasure) there is often a great want of conclusive evidence.

§ III. THE SPIRIT OF BACON'S METHOD.

We may now resume the question of Bacon's claim to the title of Father of Experimental Science. That which distinguishes his conception of philosophy from all previous conceptions is the systematization of graduated Verification, as the sole Method of research. Others before him, notably Albertus Magnus, had insisted on some parts of the experimental Method; his great predecessor and namesake, Roger Bacon, had, in the Opus Majus, insisted on experience as the truest guide, and had distributed the causes of error under four heads (Authority, Cus tom, Vulgar Prejudice, and False Science), but no one had coordinated into a compact body of doctrine all the elements of