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swell or dilate in bulk, such as red-hot iron, in which the heat is most violent.
It is also shown by the fires burning most briskly in the coldest weather.
It is also shown by this; that when the air is dilated in the thermometer uniformly and equably, without any impediment or repulsion, the heat is not perceptible. In confined draughts also, although they break out very violently, no remarkable heat is perceived, because the motion affects the whole, without any alternating motion in the particles. For which reason try whether flame do not burn more at the sides than in its centre.
It is also shown in this, that all burning proceeds by the minute pores of bodies, undermining, penetrating, piercing, and pricking them as if with an infinite number of needlepoints. Hence all strong acids (if adapted to the body on which they act) exhibit the effects of fire from their corroding and pungent nature.
The difference of which we now speak is common also to the nature of cold, in which the contracting motion is restrained by the resistance of expansion, as in heat the expansive motion is restrained by the resistance of con
Whether, therefore, the particles of matter penetrate inwards or outwards the reasoning is the same, though the power be very different, because we have nothing on earth which is intensely cold.
IV. The fourth difference is a modification of the preceding; namely, that this stimulating or penetrating motion should be rapid and never sluggish, and should take place not in the very minutest particles, but rather in those of some tolerable dimensions.
It is shown by comparing the effects of fire with those of time. Time dries, consumes, undermines, and reduces to ashes as well as fire, and perhaps to a much finer degree, but as its motion is very slow, and attacks very minute particles, no heat is perceived.
It is also shown in a comparison of the dissolution of iron and gold. For gold is dissolved without the excitement of any heat, but iron with a vehement excitement of it, although almost in the same time: because, in the former, the penetration of the separating acid is mild, and gently insinuates itself, and the particles of gold yield easily, but the penetration of iron is violent, and attended with some struggle, and its particles are more obstinate.
It is partially shown also in some gangrenes and mortifications of flesh, which do not excite great heat or pain from the gentle nature of the putrefaction.
Let this suffice for a first vintage, or the commencement of the interpretation of the form of heat by the liberty of the understanding.
From this first vintage, the form or true definition of heat (considered relatively to the universe and not to the sense) is briefly thus. "Heat is an expansive motion restrained, and striving to exert itself in the smaller particles." The expansion is modified by "its tendency to rise though expanding towards the exterior;" and the effort is modified by its not being sluggish, but active and somewhat violent. With regard to the operative definition, the matter is the "If you are able to excite a dilating or expansive motion in any natural body, and so to repress that motion and force it on itself as not to allow the expansion to proceed equally, but only to be partially exerted, and partially repressed, you will, beyond all doubt, produce heat;" without any consideration as to whether the body be of earth (or elementary as they term it) or imbued with celestial influence, luminous or opaque, rare or dense, locally expanded or contained within the bounds of its first dimensions, verging to dissolution or remaining fixed, animal, vegetable, or mineral, water, or oil, or air, or any other substance whatever susceptible of such motion. Šensible heat is the same, but considered relatively to the senses. Let us now proceed to further helps.
21. After our tables of first review, our rejection or exclusive table and the first vintage derived from them, we must advance to the remaining helps of the understanding with regard to the interpretation of nature, and a true and perfect induction; in offering which we will take the examples of cold and heat where tables are necessary, but where fewer instances are required we will go through a variety of others; so as neither to confound investigation nor to narrow our doctrine.
In the first place, therefore, we will treat of prerogative instances; 2. Of the supports of induction; 3, Of the cor-` rection of induction; 4. Of varying the investigation according to the nature of the subject; 5. Of the prerogative natures with respect to investigation, or of what should be the first or last objects of our research; 6. Of the limits of investigation, or a synopsis of all natures that exist in the universe; 7. Of the application to practical purposes, or
of what relates to man; 8. Of the preparations for investigation; 9. And, lastly, of the ascending and descending
scale of axioms.
22. Amongst the prerogative instances we will first mention Solitary instances. Solitary instances are those which exhibit the required nature in subjects that have nothing in common with any other subject than the nature in question; or which do not exhibit the required nature in subjects resembling others in every respect except that of the nature in question. For these instances manifestly remove prolixity, and accelerate and confirm exclusion, so that a few of them are of as much avail as many.
For instance let the inquiry be the nature of colour. Prisms, chrystalline gems, which yield colours not only internally but on the wall, dews, &c. are solitary instances. For they have nothing in common with the fixed colours in flowers and coloured gems, metals, woods, &c. except the colour itself. Hence we easily deduce that colour is nothing but a modification of the image of the incident and absorbed light, occasioned in the former case by the different degrees of incidence, in the latter by the various textures and forms of bodies.* These are solitary instances as regards similitude.
Again, in the same inquiry, the distinct veins of white and black in marble, and the variegated colours of flowers of the same species, are solitary instances: for the black and white of marble, and the spots of white and purple in the flowers of the stock, agree in every respect but that of colour. Thence we easily deduce that colour has not much to do with the intrinsic natures of any body, but depends only on the coarser and as it were mechanical arrangement of the parts. These are solitary instances as regards difference. We call them both solitary or wild, to borrow a word from the astronomers.
23. In the second rank of prerogative instances we will consider Migrating instances. In these the required nature passes towards generation, having no previous existence, or towards corruption, having first existed. In each of these divisions, therefore, the instances are always twofold, or rather it is one instance, first in motion or on its passage, and then brought to the opposite conclusion. These instances not only hasten and confirm exclusion but also
This very nearly approaches to Sir I. Newton's discovery of the decomposition of light by the prism.
reduce affirmation, or the form itself to a narrow compass. For the form must be something conferred by this migration, or on the contrary removed and destroyed by it. And although all exclusion advances affirmation, yet this takes place more directly in the same than in different subjects. But if the form (as it is quite clear from what has been advanced) exhibit itself in one subject it leads to all. The more simple the migration is, the more valuable is the instance. These migrating instances are, moreover, very useful in practice, for since they manifest the form, coupled with that which causes or destroys it, they point out the right practice in some subjects, and thence there is an easy transition to those with which they are most allied. There is, however, a degree of danger which demands caution, namely, lest they should refer the form too much to its efficient cause, and imbue or, at least, tinge the understanding with a false notion of the form from the appearance of such cause; which is never more than a vehicle or conveyance of the form. This may easily be remedied by a proper application of exclusion.
Let us then give an example of a migrating instance. Let whiteness be the required nature. An instance which passes towards generation is glass in its entire and in its powdered state; or water in its natural state, and when agitated to froth. For glass when entire, and water in its natural state, are transparent and not white, but powdered glass and the froth of water are white and not transparent. We must inquire, therefore, what has happened to the glass or water in the course of this migration. For it is manifest that the form of whiteness is conveyed and introduced by the bruising of the glass, and the agitation of the water. But nothing is found to have been introduced but a diminishing of the parts of the glass and water, and the insertion of air. Yet this is no slight progress towards discovering the form of whiteness, namely, that two bodies in themselves more or less transparent (as air and water, or air and glass), when brought into contact in minute portions exhibit whiteness, from the unequal refraction of the rays of light.
But here we must also give an example of the danger and caution of which we spoke. For instance; it will readily occur to an understanding perverted by efficients, that air is always necessary for producing the form of whiteness, or that whiteness is only generated by transparent bodies, which suppositions are both false, and proved
to be so by many exclusions. Nay, it will rather appear (without any particular regard to air or the like) that all bodies which are even in such of their parts as affect the sight exhibit transparency, those which are uneven and of simple texture whiteness, those which are uneven and of compound but regular texture all the other colours except black, but those which are uneven and of a compound irregular and confused texture exhibit blackness. An example has been given, therefore, of an instance migrating towards generation in the required nature of whiteness. An instance migrating towards corruption in the same nature is that of dissolving froth or snow, for they lose their whiteness and assume the transparency of water in its pure state without air.
Nor should we by any means omit to state that under migrating instances we must comprehend not only those which pass towards generation and destruction, but also those which pass towards increase or decrease, for they too assist in the discovery of the form, as is clear from our definition of a form, and the table of degrees. Hence paper, which is white when dry, is less white when moistened (from the exclusion of air and admission of water), and tends more to transparency. The reason is the same as in the above instances.
24. In the third rank of prerogative instances we will class Conspicuous instances, of which we spoke in our first vintage of the form of heat, and which we are also wont to call coruscations, or free and predominant instances. They are such as show the required nature in its bare substantial shape, and at its height or greatest degree of power, emancipated and free from all impediments, or at least overcoming, suppressing, and restraining them by the strength of its qualities. For since every body is susceptible of many united forms of natures in the concrete, the consequence is that they mutually deaden, depress, break, and confine each other, and the individual forms are obscured. But there are some subjects in which the required nature exists in its full vigor rather than in others, either from the absence of any impediment or the predominance of its quality. Such instances are eminently conspicuous. But even in these care must be taken, and the hastiness of the understanding checked, for whatever makes a show of the form, and forces it forward, is to be suspected, and recourse must be had to severe and diligent exclusion.
For example; let heat be the required nature. The