and cold; where should we seek, but in the element of fire, for the true and physical cause of cohesion ? when we descend indeed to the precise mode of its operation, it is hardly to be expected our ideas can have I take it for

a mathematical exactness.

granted they are very gross, and such as we should be ashamed of, if the sense of man extended to the first principles of matter, which God, in great wisdom, hath purposely placed out of his reach. But it is no inconsiderable step in philosophy, barely to determine the question, whether God hath chosen to act by material instruments, or immaterial influences: it being of the utmost im portance in every science to begin rightly, and have our labours directed into a proper channel.

As it will probably be much doubted whether the operations of heat and cold are to be ascribed to elementary fire, as to one and the same substance; that matter, I think, may easily be adjusted by observing what happens to a common thermometer. When you bring it near a fire, and find it rises from 55 degrees, the point of temperate, to 212, the degree of boiling water; to what substance do you impute this, and what is your name for

it? doubtless you will tell me, it is elementary fire. Suppose the thermometer to be falling 32 degrees below the point of freezing, and to be raised from thence up to temperate; during the greater half of which motion, a very severe coldness is predominant. Now, if you imagine heat and cold to be things different in their nature, yoù must provide one element to raise the therinometer through the upper parts of its scale, and another to raise it through the lower; and after all, you will never be able to guess at any point of distinction, where the one ends. and the other begins. For if your instrument, when exposed out of doors, should stand at 55 degrees in January, the air would be attended with a very sensible warmth to the body, and you would find yourself able to sit very comfortably without a fire. If it should stand at the same point in July, you would complain of shivering with cold, and perhaps order a fire to be lighted. Thus you will give the contrary appellations of hot and cold to one and the same temperature of the air; and if you realize these different ideas, which arise wholly from a deceived sensation, and search after a species of frigorific particles, acids of the air,

and

and such like, you discover nothing real in nature, but become the dupe of your own language.

Heat and cold are the names we have given to the sensations which are raised in us by the different (and sometimes by the same) impressions of elementary fire. If an effect is produced when the thermometer stands at the heat of boiling water, I call it fire that produces it: if water is frozen into ice, and the thermometer stands at 30°, I am sensible of cold in my body, but the agent in this case is elementary fire; and there may be a greater philosophical propriety than we are aware of in that expres sion of Virgil-Penetrabile frigus adurit*. As we do not look upon air to be one kind of element when the barometer sinks to 28 inches, and another when it rises to 31; the element of fire ought to be considered in the same way, under all the different stations of the thermometer, that is, all the different degrees of heat and cold.

Upon these considerations, I have followed the most eminent of the chemists, and some modern writers on electricity, in using

the word fire in its largest sense, when mean either fire, or light, or æther. We may indeed call it by any of these names, because the same fluid must be understood by every one of them: though, if we were to stand upon strictness and propriety of expression, it would be necessary, on some occasions, to use the first of these; on others, only the second or the third. When this fluid is cold and invisible, or appearing only in pellucid bubbles like air, as in the freezing experiment, let us call it æther"cæcus ignis expers luminis*."

becomes visible and lucid, it is men have agreed to call light. gives heat and burns, it is fire.

When it

what all

When it

The em

ployment most agreeable to me, is to search after things, and try to render them intelligible: if I can but be so fortunate as to make some little progress in that respect, I leave the reader to correct my expressions, and settle the application of words at his own discretion. If he has had patience enough to follow me thus far, I hope he will hear me out, while I examine the nature of another sort of power, opposite to attraction, and distinguished by the writers of these times under the name of repulsion.

Du Hamel,

CHAP,

CHAP. V.

Of the Physical Cause of Repulsion; particularly as this Principle is applied to the Elasticity of the Air.

T is needless to repeat all the instances

IT

commonly urged, either to illustrate the operation, or prove the reality of such a principle as repulsion. The best-known, and most universal of these, is the elasticity of the air; the property by which it resists any compressing force, and which the great Sir Isaac Newton, and others, have endeavoured to account for, by supposing a repulsive force to be implanted in its particles: for thus, it is presumed, they may drive each other farther off, without the intervention of any other matter *.

If repulsion is proposed to us, as a discovery of the cause of this property in the air,

"Which vast contraction and expansion (of the air) seems unintelligible, by feigning the particles of air to be "springy and ramous, or rolled up like hoops, or by any "other means than by a repulsive power," Newt. Opt,