It therefore follows, that if the caloric of nitric acid enabled it to dissolve and combine with the silver, it must have a still stronger attraction for the particles of mercury, or it would not desert the atoms of silver for them; in short, that the silver is separated from its combination with the acid by an abstraction of caloric, for the same reason that salts dissolved in water are precipitated by a reduction of temperature.

The same principle must apply to all other elections and precipitations, modified, however, by circumstances not yet fully understood.

Lead precipitates mercury, and copper lead, which is again thrown down from a state of combination with nitric acid by iron; all of which changes are attended with transitions of caloric from the solvent to the solvend.

The art of coating metals with other metals by precipitation, is founded on the above law, such as gilding, silvering, tinning, &c. If a plate of copper or iron be immersed in a solution of nitrate of mercury, a coating of the latter metal is precipitated on them, for the same reason that aqueous vapour is precipitated on cold bodies, and coheres to them in the form of frost or ice. If nitrate of silver be dissolved in water, and a plate of clean copper be inserted into the solution, a beautiful coating of silver is precipitated on the copper; or, if a polished iron rod be immersed in a solution of nitro-muriate

of gold, the latter is precipitated on it in the metallic form; for the same reason that melted tin, zinc, bismuth, silver, &c. are attracted by solid metals, and thus chemically united with them, as in the process of plating. Hence it follows, that if caloric be indispensable to the solution of metals in acids, their precipitation must be owing to its abstraction by the solid metals.

From a general view of the preceding facts and observations, it is evident, that caloric is the cause of all oxidation; that water and the strong acids are generated by its agency, and chemically united with the earthy, alkaline, and metallic oxides, forming salts and rocks: in short, that combustion, fermentation, and solution, are modified effects of the same cause. Vinous fermentation begins at about 59° F. and increases up to 77°, at which point acetous fermentation begins, and ceases at 88°. The difference between putrefaction and combustion is only in degree. The former requires a temperature of about 80°, and augments up to 135° F.; while phosphorus enters into a slow combustion at 43o, and potassium at still lower temperatures.

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CHAPTER IV.

Capillary Attraction.

That very law which moulds a tear,
And bids it trickle from its source,
That law preserves the earth a sphere,
And guides the planets in their course.

ROGERS.

CONSIDERED in all its relations and bearings, capillary attraction is one of the most important processes in nature, and is immediately connected with all the changes and modifications of both living and dead matter. It is the force by which solids and liquids are drawn and held together; which causes plates of glass, wood, metal, &c. to adhere with some force to the surface of water and other liquids, when laid flat upon them. It is by the force of capillary attraction, that the waters of the earth are conveyed through its crust in opposition to that of gravity; for water is much lighter than the earthy and rocky strata through which it descends; and has been found at the greatest depths to which miners have penetrated, forming subterraneous streams, which greatly augment after heavy rains. During this transudation of water through

the earth, it dissolves and combines chemically with a portion of its mineral ingredients; after which, it is forced to the surface by hydrostatic pressure, (on meeting with an obstruction to its horizontal progress,) forming innumerable springs and fountains of delicious water.

It is owing to the attraction of liquids for solids that the dust of our roads, which, when dry, has little or no cohesion, becomes so tenacious after rain; or that a rope when moistened with water, contracts with great force. The combination of tannin with leather, and colouring matter with clothing, are due to a modification of the same power that enables water to combine with salts, or to permeate porous rocks.

Several distinguished philosophers have recently conjectured, that a discovery of the cause which determines the rise of liquids through porous solids and capillary tubes, would unfold the latent principle of action in all molecular changes, whether chemical or vital. The trunks, stems, leaves, and flowers of all the trees and plants that adorn the earth, are composed throughout, of exceedingly small capillary tubes and pores that attract liquid nourishment from the earth, and convert it into their own substance, by the same power which causes the combination of other solids and liquids. And so of all animals. Every muscle, gland, nerve, bone, and blood vessel, is composed of imperceptibly

small capillary tubes, through which the blood is circulated by the same power which causes universal attraction.

The subject of capillary attraction has been laboriously investigated by many of the most distinguished votaries of science, for the last 150 years; but without any attempt to explain the cause of the phenomena; if we except a single suggestion of Newton, contained in his letter to Boyle, concerning the ather. He there refers capillarity to the same agent which causes menstruums to dissolve solids, -a most important hint, for which his successors have not given him any credit. It must, however, be acknowledged, that he never gave a satisfactory explanation of what causes solution. Still, his views were more just and comprehensive than those of the present day. That he also referred vital attraction to the same cause which produces ordinary capillary attraction, is evident from the following observation: "The same principle which causes a sponge to suck in water, causes the glands in animal bodies to suck in the various juices from the blood, according to their several natures and dispositions." (Opticks, page 367.)

In the time of Newton, it was ascertained experimentally by Hawkesbee,

1. That water rises to the same height in capillary glass tubes of equal diameter, whether the tubes be thin or thick.