Mitscherlich has shewn that the mutual inclination of the crystalline planes in calc spar is altered eight and a half minutes of a degree between the temperature of 32° and 212°; while in other cases it varies nearly 1o in different specimens of the same salt. For a full account of his discoveries, see Ann. de Chym. et de Phys. tom. xiv. 172; xix. 350; xxiv. 264, 355.

In accordance with the above facts, it has been discovered that on crystallizing from solution in the bisulphuret of carbon, or in oil of turpentine, at temperatures below 100°, sulphur assumes the octohedric form, with rhombic bases; but that when melted by itself and allowed to cool slowly, it takes the form of an oblique rhombic prism on solidifying at 230°. (Graham's Elements of Chemistry, p. 189.) It has also been ascertained that pure carbon occurs in the form of regular octohedrons, as in the diamond, but in six sided plates in Graphite. And Professor Johnston has recently shown that there is a large class of bodies composed of the same elements in the same proportions, which crystallize in two, if not more primitive forms. But since the time of Dr. Black, chemists have attended so little to the agency of caloric in the various transformations of matter, that they have scarcely ranked it among their elements, although it constitutes by far the largest proportion by volume of solid bodies.

The phenomena of isomerism are no less remarkable, as illustrating the influence of caloric

in changing the chemical properties of bodies. For example, if phosphoric acid be obtained dry by evaporating its solution, and then fused, it becomes what is called pyro-phosphoric acid. In these two states, although composed of phosphorus and oxygen in the same proportions, it produces totally different salts, on uniting with equal proportions of the same base. It has also been found, that crystals of tartaric and racemic acids, in their ordinary class of salts, are both composed of oxygen, hydrogen, and carbon, in the same proportions, and are so modified by heat as to form different salts on uniting with the same proportions of the same base and so of many other acids.

Among the most civilized nations of antiquity, it was supposed that everything in nature is composed of four primitive elements, which they termed earth, water, air and fire, to which Aristotle added a fifth. But the tendency of modern science has been to increase the number, until it has extended to fifty-four, all of which are regarded as simple, because chemists have not yet been able to resolve them into fewer elements. There is reason, however, to believe that none of our chemical atoms and elements are perfectly simple, if we except caloric, which also exists in a great variety of states, as will be shown hereafter; and that it is only in the subtilized form of light that we can hope to discover the ultimate constitution of ponderable matter.* * For example, if it be true that all the reputed elements are

H

In accordance with the hypothesis advanced by Newton in one of his Optical Queries, that light and common matter are mutually convertible into each other, it has been recently maintained

convertible into light, and that light is decomposable into seven primitive rays by prismatic refraction, as maintained by Newton, or into only three fundamental colours, when analyzed by absorbent transparent media, as maintained by Brewster, it will follow that what we call simple bodies are composed of one, two, three, or more of the primitive elements which constitute white light. Now it is worthy of remark, that hydrogen is the most elastic of all gaseous bodies, and affords during combustion an almost perfectly blue light; while according to the experiments of Newton, the blue rays are more refrangible than any others except the violet, which is not a simple colour according to Brewster, but composed chiefly of blue, with small proportions of yellow and red; which he regards as the three primitive rays, and the elements of all compound colours. It is therefore probable that the blue and violet rays are more refrangible than the others, for the same reason that hydrogen, of which they are formed, is the most elastic of all known bodies; viz. because it contains more caloric around its particles in proportion to their size. And it was supposed by Newton that the atoms of light diminish in size from the red to the violet extremity of the spectrum. Again, as sulphur burns with a blue, and iodine with a violet light, (which contains small proportions of the other rays,) they may be composed chiefly of hydrogen, with some other base or bases; and cannot be otherwise decomposed than by expanding them into the subtile form of light. On the other hand, during the combustion and ignition of potassium, strontium, barium, lithium, sodium, and magnesium, they afford chiefly red light, with small proportions of the other coloured rays; therefore must all be composed principally of one element, the primitive atoms of which are red, while those of hydrogen (which is the basis of sulphur, iodine, and perhaps some other bodies) are blue. Hydrochlorate of calcined soda also affords an orange coloured light; while in that of chlorine, silver, copper, and some other bodies, there is a predominance of greenish rays; so that if green light be composed chiefly of yellow and blue, as maintained by Brewster,

by Sir David Brewster, that" the particles of light are identical with the ultimate atoms of bodies, and that there is a specific affinity between definite atoms and definite rays, though we do not understand its nature." (Transactions of the Brit. Association, vol. i. p. 231.) He also observes in another work, that "when a portion of light enters a body, and is never again seen, we are entitled to say that it is detained by some power exerted over it by the particles of the body; that it enters into chemical combination with them, and produces the various chemical effects by which their colours are changed, and the juices of plants elaborated,"―to which he adds, "It is not easy to allow that such effects are produced by undulations of an æthereal medium." (Optics, chap. xxxiv.)

That every description of ponderable matter is actually convertible into light by a sufficiently intense heat, or by electricity, will appear from the following undeniable facts :

1. That the quantity of light generated by ordinary combustion, friction, or percussion, is always in proportion to the rapidity with which ponderable matter is ignited and volatilized.

2. That the colour of light thus produced always depends on the species of matter employed.

all bodies that burn with a green light must be compounds of two or more primitive elements; while others, again, which afford different proportions of all the coloured rays of the spectrum are still more complex.

3. That the electric spark (like that produced by the collision of flint and steel) consists of exceedingly minute portions of ponderable matter in a state of incandescence, as will be shown hereafter by the decisive and beautiful experiments of Fusinieri.

4. That when the electric fluid is transmitted through the vacuum of an air pump, little or no light is produced, as proved by the experiments of De Luc, and afterwards by those of Sir H. Davy.

5. That the most intense heat of a voltaic battery never produces any light, except when acting on ponderable matter; consequently, that light and heat are not identical,* as maintained by some modern theorists; and that neither of them is generated by the mere vibrations of an æthereal medium.

That caloric is a constituent of light, (as it is of all other bodies,) and is the active principle in its generation, is evident from the fact, that the most refractory gems and metals are transformed into luminous incandescent particles by a sufficiently intense heat, definite measures of which are required to produce the effect on different species of matter. It has also been proved by the experiments of Hulme, Dessaignes, Macartney, and others, that every species of phosphorescence is promoted (for a time) by moderate heat, and always extinguished by the coldness of freezing mixtures; while others have found that the light of luminiferous insects is much greater in vessels of oxygen gas than in common air. But that caloric is a distinct essence, and may exist independent of light, is manifest from the fact, that it converts solids into liquids, vapours, and gases, in the midst of perfect darkness; while it radiates from the earth during night, as it does from boiling liquids and other hot bodies, without being attended with any light.