4 decay of vegetation, constitute a natural thermometer on a grand scale. It will also be found, that organized bodies are composed chiefly of those elements which contain the largest amount of caloric around their particles; by which they are rendered proportionally active, and endowed with the faculty of entering into vital combinations, so as to form highly complex tissues: that the vital energy of animals, the activity of their functions, and the developement of their organs, are in proportion to the amount of caloric which circulates through, and is employed in combining arterial blood with the solids. A right understanding of these laws is of vastly greater importance than those of Geology, Astronomy, or even Chemistry. For until we comprehend the operations of life, health, disease, the modus. operandi of medicines and morbific agents, the healing art can never take its appropriate rank among the exact sciences; nor be rescued from the charge of mysticism, quackery, and ignorance of what is essential to the character of a philosophical physician. But as "the strength of a science is, like that of the old man's fagot, in the bond," the candid seeker after truth is requested to suspend his judgment, until he shall have carefully examined the whole of the evidence adduced in support of the author's peculiar views. 45* CHAPTER II. Atomic Constitution of Matter. "Omnia in mensura, et numero, et pondere disposuisti." BEFORE treating of the relative quantities of The same theory was adopted by Bacon, Boyle, Newton, and many other distinguished modern philosophers, who very justly conceived that if matter were infinitely divisible, all species would be confounded. And it is obvious that if the elements did not unite in definite proportions, the laws of chemistry could not be understood, nor its results be predicted with certainty. In opposition to this simple and rational view of the subject, it was maintained by Boscovich, that the minutest portions of matter consist of mere mathematical points, without extension or solidity, surrounded by alternate immaterial spheres of attraction, and repulsion,—by one of which, the said mathematical points are drawn together, and by the other kept asunder, so as to be prevented from touching. We are indebted to the researches of Wenzel, Bergman, and Richter, for the discovery that definite proportions of acids and alkalies are required for mutual saturation; and that many other chemical combinations take place only in fixed ratios. It was further maintained by William Higgins of Dublin, (in a work entitled " A Comparative View of the Phlogistic and Antiphlogistic Theories," published in 1789,) that all compounds are formed by the union of exceedingly minute atoms, which are surrounded by atmospheres of caloric; and that oxygen unites with nitrogen in proportions which are even multiples of the lowest. But to Dr. John Dalton of Manchester, was reserved the distinguished honour of reducing the atomic theory to a systematic form, in a work entitled "Chemical Philosophy," published in 1808-10,-aided by the subsequent analyses of Thomson, Berzelius, Prout, GayLussac, Dulong, and many others. The most general law of chemical affinity as announced by Dalton is, that the constituents of ponderable matter, whether simple or compound, are capable of combining with each other only in fixed or definite proportions by weight. In a practical point of view, this law has been of the highest importance to the manufacturing chemist, independent of its connexion with the whole theory of physical science. Passing over the early and imperfect analyses of Dalton, it has been established by the united labours of the most accurate experimenters of modern times, that water is always composed of 8 parts by weight of oxygen to 1 of hydrogen; that common salt is always composed of 36 parts by weight, (in round numbers) of chlorine to 24 of sodium; that carbonate of lime, whether in the form of marble, chalk, or stalactites, is invariably composed of 22 parts by weight of carbonic acid, to 28 of lime, &c. It was by thus ascertaining the relative proportions in which bodies combine chemically by weight, that Dr. Dalton was lead to a perfectly simple method of deducing the relative weights of their ultimate chemical atoms. The theory assumes, that in all cases, the lowest combining proportion of bodies, whether simple or compound, represents the weight of its particles; and that whenever a body combines with another in two or more proportions, the larger is an even multiple by some whole number of the smaller, as 2 to 1, 3 to 1, 4 to 1, &c.; fractional parts seldom occurring in well ascertained cases. By inspecting the following Table, it will be seen that the smallest combining ratio of oxygen is 8, compared with hydrogen as unity; and that when it combines with other elements in larger proportions, they are 16, 24, 32, 40, or some multiple of 8. Chlorine combines with mercury in the ratio of 36, as in chloride of mercury, (calomel), or of 72, as in the bi-chloride of mercury, (corrosive sublimate) and so of carbon, hydrogen, and other elements. I have adopted the atomic numbers employed by the majority of British chemists in preference to those of Berzelius, chiefly with a view of avoiding the inconvenience of fractions. I have also chosen hydrogen as a standard of comparison for the same reason. Berzelius has selected oxygen 100.00 as a standard, on account of its great abundance, and the vast variety of combinations which it forms; while Thomson and Ure have employed oxygen as unity, by which hydrogen is rendered 8 times less; or 0.125, carbon 0.75, sulphur 2, mercury 25 or 12, chlorine 4.5, and so on; both of which are unavoidably attended with fractions that are difficult to remember. Dr. Wollaston employed the terms chemical equivalent, and Sir Humphrey Davy combining proportion, to denote the atom of Dalton. I shall generally use the term atom, or particle, as indi |