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ELEMENTS OF MATERIA MEDICA.

The resins arc composed of carbon, hydrogen, and oxygen. In somecases they appear to be oxidized essential oils, (1 oil of turpentine + 1oxyg.) as will be shown when speaking of turpentine and copaiva resins.It is not improbable that the first degree of oxygenation of the volatileoils forms resins insoluble in cold alcohol, while the most oxygenated aresoluble in this liquid.

Class 3, Tonics. —Under the denomination of tonics are usually com-prehended those therapeutic agents which, by continued administration■ in debilitated and relaxed conditions of the body, increase gradually andpermanently the tonicity or insensible contractility of the whole sys-tem, and thereby render the fibres tenser and stronger, and give greater firm-ness and density to all the tissues and organs. They have received theirnames from tovoq , tone or vigour, on account of their strengthening or invigo-rating properties ; and by some they have been termed corroborants.

Tonics produce their proper or real tonic effects in certain conditions ofthe system only; that is, they do not invariably strengthen. In some casesthey give rise to no obvious results—in others they act as irritants and sti-mulants. In the healthy state moderate doses produce no sensible effects,or, perhaps, a slight excitement of the appetite merely, while large quan-tities give rise to nausea and vomiting. In irritation or inflammation ofthe stomach and intestines, and in febrile conditions of system, attendedwith a hot and dry skin, and a furred and dry tongue, tonics act as localirritants and excitants, and add to the severity of all the morbid symp-toms. In a weak and debilitated condition of body, tonics act verydifferently. Their immediate effects are to increase the appetite andassist digestion. After they have been administered for some time, thesoft solids (as the muscles, cellular tissue, &c.) become firmer, the muscularsystem more powerful, and the pulse stronger, though not quicker. Infact, all the functions are performed with more energy, and the patient iscapable of greater exertion.

Tonics sometimes purge, at others constipate. When diarrhoea arisesfrom, or is kept up by, a weakened state of the intestinal tube, tonics, byrestoring strength, may produce constipation. On the other hand, whenconstipation depends on a debilitated and torpid condition of thistube—a circumstance not uncommon in females, tonics, not unfre-quently, occasion alvine evacuations. Dr. Cullen having noticed howfrequently bitters act as laxatives and purgatives, has inserted them inhis list of cathartics.

Tonics are closely connected with the last-mentioned class of medi-cines : indeed, on many occasions, the so-called tonic substances actreally as stimulants. Thus in weak but irritable subjects just recoveringfrom a protracted state of fever, sulphate of quinia will frequently actboth as a local irritant and stimulant, and produce nausea, vomiting,furred tongue, a febrile state of system, headache, &c. In fact, the twoclasses (tonics and stimulants) mutually approach and gradually pass theone into the other, and several substances may with equal propriety bearranged under either.

Tonics are also closely related to the cerebro-spinants. Several of thevegetable bitter tonics specifically affect the cerebro-spinal system (forexample, quassia); while some of the cerebro-spinants (as strychnia), invery small doses, act as tonics. Moreover, the beneficial influence ofsome of the vegetable tonics (as cinchona) in intermittent diseases, should