416 ADROPHYSICS. 



Of chemical agents, the antacids are the alkalies; as ammonia, 

 potassa, soda, lime, and magnesia ; or their carbonates ; which serve 

 directly to neutralize acids, and prevent their injurious effect. On 

 the other hand, the antalkalies are the acids; as acetic, tartaric, 

 sulphuric, nitric, or muriatic; which however are seldom required, 

 unless it be as stimulants, or tonics. Acids, and alkalies, are some- 

 times used as antilithics ; but not without careful discrimination: the 

 tonics and diuretics being perhaps preferable. The chief disinfectants, 

 are chlorine ; or in its place the chloride of soda, or of lime ; and 

 sulphurous, nitric, and muriatic acids, charcoal, creosote, and smoke. 

 Of mechanical agents, in medicine, we may name as demulcents, 

 gum Arabic, or gum tragacanth ; Iceland, or Irish moss ; flaxseed, 

 barley, oat-meal, or starch ; sassafras pith, or slippery elm bark ; 

 and olive oil, lard, or spermaceti. When the action of diluents is 

 required, water, and the most simple beverages, are those generally 

 employed. 



3. Pharmacy, or Pharmaceutics, is that division of Pharmacology 

 which relates to the selection, preservation, and preparation of 

 medicines ; constituting the art of the apothecary. In purchasing 

 mineral substances, it is of course highly important that they should 

 be pure and genuine ; which can only be ascertained by chemical 

 tests. In procuring drugs from plants, botanical knowledge is also 

 necessary, in order to identify the species, to which chemical tests 

 would be inadequate. It is moreover important that the plants, or 

 their parts, should have been gathered in the right season, in a sound 

 state ; and that they should have been properly preserved. In 

 general, the best seasons for gathering medicinal plants, are the 

 spring and autumn : but flowers should be gathered when just 

 expanded ; and aromatic herbs, when just in flower. 



Among the chemical processes, employed in preparing medicines, 

 are sublimation, or the conversion of a solid into vapor ; pulveriza- 

 tion, or the reducing of a solid to a powder ; solution, or the 

 dissolving of a solid in a liquid ; maceration, or the soaking of a 

 vegetable substance, for a long time, in a cold liquid, to dissolve some 

 soluble principle ; digestion, or the same process at a temperature 

 between 90 and 100 ; decoction, or the same process briefly 

 conducted at a boiling heat ; distillation, or the heating of any 

 substance, in a retort, or close vessel, with a receiver to condense the 

 vapors which pass over ; lixivation, or leaching, to extract any 

 soluble substance by means of a liquid filtering through it ; crystalli- 

 zation, either by the evaporation of a liquid, or the cooling of a 

 melted substance ; and calcination, or the exposure of any solid to 

 a strong heat, to expel a vaporizable ingredient. 



In prescribing medicines, and preparing prescriptions, the measures 

 used, are, for solids the grain, (gr.) of which 5760 make a pound 

 Troy, and 7000 make a pound avoirdupois ; the scruple, @), or 20 

 grains ; the drachm, (3), which is three scruples, or 60 grains ; and 

 the ounce, (^), which is 8 drachms, or 480 grains : the ounce and 

 pound of apothecaries' being the same as those of Troy weight, but 

 differently subdivided. For liquids, the wine gallon, (c), is divided 

 into eight pints, (o), of 28.875 cubic inches each ; so that a pint 



