Independent of the knowledge of diseases and the treatment of them, much of the success of the practitioner depends on circumstances connected altogether with the form in which the remedies are exhibited. In prescribing a medicine, even the best calculated to fulfil the object of the practitioner, it is necessary to consider the age, sex, temperament, habits, and idiosyncrasy of the patient, before the dose can be properly apportioned; and as far as the medicine itself is regarded, the most convenient and agreeable form of exhibiting it; whether it should be given alone, or combined with other ingredients, and how far these are likely to impede, modify, or facilitate its operation. An attention to these circumstances is absolutely requisite to prevent the errors which too frequently occur in forming a prescription.

1. Circumstances connected with the State of the Patient. Age. -Here it must be observed, that the doses of the medicines described in the foregoing pages are those adapted for an adult; but, as in the two extremes of life, childhood and old. age, the body is weaker, and in early youth more susceptible of all impressions, these quantities cannot be administered with safety in every case; and thence the judgment of the prescriber must be exercised. Under ordinary circumstances the following table, originally drawn up by Gaubius, may be considered as a sufficient guide for the young practitioner: -

Ages.

Proportional Quantities.

Doses.

For an adult

Suppose the dose to be - -

ONE

or 1 drachm.

Under 1 year

Will require only - -

1/12

- 5 grains.

2 years

-----

1/8

- 7 1/2 grains.

9--

-----

1

- 10 grains.

4--

-----

1/4

- 15 grains.

7--

-----

1/2

- 1 scruple.

14--

-----

1/2

- 1/2 drachm.

20--

-----

2/5

- 2 scruples.

Above 21--

The full dose

- 1 drach

65--

The inverse gradation of the above.

Sex,__Although some women possess as much bodily strength and vigour of constitution as the majority of men, yet the general greater delicacy and sensibility of the female frame, at every period of life, require not only caution in apportioning the doses of active medicines, which should be less than those ordered for men of the same age; but the medicines themselves should be such as are likely to fulfil the indications required, without much violence. The state of the uterine system likewise must not be overlooked in prescribing for a female. Thus the employment of aloetic and drastic purgatives, cinchona bark, sulphuric acid, and astringents, should be suspended during the period of the catamenia.

Temperament. - It is undoubtedly true, that persons of different temperaments or original conformations of body are differently affected by the operation of medicines. Stimulants more readily affect those of a sanguine than those of a phlegmatic temperament: and, therefore, smaller doses are required. In the phlegmatic, also, the bowels are generally torpid, and require both a description of purgatives and such doses of them to excite the proper peristaltic motion as would induce either visceral inflammation or be followed by an alarming state of debility, were they administered to those of a sanguine temperament. Hence the necessity of attending to this circumstance in prescribing.

Habits - have a considerable influence in modifying the operation of medicines. Persons addicted to the use of spirits, narcotics, and other stimulants, are less easily excited both by medical stimulants and narcotics; and the knowledge of the habits of a patient, as far as the exhibition of purgatives is concerned, is absolutely necessary for the prescriber, many people being in the almost daily habit of taking this class of remedies without consulting a medical practitioner. In the first of these cases, larger doses of stimulants and narcotics are required to produce the ordinary effects of these remedies; but in the second a change of the purgative usually taken will generally be sufficient. In the employment of medicines, also, which require to be long continued, the beneficial effect is soon lost if the doses be not increased.

Idiosyncrasy. - Many persons have a peculiarity of disposition, unconnected with temperament, which renders them liable to be affected by substances taken into the stomach, either in the form of food or of medicine, in a manner different from the majority of mankind. Such a state can be discovered only by accident or by time; but when it is known, it must be attended to by the practitioner. Instances in which opium proves deleterious in every form and dose, are not unfre-quent. I knew a lady, in whom the smallest dose of squill excited an erythematic eruption over the whole body; spirit of turpentine, also, frequently produces a similar eruption; and many examples of the same kind might be quoted. But besides these guides in forming a prescription, the choice of the medicine must occasionally depend on the circumstance of the patient being more or less immediately under the eye of the prescriber. Thus, if the patient can be seen every day, or frequently, by the practitioner, the most active medicine which the nature of the case requires should be chosen; but if he cannot be frequently seen, or is not resident in the same place, the practitioner should choose a remedy of the same class, but less likely to have a sudden or violent effect.

Thus, in prescribing for ascites under such circumstances, squill or acetate of potassa, or bitartrate of potassa, must be preferred to elaterium; for intermittent fever, cinchona bark to the arsenical solution; and so in other cases, when the patient is not under the eye of the prescriber.

2. Of the Form and Composition of extemporaneous Prescriptions.-- In every prescription simplicity should be kept in view, and when one medicine will answer the intention of the prescriber, it ought to be preferred. The nauseous taste, however, and the other qualities of the great majority of drugs, require the addition of others to modify their action; but, although medicines are more generally prescribed in a compound form, yet the practice of accumulating a great variety of ingredients in one prescription must be avoided.

Medicines exhibited in the fluid form operate sooner, and with more certainty than in the solid state : but in choosing the vehicle or solvent, the taste of the patient ought not to be overlooked. Thus, for those to whom peppermint-water is not disagreeable, the nauseous taste of sulphate of magnesia is more completely concealed by that vehicle than any other; if cinchona bark in powder be ordered, milk effectually covers its taste, provided the dose be taken the moment it is mixed; and if aloes, the most nauseous article of the materia medica, be prescribed in a fluid form, a solution of extract of liquorice renders it by no means unpalatable. Medicines which, when given alone, produce griping, require the addition of aromatics to correct that quality; and when they operate with violence, mucilages and demulcents are sometimes necessary to obtund their acrimony, or narcotics to moderate their action. In prescribing purgatives, it is also necessary to consider the particular part of the alimentary canal on which they more immediately act. Thus rhubarb acts chiefly on the pylorus and duodenum, aloes on the rectum, and calomel and jalap on the larger intestines. Another reason for ordering medicines in a compound form, is the necessity of producing two or more effects at one time.

Thus the same dose may be required, in a case of colic, for example, to allay pain and to open the bowels; or, in fever, to determine to the surface, to allay irritation, and to produce sleep. But in combining medicines, care must be taken not to bring together incompatibles, or substances that decompose each other, or chymically combine, and, consequently, alter the nature of the mixture, or render it inert; unless the resulting compound be the remedy on which the practitioner relies. Thus, acids and alkalies are incompatible, unless the neutral salt they produce be the remedy required; and astringent vegetable infusions and decoctions destroy the emetic and diaphoretic property of tartar emetic. Hence the necessity of a knowledge of chymistry to the medical practitioner.

In writing a prescription, the first object is the principal or most active ingredient, which is called the basis; the next, the adjuvans, or that which is designed to promote the action of the basis; the third, the corrigens, or that intended to correct or modify its action; and the last, the vehiculum, or that substance in which the more active ingredients are to be exhibited, and which, consequently, gives the formula its peculiar character. It has been usually regarded as a proper rule in writing a prescription to place the basis first, and the other articles in the form in which they have been enumerated; but this must depend on the mode best fitted for compounding the medicine. Thus, salts and other soluble solids should be placed before the menstruum in which they are to be dissolved; and volatile substances should always be placed last, as they are necessarily the last ingredients added in the manipulation of the compound. Finally, the names of each ingredient should be written at full length, in a legible hand, and the symbols of the quantities distinctly marked; and no prescription should pass from the hand of the prescriber without being deliberately read over, and its correctness ascertained.

To facilitate the art of prescribing to the young practitioner, I have added a Table of Incompatibles, and a few examples of the usual Forms of extemporaneous Prescriptions.

No. IV. Table Of Incompatibles

Substances

Incompatible with

Acidum Citricum - -

Alkalies.

Alkaline Solutions.

-- Sulphates.

-- Carbonates.

-- Tartrates.

Soaps.

Earthy Carbonates.

Acetates.

Metallic Carbonates.

Substances

Incompatible with

Acidum Hydrochloricum -

Alkalies.

Tartrate of Potassa.

Sulphuret of Potassium.

Most Earths.

-Oxides.

--Carbonates of the incompatible Oxides.

Tartarized Antimony.

Tartrate of Iron.

Nitrate of Silver.

Solutions of Acetate and Diacetate of Lead.

--Nitricum - -

Alkalies.

Carbonates of Alkalies.

Acetates of Alkalies.

Earths.

Oxides.

Sulphate of Iron.

Solutions of Acetates of Lead.

Sulphurets,

Cyanides.

Iodides.

Charcoal.

Phosphorus.

Sugar.

Alcohol and Spirits.

Volatile Oils.

--Sulphuricum - -

Alkalies.

Alkaline Carbonates.

Some Earths.

- Earthy Carbonates.

Solution of Chloride of Calcium.

Barytic Salts.

Oxides of Metals.

Solutions of Acetates of Lead.

-- Tartaricum - -

Alkalies.

Carbonates of Alkalies.

Salts of Potassa.

Most Earths.

-- Carbonates of Earths.

Salts of Lime.

- Lead.

Ammonias Liquor - -

All Acids.

Saline Solutions of most Earths,

except Baryta and Lime.

-- Sesquicarbonas -

Acids.

- Potassa.

Soda.

-- Carbonas - -

Carbonate of Soda.

-- Potassa.

Bisulphate of Potassa.

Bitartrate of Potassa.

Lime.

-- water.

Solution of Chloride of Calcium.

Magnesia.

Sulphate of Magnesia.

Alum.

Salts of Iron, except Potassio-Tartrate of Iron.

Sulphate of Zinc.

Bichloride of Mercury.

Acetate of Lead.

Diacetate of Lead.

Substances

Incompatible with

Ammoniae Acetatis Liquor

Acids.

Potassa.

Soda.

Carbonate of Potassa.

------------------Soda.

Lime-water.

Magnesia.

Sulphate of Magnesia.

Bichloride of Mercury.

Sulphate of Iron.

-----------------Copper.

------ Zinc.

Nitrate of Silver.

Acetate of Lead.

Diacetate of Lead.

Piperina -

Carbonates of Alkalies.

Ferro-cyanide of Potassium.

Aqueous Solutions.

Potassae Carbonas

Acids.

Acidulous Salts.

Hydrochlorate of Ammonia.

Acetate of Ammonia.

Lime-water.

Chloride of Calcium.

Sulphate of Magnesia.

Disulphate of Quina.

Alum.

Tartarized Antimony.

Nitrate of Silver.

Ammoniated Copper.

------------------Iron.

Tincture of Ammoniated Iron.

-----------------Chloride of Iron.

Sulphate of Zinc.

Calomel.

Bichloride of Mercury.

Acetate of Lead.

Diacetate of Lead.

Iodide of Iron.

-------------Zinc.

------- Arsenic.

---- Carbonatis Liquor

The same as with Potassae Carbonas.

------Bicarbonas - -

Nearly the same as with Potassae Carbonas, except

Sulphate of Magnesia.

Disulphate of Quina.

Calomel, unless heated.

----- Liquor - -

Acids.

Acidulous Salts.

Carbonate of Ammonia.

Acetate of Ammonia.

Hydrochlorate of Ammonia.

Preparations of Earths and of Oxides of Metals

held in Solution by Acids.

Calomel.

Bichloride of Mercury.

-----Acetas - -

Sulphuric Acid.

Hydrochloric --.

Nitric-------.

Sulphate of Soda.

---------------Magnesia.

Most Metallic Salts.

-------Earthy Salts.

Substances

Incompatible with

Potassae Tartras - -

Most Acids.

-------Acidulous Salts.

Lime-water.

Chloride of Calcium.

Salts of Lead.

----- Silver.

----Bisulphas - -

Alkalies.

Earths.

Carbonates of Earths.

Oxides.

------ Sulphas - -

Tartaric Acid.

Baryta Water.

Chloride of Barium.

-----------------Calcium.

Acetate of Lead.

Diacetate of Lead.

Quinae Disulphas - - -

Chloride of Barium.

-----------------Calcium.

Tincture of Galls.

Acetate of Lead.

Diacetate of Lead.

Sodae Carbonas - - -

Acids.

Acidulous Salts.

Hydrochlorate of Ammonia.

Earthy and Metallic Salts.

Lime-water.

------ Bicarbonas - -

The same as those of Sodas Carbonas,

------ Sulphas - -

Carbonate of Potassa.

Solution of Baryta.

Barytic Salts.

Chloride of Calcium.

Nitrate of Silver.

Acetate of Lead.

Diacetate of Lead.

------- Potassio-tartras - -

Most Acids.

---- Acidulous Salts, except

Bitartrate of Potassa.

Barytic Salts.

Salts of Lime.

Acetate of Lead.

Diacetate of Lead.

Alum - -

Alkalies.

Carbonates of Alkalies.

Tartrate of Potassa.

Lime. Carbonate of Lime.

Chloride of Barium.

Magnesia.

Carbonate of Magnesia.

Acetate of Lead.

Barii Chloridum - - -

Sulphates.

Alkaline Carbonates.

Earthy Carbonates.

Creta Preparata -

Acids.

Acidulou s Salts.

Preparati ons of,Ipecacuanha

Calx - -

Acids.

Acidulous Salts.

Alkaline Carbonates.

Ammoniacal Salts.

Borates.

Metallic Salts.

Astringent Vegetable Infusions.

Substances

Incompatible with

Calcis Liquor - -

The same substances as with Calx.

Calcii Chloridum - -

Sulphuric Acid.

Sulphates, except of Lime.

Potassa.

Soda.

Carbonate of Potassa.

------------------Soda.

----- Ammonia.

------------------Magnesia.

Magnesias Sulphas - -

Potassa.

Soda.

Ammonia.

Carbonate of Potassa.

---- Soda.

Lime-water.

Chloride of Calcium.

_Acetate of Lead.

---- Carbonas - -

Acids.

Acidulous Salts.

Hydrochlorate of Ammonia.

Lime-water.

Metallic Salts.

Magnesia - - -

Same as with Magnesi Carbonas, except

Lime-water.

Antimonii Potassio-tartras

Its solution is incompatible with Alkalies.

Alkaline Carbonates.

Some Earths.

Lime-water.

Chloride of Calcium.

Some Metals.

------- Oxides of Metals.

Acetates of Lead.

Infusions of Cinchona.

----- Rhubarb.

------Catechu.

Argenti Nitras - -

Spring Water.

River Water.

Potassa.

Soda.

Carbonate of Potassa.

----------------- Soda.

Soaps.

Lime-water.

Sulphuric Acid,

and Salts containing these Acids.

Hydrochloric -----

Tartaric -----

Carbonate of Ammonia.

Liquor Potassae Arsenitis.

Sulphureted Hydrogen.

Hydrosulphurets.

Astringent Vegetable Infusions.

Decoction of Yellow Cinchona Bark.

Liquor Potassae Arsenitis -

Acids.

Acidulous Salts.

J Lime-water.

Chloride of Calcium.

Sulphate of Magnesia.

Alum.

Sulphate of Iron.

Substances

Incompatible with

Liquor Potassae Arsenitis -

Chloride of Iron.

Iodide of Iron.

Nitrate of Silver.

Sulphate of Copper.

Sulphureted Hydrogen and its Compounds.

_ Decoction of Cinchona.

Cupri Ammonio-sulphas - -

Acids.

Potassa.

Soda.

Lime-water.

Ferri Sulphas - -

Potassa.

Soda.

Ammonia.

Carbonate of Potassa.

-----------------Soda.

Baryta.

Chloride of Barium.

Salts of Baryta.

Lime-water.

Chloride of Calcium.

Soaps.

Nitrate of Silver.

Acetates of Lead.

Astringent Vegetable Infusions and Decoctions.

-------Sesquioxydum - -

Acids.

Acidulous Salts.

------Sesquichloridi Tinctura

Alkalies.

Carbonates of Alkalies.

Lime-water.

Carbonate of Lime.

Magnesia.

Carbonate of Magnesia.

Astringent Vegetable Bodies.

Solution of Gum Arabic.

Ferri Ammonio-chloridum

Alkalies.

Alkaline Carbonates.

Lime-water.

Astringent Vegetable Bodies.

------Vinum -

The same as with Ferri Sesquichloridi Tinctura.

Hydrargyrum cum Creta -

Acids.

Acidulous Salts.

Hydrargyri Binoxydum -

Acids.

Acidulous Salts.

Sulphureted Hydrogen.

---Bichloridum -

Potassa.

Soda.

Ammonia.

Carbonate of Potassa

Carbonate of Soda.

-----------------Ammonia.

Sulphuret of Potassium.

All Hydrosulphurets.

Soap.

Lime-water.

Potassio-tartrate of Antimony.

Nitrate of Silver.

Acetates of Lead.

Infusion of Bitter Vegetables.

----------------- Astringent Vegetables.

Substances

Incompatible with

Hydrargyri Chloridum -

Potassa.

Soda.

Ammonia.

Alkaline Carbonates.

Hydrosulphurets.

Lime-water.

Salts of Iron.

------ Lead.

--------- Copper.

Plumbi Acetas - -

"Sulphuric Acid.

Hydrochloric -------

Carbonic -------

Citric -------

Tartaric -------

Potassa.

Soda.

Ammonia.

Chloride of Sodium.

Liquor Ammoniae Acetatis.

Lime-water.

Carbonate of Lime.

Sulphate -------------

Solution of Sulphureted Hydrogen.

Hard Water usually.

All Vegetable Infusions containing Gum.

Astringent Infusions.

Strychnia.

----- Diacetatis Liquor -

Same as with Plumbi Acetas.

Zinci Sulphas - -

Alkalies.

Alkaline Carbonates.

Lime-water.

Hydrosulphurets.

Astringent Vegetable Infusions.

----- Oxydum - -

Acids.

Acidulous Salts.

Alkalies.

Potassii Sulphuretum -

Acids which combine with Potassa and expel Sulphureted Hydrogen Gas.

Solutions of most of the Metals.

Infusum Anthemidis -

Salts of Iron.

----------Mercury.

----- Silver.

------ Lead.

----- Amoraciae Comp - -

Alkaline Carbonates.

Salts of Silver.

---------- Mercury.

----Calumbae - -

Lime-water.

Acetates of Lead.

Bichloride of Mercury.

------ Caryophyllorum

Lime-water.

Solutions of the Preparations of Iron.

--------------------------------------------Zinc.

---------------- Lead.

------------------------------------------- Silver.

-------------------------------------- Antimony.

----------Cascarillae

The same as with Infusum Caryophyllorum.

------ Cuspariae

Solutions of the Salts of most Metals.

----- Digitalis

-------------------Iron.

Probably by those of most other Metals.

----- Gentianae Comp - -

Solution of Acetate of Lead.

-------- Persulphate of Iron.

Substances

Incompatible with

Infusum Lini Compositum

Preparations of Lead.

--------------------Iron.

Most Metallic Salts.

----------Rhei - - -

The stronger Acids.

Metallic Solutions.

Some astringent Infusions.

----------Rostt- Compositum -

Alkalies.

Earths and all substances which combine with Sulphuric Acid, or are acted upon by small quantities of it.

Acetate of Lead.

_ Sulphate of Iron.

----- Sennae Compositum

Strong Acids.

Lime-water.

Most Metallic Salts.

------ Simarobae - -

Alkaline Carbonates.

Lime-water.

Salts of Lead.

----------Silver.

----------Mercury.

Decoctum Aloes Compositum

Acids.

Acidulous Salts.

Earthy Salts.

Metallic Salts.

All substances which are decomposed by, or which decompose Carbonate of Potassa.

-------------Cydoniae - -

Acids,

Alcohol.

Most Metallic Solutions.

------------- Quercus - -

Alkaline Solutions.

Most Metallic Salts.

Solutions of Isinglass.

Decoction of Yellow Bark.

-------- Sarzae - -

Lime-water.

Acetate of Lead.

Mistura Ferri Composita -

Acids

which dissolve the Proto-carbonate of Iron.

Acidulous Salts

_ Vegetable Astringents.

Spiritus Ammoniae Aromaticus

Acids.

Acidulous Salts.

Earthy --------

Lime-water.

Metallic Salts.

------------- Foetidus

The same as in Sp. Ammoniae Arom.

--------- Succinatus

Acids.

Acidulous Salts.

Earthy -------

Metallic -------

--------- Camphorae - -

Water.

Tinctura Opii - -

Potassa and its Carbonate.

Soda -------------

Ammonia ---------

Most Metallic Salts.

Infusion of Galls.