This second part of the table of secondary salts consists of three columns. In the middle column are set down the names of the secondary salts employed in medicine, in the same order as in the former table; and in the adjoining columns on each side are noted those substances employed in medicine which are capable of effecting a decomposition of each salt; those in the lefthand column being such as decompose the salt by what is called single affinity, in consequence of that substance having a superior attraction for the acid or the base of the salt; while the substances in the right-hand column are secondary salts, between which and the opposite salt in the middle column such an action may take place as to effect their mutual decomposition.

With tables of this kind before him, a prescriber will avoid several mistakes into which he might be betrayed from a deficiency of chemical knowledge. Thus, knowing the solubility of any salt, he will not prescribe a greater quantity of it than is capable of being retained in solution in the watery part of any draught or mixture which he is to order. For instance, knowing that sulphate of potash requires sixteen parts of water at 60° for its solution, he will, if he proposed to prescribe a draught containing two drams of this salt, be aware that such a quantity would require at least four ounces of water; but this making the draught too large is a great objection to giving the medicine in that form. Or suppose that he wished to give half an ounce of supertartrate of potash (crystals of tartar), by way of laxative; he sees, that to dissolve this quantity it would require at least two pounds of water, and therefore that he cannot order it in the form of solution, though, when mixed up with syrup into an electuary, it affords a good into an electuary, it affords a good and efficacious cooling laxative. Again, knowing that sulphate of soda effloresces in the air, and thereby loses nearly half its weight, he will take care always to prescribe it in the form of crystals; and if he is to order a laxative draught containing one ounce of this salt, he must prescribe at least three ounces of liquid.

The information conveyed in the second column respecting the deliquescence or efflorescence of certain salts, or the readiness with which they imbibe water from the atmosphere, or part with their water of crystallization, is extremely useful in pointing out the proper forms of exhibition. Seeing, for instance, that acetate of potash (diuretic salt) is a deliquescent salt, no one would think of prescribing it in the form of pills; while, on the other hand, carbonate of soda being efflorescent, is well adapted to that form, and accordingly has been 3

so prescribed by Dr Beddoes; (see Kirby's Tables, formula 153.).

Knowing the proportional quantities of the component part of any salt, we can, by calculation, ascertain pretty nearly how much of the one is required to decompose the other, and thus employ no more of either than is necessary. Thus, suppose it were required to decompose 100 grains of green sulphate of iron by carbonate of soda, in order to procure the greatest possible quantity of carbonate of iron. We find by the first table, that 100 grains of the sulphate contain 28 grains of oxide of iron, and to saturate this, we find by computation, that there are required 9 grains of carbonic acid. Now, on examining the composition of carbonate of soda, we find that 100 grains of this salt contain about 14 grains of carbonic acid, and consequently, that about 60 grains of carbonate of soda are sufficient to decompose 100 grains of green sulphate of iron.

Further, knowing the substances that are capable of decomposing any particular salt, a prescriber will not order of these substances in the same formula with any that salt, unless some manifest advantage were to be the result of their mutual action. He knows that sulphate of zinc and acetate of lead decompose each other, and that the acetate of zinc formed by their mixture, is a better remedy in cases of ophthalmia than either of the former salts. Here then is an advantage. Tartrate of antimony and potash is a good remedy in fever, so is decoction of Peruvian bark; but we find by the tables, that this salt is decomposable by gallic acid, and we know that decoction of cinchona contains this acid, especially after having stood for some time. It would therefore be improper to prescribe these remedies in conjunction, as has sometimes been recommended, because the salt would be so much altered by the decomposition as to be no longer the medicine we propose to administer. A similar instance of unscientific prescription, arising from a want of chemical knowledge, occurs in a formula attributed to Mr Coleman, and published in the fifth edition of the Pharmacopacia Chirurgica, p. 58. under the title of Collyrium hydrargyri muriati cum calce. It is composed of a scruple of muriate of mercury dissolved in an English pint of boiling distilled water, with the addition of two drams of quicklime, and after the whole is completely mixed, we are directed to filter the clear liquor through paper. The author of this Pharmacopoeia seems aware that "the different elective attractions operating in the mixture of the lime with the solution of muriate of mercury, are such as produce

Previous produce a new chemical arrangement, in which the actiRequisites. vity of the ingredients is mutually diminished. The fact is, that the large quantity of lime here directed will completely decompose the muriate of mercury, so that the clear liquor will contain nothing but uncombined lime, and muriate of lime. Hence the muriate of mercury is an unnecessary ingredient, and if the medicine be efficacious as a collyrium, it would be better to form it at once by the addition of a small quantity of muriate of lime to lime-water.

32

Dover's powder.

33 Berat sponge.

sistence.

A physician who is familiar with the principles of chemistry will not direct a chemical medicine to be prepared of more ingredients, or in a more operose manner, than is requisite to produce the desired effect. When Dr Dover first gave to the public the composition of his sudorific powder, he ordered it to be prepared in the following manner. Four ounces of nitre, and the same quantity of vitriolated tartar (sulphate of potash), are to be thrown into a red-hot crucible, and kept stirring till the deflagration ceases. To the mixture, while hot, is to be added an ounce of sliced opium. The whole is then to be reduced to powder and well mixed with an ounce of powdered ipecacuanha, and the same quantity of powdered liquorice root. It is well known to the chemists of the present day, that nitrate of potash, when thrown on an ignited combustible body, deflagrates, and is decomposed; but that it does so when thrown into an ignited crucible, with an incombustible body, such as the sulphate of potash, we can scarcely conceive. If it does, the effect must be, that the nitric acid is carried off, and there remains the potash, which is an unnecessary ingredient in the composition. Again, the only use of heating the salt, would be to dry the opium and thus render it more easily pulverised; but as dried opium is always kept in the shops, and by means of sulphate of potash, is very easily reduced to powder, that part of the operation is superfluous. Accordingly, a powder equally efficacious, and much less operose, is prepared by rubbing together sulphate of potash, opium, and ipecacuanha, forming the present pulvis ipecacuanha et opii, Ed. or pulvis ipecacuanha compositus, Lond.

From the same want of chemical knowledge, some medicines have been extolled as efficacious remedies, from not knowing their real nature. Thus burnt sponge has long been celebrated for the cure of scrofula. We do not altogether deny its efficacy in this complaint; but as burnt sponge is composed almost entirely of charcoal, with the addition of a little carbonate of soda, a powder composed of these ingredients must be equally efficacious.

34 Errors in reUnder this head we may notice an error which is frespect to con-quently made by prescribers who have not been accustomed to see and prepare the remedies which they prescribe. We have often seen a mass for pills ordered to be prepared of such ingredients as are naturally too hard to form into pills, as for instance, extract of cinchona, and extract of liquorice, and yet there has been directed a quantity of liquorice powder, to form the mass of a proper consistence. Sometimes again, the matters directed are already too soft, or become too soft by mixture, as when aloes and extract of gentian are directed to be beaten together with a proper quantity of syrup, to form a mass for pills. See the Edin. Phar. edit. 1783.

55 Water.

We shall conclude this part of our subject with re

marking, that it is of consequence in a chemical point Previous of view, to prescribe as the constituent of a liquid me- Requisites. dicine, such water as will not decompose any of the other ingredients. It is common to order the water by the name of aqua pura, or aqua fontana. Now, if this water be hard, i. e. impregnated with sulphate of lime, &c. it will decompose many of the secondary salts, and thus diminish their efficacy. Acetate of lead, for instance, is always decomposed by hard water, and a turbid liquor is thus formed, which by standing deposits a sediment. It would therefore be better in all cases to prescribe distilled water, or where this is not likely to be found, as in small country towns, soft water.

III. We have thus considered at some length the previous knowledge required by a practitioner before he can pretend to prescribe for his patient in a scientific manner. We shall now endeavour to apply the obser vations that have been made, and from the application deduce some general rules for extemporaneous prescrip

tion.

36

37

hints.

When a practitioner is called to a patient, he will Practical first examine into the symptoms and causes of the malady under which the patient labours, and attend to the age, sex, and peculiar habit of the patient. He will then consider whether or not a cure is probable, or whether it may be in his power only to relieve the distressing symptoms. If a cure appears to be practicable, he will proceed to form his indications, and in conformity with these he will prescribe the remedies that seem best adapted to the case. It is this method of procedure that distinguishes the scientific practitioner from the ignorant empiric. The latter, from a superficial view of the most obvious symptoms, hastily determines the nature of the complaint, which he probably contrives shall be some one of which he has witnessed many cases, or for the cure of which he is in possession of some favourite remedy. Having resolved what the disease shall be, he has nothing to do but apply his remedy, and this he does without considering whether existing circumstances may not render the administration of it im

proper.

To return from this digression, we shall endeavour to give an example as simple as will answer our purpose, to illustrate the above method of procedure. We shall suppose that a practitioner is sent for to a middle-aged man, in moderate circumstances, who has been for some days labouring under a tertian intermittent fever, with which he had never before been affected, but had commonly been strong and healthy. The practitioner sees nothing in the circumstances of the case which can lead to an unfavourable prognosis, and he therefore has little hesitation in pronouncing, that the fever will probably soon be removed. Considering the indications usually laid down in practical writers on intermittents, he proceeds to prescribe the remedies which appear best suited to the case in point. Thus the indications given by Dr Cullen are,

1. In the time of intermission to prevent the recurrence of paroxysms.

2. In the time of paroxysms to conduct these so as to obtain a final solution of the disease.

3. To take off certain circumstances which might prevent the fulfilling of the two first indications *.

*Cullen's

In considering the first indication, the practitioner First Lines, reflects on the effect of the usual exciting cause of an in- § 228. termittent,