Of compounds, compound remedies, and purity

[John Harvey]

In discussion of whether Hahnemann intended the phrase "single, simple medicinal substance" to comprehend medicinal mixtures, some confusion has become evident on the list in distinguishing compounds from compound (including complex) remedies and in distinguishing compounds from elements contaminated by impurities. In order to enable clearer conversation of that subtopic, I thought I'd outline the difference.
Compound remedy
A compound remedy is the term that those who use medicinal mixtures use to differentiate the remedy from a remedy prepared from a "single, simple medicinal substance". The usual procedure is to make the mixture in crude form and give it a new name. It is then put it through the desired certain number of potency steps and treated as though (a) the potentised form corresponds to the crude form in the same way that potentised single, simple medicines correspond to their crude form and (b) there is some basis for its prescription.
Combination remedy
Medicinal mixtures made between single, simple medicinal substances that have already been potentised (often to the sixth decimal potency) are sometimes differentiated from other compound remedies by calling them combination remedies. They often are mixed with other substances in crude form that can target the symptom of concern.
Disclaimer
Obviously, any medicine prescribed either for a single symptom or without a knowledge of its pathogenesis is not homoeopathic practice, which requires prescription for a totality of symptoms.
Compound
A compound is an entirely different matter from both of these. A compound is a chemical combination of two or more elements into a single type of molecule. Examples include hydrochloric acid, a compound of hydrogen and chlorine (HCl); sodium hydroxide, a compound of sodium and the hydroxyl radical, which itself is oxygen and hydrogen (NaOH); glucose, a compound of carbon, oxygen, and hydrogen (C6H12O6); ethanol, another compound of carbon, oxygen, and hydrogen (CH3CH2OH); water, a compound of hydrogen and oxygen (H2O); and hydrogen peroxide, another compound of hydrogen and oxygen (H2O2).
Notice that various combinations of the same few types of atoms are possible, and that they result in quite different types of chemicals. The differences are marked, for instance, between the properties of water and those of hydrogen peroxide, though both are composed entirely of hydrogen and oxygen.
You'll notice that the purity of an unknown substance cannot be gauged in any way by the number of different elements in it, nor by their ratios. Pure water, for instance, has not one but two elements, hydrogen and oxygen, in a ratio of 2:1; but pure hydrogen peroxide also has two elements, in a ratio of 1:1.
Notice too that the marked differences between the properties of water and of hydrogen peroxide -- though both molecules consist entirely of hydrogen and oxygen -- show that the properties of the two molecules cannot possibly be predicted from the properties of hydrogen and oxygen; if they could, then water and hydrogen peroxide would have the same properties.
This latter observation applies as well, and particularly commonly, to compounds that are a little more complex. Sulphurous acid (H2SO3) and sulphuric acid (H2SO4), for example, though they differ by just one oxygen molecule, are remarkably different acids. Sucrose and lactose, with very different effects on health, are both C12H22O11, differing only in their structure.
Even the one structure may differ between two types only in the way in which it bends. There are, for instance, two forms of glucose, right-handed and left-handed; they are mirror images of each other and have somewhat different effects in the body. The thalidomide disaster is alleged to have been due largely to the contamination, in a generic form of the drug, with the original drug's mirror image, due to use of a different method of production.
Notice one more thing: that the word compound does not require reference to a particular sample of matter. It refers rather to a single type of matter, of which a particular sample may wholly or partly consist.
Purity
The purity of a particular sample of an element or compound is a measure of the proportion of the matter under consideration that consists of that element or compound. If water is 100% pure, then all of it consists of water molecules. If it is 50% pure by mass, then half of its mass is due to other atoms or molecules.
Virtually no element or compound [see "Substance (iii)", under separate post "Of substance"] on earth can be obtained in 100% purity: there will always be slight admixtures of various contaminants. Fortunately for the predictability of "single, simple medicinal substances", the threshold above which the proportion of a contaminant will significantly alter the medicinal effects of the single substance sought is in most instances not difficult to stay below.
That threshold varies, of course, depending upon the medicinal activity of the substance. You may have as much pure quartz sand as you like in an (unpotentised) substance without altering its medicinal properties; on the other hand, one millionth of a gram of plutonium is fatal to humans, so a rather lower threshold is necessary of this contaminant. Most contaminants' thresholds are somewhere between these two extremes.
Partly because contamination is unavoidable, substances prepared for medicinal use are expected to be prepared in the same way each time, which, if it cannot eliminate the contamination, at least standardises it, keeping it consistent from preparation to preparation. That is one of the important functios of a pharmacopoeia.
Cheers --
John
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"Nothing is so fatal to the progress of the human mind as to suppose that our views of science are ultimate; that there are no mysteries in nature; that our triumphs are complete; and that there are no new worlds to conquer."

— Sir Humphry Davy, in "An Account of some Galvanic Combinations", Philosophical Transactions 91 (1801), pp. 397–402 (as quoted by David Knight, Humphry Davy: Science and Power, Cambridge, 1998, p. 87)