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 some circumstances, which are scarce discernable, 'till the work be over: or from the diversity of Materials, whereof some may be genuine, some sophisticated, some mix'd, some mix'd some fresh, some may have lost their virtue. And this is chiefly remarkable, in Chymical Operations, wherein if the dissolvents be ill prepar'd, if the Spirts be too much, or too little purify'd, if there be the least alteration, in the degrees of Fire, the quantity of Matter, or by the negligence of those that attend it, the whole course will be overthrown, or chang'd from its first purpose.

But what is now to be concluded from hence? Shall this Instability, and Casualty of Experiments, deter us from labouring in them at all? or should it not rather excite us to be more curious and watchful in their Process? It is to be allow'd, that such undertakings are wonderfully hazardous and difficult; why else does the Royal Society endeavour to preserve them from degenerating, by so many forewarnings, and Rules, and a Method so severe? It is granted, that their event is often uncertain, and not answerable to our expectations. But that only ought to admonish us, of the indispensible necessity of a jealous, and exact Inquiry. If the uncertainty proceeded from a constant irregularity of Nature, we had reason then to despair; but seeing it for the most part arises only from some defect or change in our progress, we should thence learn, first to correct our own miscarriages, before we cease to hope for the success.

Let then the Experiment be often renew'd. If the same kinds and proportions of Ingredients be us'd, and the same circumstances be punctually observ'd, the effect without all question will be the same. If some little variation of any of these, has made any Rh