He will make use of no Medicines but the choicest, and when they are in their full vigour, and such as are durable, and after once or twice Tryal of them, will seldom fail in his expected success; which cannot be certainly had without some tryal. For though a man buy the choicest ingredients, viz. Sena, which may appear to the Senses very good, yet he cannot positively say, how well, nor yet what quantity of it will work, till he hath made use of it. But afterwards he may confidently apply the whole parcel he hath bought to his purpose. The like may be instanced in a crop of Wheat or Barley, which the skillfullest Husband-man cannot tell how they will yield for Bread, or Malt, till he hath used them. Now how is it possible that a Physician can with any certainty make use of several Shops, since there is so great difference in the ingredients? and 'tis certain the same Medicine made by several Apothecaries, shall differ much in colour, smell, and tast, and consequently effect too; which cannot proceed from any other cause then the difference of the ingredients themselves, or by omitting some ingredients, or by substituting one thing for another; or by distinct ways of preparing them. The same also may be said of Compositions, much more of Chymical Medicines so much sophisticated, and of so much danger and hazard, if not well prepared, which he cannot discover till he hath seen the effect of them, unless it be such as he makes himself, nor those neither till he hath made some tryal of them. He will much inlarge Materia Medica, Chymistry and Pharmacy, and discover the grounds of them, and wherein the efficacy of remedies lyes, and thereby lay open a whole Ocean for new discoveries, and by the by observe many useful products and Phenomena of Nature, to the great improvement of his Art, and sound Natural Philosophy, which are not taken notice of by Apothecaries, and their Servants; for all which they have neither will nor skill. As to the improvement of Medicines, this may be added, by the experimenting Physician, that in distill'd waters he will consider and find which of them will afford any virtue, which only phlegm equivalent but to Conduit-water, which of them will keep long, and in perfection, which soon or in what time decay, and spend them accordingly, and in compound distill'd waters, will find cause to lay aside many simples as nothing conducing, or rather weakning the efficacy of the Medicine designed; whereby much charge and trouble will be spared, and better compositions be made. He will gain and keep to himself Patients, who have diseases they are unwilling should be known by Apothecaries and their Boys, and all such as have a mind to turn over their File. The Patient will have better opinion of the Medicines, and confidence in the use or them, and the Physician more satisfied in his Conscience, and better assured of the success.