Persius has justly observed, that knowledge is nothing to him who is not known by others to possess it[1]: to the scholar himself it is nothing with respect either to honour or advantage, for the world cannot reward those qualities which are concealed from it; with respect to others it is nothing, because it affords no help to ignorance or errour. It is with justice, therefore, that in an accomplished character, Horace unites just sentiments with the power of expressing them; and he that has once accumulated learning, is next to consider, how he shall most widely diffuse and most agreeably impart it. A ready man is made by conversation. He that buries himself among his manuscripts, "besprent," as Pope expresses it, "with learned dust," and wears out his days and nights in perpetual research and solitary meditation, is too apt to lose in his elocution what he adds to his wisdom; and when he comes into the world, to appear overloaded with his own notions, like a man armed with weapons which he cannot wield. He has no facility of inculcating his speculations, of adapting himself to the various degrees of intellect which the accidents of conversation will present; but will talk to most unintelligibly, and to all unpleasantly. I was once present at the lectures of a profound philosopher, a man really skilled in the science which he professed, who having occasion to explain the terms _opacum_ and _pellucidum_, told us, after some hesitation, that _opacum_ was, as one might say, _opake_, and that _pellucidum_ signified _pellucid_. Such was the dexterity with which this learned reader facilitated to his auditors the intricacies of science; and so true is it, that a man may know what he cannot teach. Boerhaave complains, that the writers who have treated of chymistry before him, are useless to the greater part of students, because they presuppose their readers to have such degrees of skill as are not often to be found. Into the same errour are all men apt to fall, who have familiarized any subject to themselves in solitude: they discourse, as if they thought every other man had been employed in the same inquiries; and expect that short hints and obscure allusions will produce in others the same train of ideas which they excite in themselves.