The mariner's compass, printing, &c. were discovered by chance. The fire and steam-engines are deductions of pure science in all their gradations. I do not wonder that Tertius and yourself find that metaphysical subjects leave little impression on the mind. If you ask Mr. Stewart to account for this, he will explain it to you. In early life our attention is employed on external objects, and not at all on the operations of our own minds; the study of which requires habits of abstraction and attention that cannot be acquired without considerable practice. Yet these habits of attention and abstraction are valuable in many other points of view; particularly as increasing the power of volition (as Locke calls it), or that faculty by which we regulate our own thoughts and conduct, -a power of the utmost importance both to science and virtue. Do not, however, be too anxious to comprehend things all at once. You must get initiated now, and as inclination or leisure prompts, you can pursue the subject in future. I was a year older than you before I attended to metaphysics at all. But they took hold of me a good deal, and for several years engaged much of my attention. The chief difficulty I found in pursuing the study arose (I should perhaps say arises) from the inaccuracy and the imperfection of language. This leads

people engaged in such studies to labour at overcoming the difficulty; which obliges them to appreciate words and sentences carefully, in order that they make the expression equivalent to, and exactly representative of, the thought. Hence, he who has studied metaphysics, obtains a superior command both of thoughts and language on other subjects, which is, by the way, the chief practical use of the study. Metaphysics, indeed, involve the principles of general grammar, a very interesting subject, which the work of Horne Tooke has brought into fashion among us. And this leads me to an anecdote which Tooke mentioned the other day in company with a friend of ours. He stated that during the contest for Westminster he had said some severe things on the hustings of Mr. Fox when absent, and was doubtful how he might take them. Next day Fox appeared, and soon seemed absorbed in thought, walking backwards and forwards on the back part of the hustings, while the people were voting; and often looking him (Tooke) in the face in a very strange way, but without saluting him. He had no doubt he was meditating a philippic against him, to be poured out from the hustings at the end of the day's poll. At length, however, he came up to Mr. Tooke rather courteously, and desired to say a word to him on one

side; it was to consult him on some difficulty that had arisen in the course of his meditation, which had been on the various coincidences and differences between, which and that. I will not be sure that I state the point exactly, but it was this or something similar. Indeed, Fox is most deep and most clear on all the principles of general grammar, and the derivation of words; and it may serve to show the habits of his study, as well as the kindness of his temper, that the sight of Tooke, instead of suggesting the hostility of the man, should have excited a train of thought arising out of his "winged words."

I entirely agree with you in your reflections on the suspension of the slave-trade: Heaven grant that it may be suspended for ever! - I heartily rejoice in the rise of the negro commonwealth in St. Domingo, one of the most extraordinary occurrences of these extraordinary times. I hope the black man will be found capable of all the blessings of civilisation, and of all the relations of peace and amity; and I earnestly wish him fair play.

Professor Robison was mistaken, I think, in his assertion that the skin absorbs moisture from immersion in cold water. It is the common opinion, but I believe it is erroneous. I have treated on the subject in my Medical Reports, where the

partial relief of the thirst from this immersion is explained on other principles.

Habits

The observations you make in your letter of the 11th of March, on the difficulty of fixing the attention on metaphysical speculations, and the causes of this difficulty, are ingenious and just. I have no doubt that you will get over it as much as is necessary. In the power of attention you will find your mind improve for some years to come; in apprehension, considered simply, you are come, I suppose, to your acme. of deep study, of whatever nature the study may be, are apt to absorb the mind, and to withdraw the attention from present objects; while the qualities that please in conversation and in the intercourse of society, require a person to be, at the moment at least, alert and at home. The study of subjects that do not easily enter into conversation is more apt than any other to unfit a man for society; because, besides the effect of abstraction itself, the subjects are of a kind

that furnishes no materials for the usual intercourse of life, nothing that the memory can store up for common talk, nothing that the imagination can new model and embellish. But this applies much more to mathematics than metaphysics. The mathematician does not even associate language with his thoughts, figures are the medium of his reflections. On the contrary, the metaphysician, except when he is unprofitably engaged in the first principles of thought, is perpetually conversant with subjects that border on, and that actually form, the topics of the best conversation; such as enquiries into the beautiful and sublime, into the powers of memory, into the phenomena of the imagination, into the structure of language, into all the principles of human action: and if, along with the knowledge requisite for such discussions, he has acquired a suitable command of easy and clear expression, he has obtained the power of not only rendering himself useful and instructive in conversation, but agreeable and even splendid in a high degree. Johnson and Burke of the last age, and Mackintosh of the present, are acknowledged to have surpassed all their contemporaries in conversation; though the two last, at least, may be considered as profound metaphysicians, and the first a metaphysician of no ordinary degree. Such men possess