One of the new friends Smith made during his last visit to London was Sir Joseph Banks, President of the Royal Society, who seems to have shown him particular attentions; and shortly after his return he gave a young Scotch scientific man a letter of very warm recommendation to Sir Joseph.  The young man of science was John Leslie, afterwards Sir John, the celebrated Professor of Natural Philosophy in Edinburgh University.  Leslie, who belonged to the neighbourhood of Smith’s own town of Kirkcaldy, had been employed by him for the previous two years as tutor to his cousin and heir, David Douglas, and being thus a daily visitor at Smith’s house, had won a high place in his affections and regard.  Accordingly when Leslie in 1787 gave up his original idea of entering the Church, and resolved to migrate to London with a view to literary or scientific employment, Smith furnished him with a number of letters of introduction, and, as Leslie informed the writer of his biography in Chambers’s Biographical Dictionary, advised him, when the letter was addressed to an author, to be always sure to read that author’s book before presenting it, so as to be able to speak of the book should a fit opportunity occur.  The letter to Sir Joseph Banks runs as follows:—­

SIR—­The very great politeness and attention with which you was so good as to honour me when I was last in London has emboldened me to use a freedom which I am afraid I am not entitled to, and to introduce to your acquaintance a young gentleman of very great merit, and who is very ambitious of being known to you.  Mr. Leslie, the bearer of this letter, has been known to me for several years past.  He has a very particular happy turn for the mathematical sciences.  It is no more than two years and a half ago that he undertook the instruction of a young gentleman, my nearest relation, in some of the higher parts of these sciences, and acquitted himself most perfectly both to my satisfaction and to that of the young gentleman.  He proposes to pursue the same lines in London, and would be glad to accept of employment in some of the mathematical academies.  Besides his knowledge in mathematics he is, I am assured, a tolerable Botanist and Chymist.  Your countenance and good opinion, provided you shall find he deserves them, may be of the highest importance to him.  Give me leave, upon that condition, to recommend him in the most anxious and earnest manner to your protection.  I have the honour to be, with the highest respect and regard, sir, your most obliged and most obedient humble servant,

     ADAM SMITH.[348]

     EDINBURGH, 18th December 178(sic). 
     Sir JOSEPH BANKS.

Why does so large a proportion of Smith’s extant letters consist of letters of introduction?  Have they a better principle of vitality than others, that they should be more frequently preserved?  There certainly seems less reason to preserve them, but then there is also less reason to destroy them.