The letters you're referring to were written in 1753 and 1755, and has he got older, his views on race softened to the point that he was an outright abolitionist by the end of his life. In 1763 he wrote the following in a letter:
This is chiefly to acquaint you, that I have visited the Negro School here in Company with the Revd. Mr. Sturgeon and some others; and had the Children thoroughly examin’d. They appear’d all to have made considerable Progress in Reading for the Time they had respectively been in the School, and most of them answer’d readily and well the Questions of the Catechism; they behav’d very orderly, showd a proper Respect and ready Obedience to the Mistress, and seem’d very attentive to, and a good deal affected by, a serious Exhortation with which Mr. Sturgeon concluded our Visit. I was on the whole much pleas’d, and from what I then saw, have conceiv’d a higher Opinion of the natural Capacities of the black Race, than I had ever before entertained. Their Apprehension seems as quick, their Memory as strong, and their Docility in every Respect equal to that of white Children.1 You will wonder perhaps that I should ever doubt it, and I will not undertake to justify all my Prejudices, nor to account for them.
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I'm not sure that really that excuses being so racist that he thought _Germans_ weren't sufficiently white for america in his 50s, but he did change his views over time.
This is chiefly to acquaint you, that I have visited the Negro School here in Company with the Revd. Mr. Sturgeon and some others; and had the Children thoroughly examin’d. They appear’d all to have made considerable Progress in Reading for the Time they had respectively been in the School, and most of them answer’d readily and well the Questions of the Catechism; they behav’d very orderly, showd a proper Respect and ready Obedience to the Mistress, and seem’d very attentive to, and a good deal affected by, a serious Exhortation with which Mr. Sturgeon concluded our Visit. I was on the whole much pleas’d, and from what I then saw, have conceiv’d a higher Opinion of the natural Capacities of the black Race, than I had ever before entertained. Their Apprehension seems as quick, their Memory as strong, and their Docility in every Respect equal to that of white Children.1 You will wonder perhaps that I should ever doubt it, and I will not undertake to justify all my Prejudices, nor to account for them. ---
I'm not sure that really that excuses being so racist that he thought _Germans_ weren't sufficiently white for america in his 50s, but he did change his views over time.