After Mr. Pratt had worked three years for his New York firm, in connection with two other gentlemen he bought the paint-and-oil business of his employers, and the new firm became Raynolds, Devoe, & Pratt. For thirteen years he worked untiringly at his business; and in 1867 the firm was divided, the oil portion of the business being carried on by Charles Pratt & Co. In the midst of this busy life the influence of the Mercantile Library of Boston was not lost. He had become associated with the Mercantile Library of New York, and both this and the one in Boston had a marked influence on his life and his great gifts. When the immense oil-fields of Pennsylvania began to be developed, about 1860, Mr. Pratt was one of the first to see the possibilities of the petroleum trade. He began to refine the crude oil, and succeeded in producing probably the best upon the market, called "Pratt's Astral Oil." Mr. Pratt took a just pride in its wide use, and was pleased, says a friend, "when the Rev. Dr. Buckley told him that he had found that the Russian convent on Mount Tabor was lighted with Pratt's Astral Oil. He meant that the stamp 'Pratt' should be like the stamp of the mint,--an assurance of quality and quantity." For years he was one of the officers of the Standard Oil Company, and of course a sharer in its enormous wealth. Nothing seemed more improbable when he was spending a year at Wilbraham Academy, living on a dollar a week, than this ownership of millions. Now, as then, he was saving of time as well as money. Says Mr. James McGee of New York, "He brought to business a hatred of waste. He disliked waste of every kind. He was not willing that the smallest material should be lost. He did not believe in letting time go to waste. He was punctual at his engagements, or gave good excuse for his tardiness. Speaking of an evening spent in congratulations, he said that it was time lost; it would have been better spent in reviewing mistakes, that they might be corrected. It is said that a youth who had hurried into business applied to Mr. Pratt for advice as to whether he should go West. He questioned the young man as to how he occupied his time; what he did before business hours, and what after; what he was reading or doing to improve his mind. Finding that the young man was taking no pains to educate himself, he said emphatically, 'No; don't go West. They don't want you.'" Active as Mr. Pratt was in the details of a great business, he found time for other work. Desiring an education, which he in his early days could not obtain, he provided the best for his children. He became deeply interested in Adelphi Academy, Brooklyn, was a trustee, and later president of the Board. In 1881 he erected the wing of the main building; and six years later, in 1887, he gave $160,000 for the erection of a new building.