Dr. Brown had nominated him as one of his literary executors, and his family were urgent in their request that he should write their father's Life. With great reluctance he consented, and for eighteen months this task absorbed the whole of his leisure, to the complete exclusion of the work on "The Difficulties of Christianity," with which he had already made some progress. The undertaking was a labour of love, but it cannot be said to have been congenial. Memoir writing was not to his taste, and in this case he had made a stipulation that still further hampered him and made success very difficult. This was that he should omit, as far as possible, all personal details, and leave these to be dealt with in a separate chapter which Dr. Brown's son John undertook to furnish. This chapter was not forthcoming when the volume had to go to press, and was separately issued some months later. When the inspiration did at length come to "Dr. John," it came in such a way as to add a new masterpiece to English literature, and one which, while it gave a wonderfully living picture of the writer's father, disclosed to the world as nothing else has ever done the true _ethos_ and inner life of the Scottish Secession Church. The Memoir itself, of which this "Letter to John Cairns, D.D." is the supplementary chapter, is a sound and solid bit of work, giving an accurate and interesting account of the public life of Dr. Brown and of the movements in which he took part. It is, as William Graham said of it, "a thoughtful, calm, conclusive book, perhaps too reticent and colourless, but none the less like Dr. Brown because of that." No sooner was this book off his hands than Cairns was urged to undertake another biographical work--the Life of George Wilson. But this, in view of his recent experience, he steadfastly refused to do, and contented himself with writing a sketch of his friend for the pages of _Macmillan's Magazine_. When, however, Wilson's biography was taken in hand by his sister, Cairns promised to help her in every possible way with his advice and guidance, and this he did from week to week till the book was published. This help on his part was continued by his seeing through the press Wilson's posthumous book, _Counsels of an Invalid_, which appeared in 1862. With the completion of this task he seemed to be free to return to his theological work, and he did return to it; but his release turned out to be only a brief respite. In 1863 the ten years' negotiations for Union between the Free and United Presbyterian Churches, in which he felt impelled to take a prominent and laborious part, were begun, and they absorbed nearly all of his leisure during what might have been a productive period of his life. When he emerged from them he was fifty-four years of age, he had passed beyond the time of life when his creative powers were at their freshest, and the general habits of his life and lines of his activity had become settled and stereotyped.