“There are a few of us who could scarcely do more than we are doing of our own regular order of work, but there may yet be spare moments for little extra efforts of another sort which in the aggregate, in the run of a year, might produce a great total of real practical result. We must, like goldsmiths, carefully sweep our shops, and gather up the filings of the gold which God has given us in the shape of time. Select a large box and place in it as many cannon-balls as it will hold, it is after a fashion full, but it will hold more if smaller matters be found. Bring a quantity of marbles, very many of these may be packed in the spaces between the larger globes; the box is full now, but only full in a sense, it will contain more yet. There are interstices in abundance into which you may shake a considerable quantity of small shot, and now the chest is filled beyond all question, but yet there is room. You cannot put in another shot or marble, much less another cannon-ball, but you will find that several pounds of sand will slide down between the larger materials, and even then between the granules of sand, if you empty pondering there will be space for all the water, and for the same quantity several times repeated. When there is no space for the great there may be room for the little; where the little cannot enter the less can make its way; and where the less is shut out, the least of all may find ample room and verge enough.”

"A Spur for a Free Horse" in The Sword and the Trowel (February, 1866) http://www.spurgeon.org/s_and_t/spur.htm