When a man, instead of burning up three hundred pounds of carbon a year, has got down to two hundred and fifty, it is plain enough he must economize force somewhere. Now habit is a labor-saving invention which enables a man to get along with less fuel,--that is all; for fuel is force, you know, just as much in the page I am writing for you as in the locomotive or the legs that carry it to you. Carbon is the same thing, whether you call it wood, or coal, or bread and cheese. A reverend gentleman demurred to this statement,--as if, because combustion is asserted to be the sine qua non of thought, therefore thought is alleged to be a purely chemical process. Facts of chemistry are one thing, I told him, and facts of consciousness another. It can be proved to him, by a very simple analysis of some of his spare elements, that every Sunday, when he does his duty faithfully, he uses up more phosphorus out of his brain and nerves than on ordinary days. But then he had his choice whether to do his duty, or to neglect it, and save his phosphorus and other combustibles. It follows from all this that THE FORMATION OF HABITS ought naturally to be, as it is, the special characteristic of age. As for the muscular powers, they pass their maximum long before the time when the true decline of life begins, if we may judge by the experience of the ring. A man is "stale," I think, in their language, soon after thirty,--often, no doubt, much earlier, as gentlemen of the pugilistic profession are exceedingly apt to keep their vital fire burning WITH THE BLOWER UP. --So far without Tully. But in the mean time I have been reading the treatise, "De Senectute." It is not long, but a leisurely performance. The old gentleman was sixty-three years of age when he addressed it to his friend T. Pomponius Atticus, Eq., a person of distinction, some two or three years older. We read it when we are schoolboys, forget all about it for thirty years, and then take it up again by a natural instinct,--provided always that we read Latin as we drink water, without stopping to taste it, as all of us who ever learned it at school or college ought to do. Cato is the chief speaker in the dialogue. A good deal of it is what would be called in vulgar phrase "slow." It unpacks and unfolds incidental illustrations which a modern writer would look at the back of, and toss each to its pigeon-hole. I think ancient classics and ancient people are alike in the tendency to this kind of expansion.