A most determined and thorough-going attempt to hunt down the secret of vitality, and to determine how far its phenomena can be interpreted in terms of mechanics and chemistry, is to be found in Professor H. W. Conn's volume entitled "The Living Machine." Professor Conn justifies his title by defining a machine as "a piece of apparatus so designed that it can change one kind of energy into another for a definite purpose." Of course the adjective "living" takes it out of the category of all mere mechanical devices and makes it super-mechanical, just as Haeckel's application of the word "living" to his inorganics ("living inorganics"), takes them out of the category of the inorganic. In every machine, properly so called, all the factors are known; but do we know all the factors in a living body? Professor Conn applies his searching analysis to most of the functions of the human body, to digestion, to assimilation, to circulation, to respiration, to metabolism, and so on, and he finds in every function something that does not fall within his category--some force not mechanical nor chemical, which he names vital. In following the processes of digestion, all goes well with his chemistry and his mechanics till he comes to the absorption of food-particles, or their passage through the walls of the intestines into the blood. Here, the ordinary physical forces fail him, and living matter comes to his aid. The inner wall of the intestine is not a lifeless membrane, and osmosis will not solve the mystery. There is something there that seizes hold of the droplets of oil by means of little extruded processes, and then passes them through its own body to excrete them on an inner surface into the blood-vessels. "This fat absorption thus appears to be a vital process and not one simply controlled by physical forces like osmosis. Here our explanation runs against what we call 'vital power' of the ultimate elements of the body." Professor Conn next analyzes the processes of circulation, and his ready-made mechanical concepts carry him along swimmingly, till he tries to explain by them the beating of the heart, and the contraction of the small blood-vessels which regulate the blood-supply. Here comes in play the mysterious vital power again. He comes upon the same power when he tries to determine what it is that enables the muscle-fibre to take from the lymph the material needed for its use, and to discard the rest. The fibre acts as if it knew what it wanted--a very unmechanical attribute.