And nature would be quite as intractable in the case of "force-correlation," another of Mr. Spencer's redoubtable phrases. This term is quite recent in its application to animate objects, nor has it been long applied to inanimate. It is claimed to be a recently discovered force, and is one that the materialists have seized upon as the Herculean club with which to smite all vital theories to the earth. Its meaning, so far as it has any, is not difficult to get at. The simplest way to explain it, however, is the best. The reader is to understand that when he rubs two flat sticks together, the heat thereby engendered is not the result of friction, as all the world has heretofore supposed, but that the amount of force expended in rubbing the right-hand stick against the left-hand stick, is, by some law of versability, not over-well defined, transferred to the two sticks, and gets so entangled between their surfaces that it can only reappear in another and altogether different kind of force. When it leaves the hands and passes into the two sticks, it is, as the materialists assert, vital force. But as no force can be annihilated, the conclusive assumption is that it still exists somewhere. All of it, in the first place, went into the two flat sticks, and, when there, _ceased to be vital force._ Some of it disappeared, of course, in overcoming the inertia of the sticks, but the bulk of it became entangled with the superficial molecules of the two sticks, and reappeared as _heat_--another name for molecular force. This is what is meant by the "differentiation" of vital force into molecular force, and _vice versa_. But by what process of rubbing, under this law of versability, molecular force can be reversed, or differentiated back into vital force, Mr. Spencer has not condescended to inform us. The simple truth is, and the materialists will be forced to admit it in the end, that there is no verification of this theory beyond that of mere force-equivalence. For instance, it has been experimentally determined that a certain amount of fuel expended in heat is equivalent to a certain amount of mechanical force, not mechanical _work_, as M. Carnot puts it. For force is not expended in work until it is actually generated, and the amount generated, not that expended in work, is the real equivalence of the heat produced from fuel. Another problem is presented when it comes to determining the amount of generated force necessary to run a piece of machinery which shall accomplish a given amount of mechanical work.