_Van Helmont._—Finally, the theory was laid down by Van Helmont, physician, chemist, experimentalist, and philosopher, endowed with a rare and penetrating intellect. Here we find many profound truths combined with fantastic dreams. Refusing to admit the direct action of an immaterial agent, such as the soul, on inert matter, on the body, he filled up the abyss which separated them by creating a whole hierarchy of immaterial principles which played the part of mediators and executive agents. At the head of this hierarchy was placed the thinking and immortal soul; below was the sensitive and mortal soul, having for its minister the _principal archeus_, the _aura vitalis_, a kind of incorporeal agent, which is remarkably like the vital principle, and which had its seat at the orifice of the stomach. Below again were the subordinate agents, the _blas_, or _vulcans_ placed in each organ, and intelligently directing its mechanism like skillful workmen. These chimerical ideas are not, however, so far astray as the theory of vital properties. When we see a muscle contract, we say that this phenomenon is due to a vital property—_i.e._, a property without any analogue in the physical world, namely _contractility_, in the same way the nerve possesses two vital properties, _excitability_ and _conductibility_, which Vulpian proposed to blend into one, calling it _neurility_. These are mere names, serving as a kind of shorthand; but to those who believe that there is something real in it, this something is not very far from the _blas_ of Van Helmont. _Vulcans_, hidden in the muscle or the nerve, are here detected by attraction, there by the production and the propagation of the nervous influx; that is to say, by phenomena of which we as yet know no analogues in the physical world, but of which we cannot say that they do not exist. _X. Bichat and G. Cuvier: Vital and Physical Properties Antagonistic._—The archeus and the blas of Van Helmont were but a first rough outline of vital properties. Xavier Bichat, the founder of general anatomy, wearied of all these incorporeal entities, of these unsubstantial principles with which biology was encumbered, undertook to get rid of them by the methods of the physicist and the chemist. The physics and the chemistry of his day referred phenomenal manifestations to the properties of matter, gravity, capillarity, magnetism, etc. Bichat did the same. He referred vital manifestations to the properties of living tissues, if not, indeed, of living matter. Of these properties as yet but very few were known: the irritability described by Glisson, which is the excitability of current physiology; and the irritability of Haller, which is nothing but muscular contractility. Others had to be discovered.