The nature of heat is to expand all fluids. The blood in the lungs must, therefore, expand; and if it expands, it must move; and if it moves, it must, from the organism of the parts, move to the left ventricle of the heart, into which the valvular system opens to give it a free passage--whereas the valves of the right close against it. "Eureka!" I mentally exclaimed; "I have found the _primum mobile_ of the circulation of the blood." I had for years disbelieved that the heart's slight mechanical impulse was that cause. In teaching Paley's "Natural Theology," my mind had come in contact with the passage in which he describes the heart's more than Herculean labors; and I said, "This is altogether too much--the heart alone cannot perform all this--there must be some other power," and an abiding desire to know what that power could be, prepared me for receiving this great idea. But my mind was agitated by it, as the sea is, when a great rock is thrown into its waters. The cholera was then raging around me; and as I prepared to flee from it to a mountain air, I confided to a scientific friend, Professor Twiss of West Point, my hypothesis, which I regarded as probably the incipient germ of an important discovery. But there was first the former theory to be disproved; and then there were new points to be investigated and established. In the ensuing winters of 1833, 4, and 5, I gave much attention to the subject, and employed professors in my school in the departments of chemistry and natural philosophy, who assisted me,--particularly by their ingenuity in the construction of such simple pieces of apparatus as were needed. Thus we proved that, although the heart's action gives pulsation, it does not necessarily give circulation. By an endless india-rubber tube, filled with water, coiled upon a table and struck repeatedly at one point, a pulsation was produced throughout, but no circulation. By affixing the tube to a vessel of water, and laying it on an inclined plane, the water ran through it in an equable current, making circulation with pulsation. Clasping the hand upon the tube in successive contractions, the fluid passed on _per saltem_, producing circulation and pulsation united, but no acceleration of the current. Now, add valves to the tube on each side of the opening hand, and you will have the current--which is moving by gravitation, accelerated by the hand's impulse, as the blood's current, first moved by respiration, undoubtedly is by the heart's beat.