and yet is unrestricted by any law limiting its efficiency such as fetters the heat-engines.

Messrs. Becquerel and Breschet and later investigators have found, by actually introducing slender, needle-like thermometers into the flesh, that the temperature of the body is substantially the same in all parts. The muscles are one or two degrees higher in temperature than the skin tissue, and, during exercise, they may rise a degree or two higher still; but there is no point in the body, so far as can be ascertained, at which the heat exceeds the mean to any important extent. Hirn and others have shown that the human machine has at least the efficiency of the best steam engines; that is to say, it converts as large a proportion of its supply of energy into work as the best heat-engines. This would, were it a heat-engine, a thermo-dynamic machine of similar character, compel the provision of steam-boiler temperatures within the body-a simple impossibility in a mass composed mainly of fluids evaporating at the boiling-point under atmospheric pressure and of tissues altered by temperatures not greatly in excess of that standard. Experiment also shows the arterial blood to be but two or three degrees, at most, above the temperature of the venous; cold-blooded animals, as the fishes, usually exhibit no greater excess of heat over the fluid in which they live, and the molluscs practically coincide in temperature with the water about them. Exercise, increasing the temperature of all living mechanisms, notwithstanding the increased amount of energy drawn from the store and converted into work, raises the temperature of the whole machine and causes large increase in the quantity of heat conducted, radiated, and exhaled; but this very possibly comes mainly of the increased heart action, accelerating the flow of the currents in the arteries and veins and the conversion of its friction-work into heat. The fact gives no clue to the secrets of the vital sphinx.

Galvani showed that the nerves could be traversed and the muscles contracted by a current of voltaic or of high-tension electricity, and many later investigators confirmed his statements, while still others have shown that currents actually traverse the muscular and nerve tissues which originate in the body, and which, reversing Galvani's experiment, reveal their nature by all the tests familiar to the physicist as detecting the presence and measuring the action of electric currents from artificial sources.

The potentialities of the animal machine are developed in this direction most strikingly in the gymnotus and many other creatures, in which the currents are produced in great power and intensity, and directed, at will, in the capture of their prey or in self-defence, sometimes disabling a man or felling an ox by their powerful discharges. The fact of the transformation of energy is well illustrated in these cases by the exhaustion of the creature when it has continued this discharge of electrical energy a short time. To this extent, certainly, these animals are electro-dynamic machines. The electric eel and the torpedo are but the two best known of about fifty such electro-dynamic animal machines already discovered, and it is thought by some authorities that all animals possess this power of producing and applying electric energy in less degree. Faraday found that the gymnotus, the electric eel, has a storage power equal to that of fifteen large Leyden jars. It is probably well-established that every muscle and nerve of every animal is traversed by energy closely related to the electric current, and Daguin calls this "l'électricité vitale."

Perhaps the most beautiful and striking of all the potentialities of the animal machine is that exhibited by those in which energy, whatever its nature, developed and stored in the body, or applied in its various curious and intricate operations, is made to take the form of light. In all the practical work of the engineer and of the man of science, light is only obtainable by the production of high temperatures, and the brighter the light the higher must be the temperature of its source. At 700 degrees to 800 degrees Fahrenheit a red glow only appears; at 1,000 degrees almost white light is produced; at 1,500 degrees to 2,000 degrees the radiance becomes brilliant, and the dazzling lights of the electric arc and of the sun indicate temperatures measured by thousands of degrees. Yet, on the other hand, we find light produced in nature without perceptible heat. Moonlight is sunlight almost freed from heat; its warmth is not only insensible, but so feeble that it is only by Professor Langley's famous "bolometer," an instrument of unimaginable delicacy, that science can measure it at all. The phosphorescence of the decaying stump or fallen tree in the forest, and that of the animalcule with which the waters of tropical seas so frequently teem, are examples of the production of light without heat, and, in the case of the latter at

all events, at very nearly the temperature of the animal organism and of the surrounding sea.

The glow-worm and the firefly are provided with apparatus especially designed by nature for the production of this lowtemperature light, free from heat, and Professor Langley and his colleague, Mr. Very, have shown by bolometric tests that this light from the animal machine is comparable only with moonlight in its freedom from heat In other and, for our present purposes, much more significant, words, the vital machine converts energy into pure light without wasting it in the form of heat. In the familiar forms of artificial light, the heat, which is not only wasted and thrown away, but is a source of great annoyance and a real evil in many ways, constitutes an enormous proportion, often ninety-nine per cent., of the energy expended, and a proportional part of its cost is thus thrown away with it. We pay one or five per cent. of our bills for the light received, the balance for the heat wasted; very much as, in the heat-engines, we pay often a similar proportion for power received and a balance of equal proportionate amount for the heat thrown away unutilized; one or five cents in every dollar paying for value received, ninety-five or ninety-nine cents being paid for wasted, yet no less costly, energy. Gas and bright oil-flames, electric glow and arc lamps, respectively, return us one, two, and ten per cent. of the energy transmitted to them for transformation into light; while the moon and the glow-worm or firefly give us light without sensible heat. The animal as a lightproducing machine is thus many times more efficient than the apparatus of the electrical engineer; it seems possible that we may find this vital mechanism but little less economical as a motive machine, and serving as a guide to the construction of artificial apparatus that shall displace the heat-engines and utilize the enormous proportion of their energy-supply now wasted.

Scientific investigation, on the other hand, has discovered facts which decidedly conflict with our theories of thermo-electric and electro-dynamic transformations of energies in the vital machine. The velocity of the nerve impulses is but about 90 feet per second in cold-blooded, and two to three times that speed in warm-blooded, animals; while electric currents are ordinarily inconceivably more rapid; thus indicating peculiar conditions of electrical conduction, if this be the mode of communication along

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the nerve, or, perhaps more probably, something like a mechanical or a wave-like transmission, molecule acting by contact with molecule, as often observed in muscular movements. The originating form of energy and method of energy-conversion in the brain and spinal cord remain, in any event, as obscure as ever. It is found, also, that light and heat may be produced, and sometimes are produced, by the combustion in the animal system of certain fatty fuels capable of oxidation at these low temperatures, with liberation of heat and light at the same low intensity, thus safely burning without danger to the tissues. The probability that this may be a customary method of production of heat in the animal machine is thus indicated, and the mystery in such case is that of nature's method of producing such fuels from foods and of conducting their combustion.

Summarizing the argument: the animal machine, the vital prime motor in which we live, is supplied daily with an amount of energy in its food equivalent, dynamically, to the potential energy of a pound of coal. This is, in turn, the equivalent of one-fifth of a horse-power for twenty-four hours. A day's work is at most one-eighth of a horse-power for one-third of a day, at steady labor, one twenty-fourth as an average for the twenty-four hours. Thus measured by the labor of a working man, the animal machine utilizes one-fifth of the energy supplied it, just the efficiency of the best steam engines that the greatest inventors and best mechanics of our time have been able to produce. But it does much more than this. The brain takes from one-fifth to onetenth of the original stock of energy; all the work of digestion, respiration, and circulation, and of every muscular movement, voluntary and involuntary, and all that of reconstructing and repairing tissue of muscle and nerve and bone, must be added, and the efficiency of this prime mover is thus very far in excess of twenty per cent. and of the performance of the best engines. The experiments of Hirn, showing the rejected heat-energy to be twice as great, proportionally to oxygen inhaled, when at rest as when at work, indicates the total efficiency to be about fifty per cent. or two and a half times as great as in the best engines of human construction; the production of power being the gauge. Langley has shown that, where the animal machine produces light, it does so at a cost, substantially all heat being eliminated, of probably a small fraction of one per cent.

of that of our familiar lights; and other investigations show that, where adapted to the production of electricity of high tension, as in the gymnotus, it does this by consuming food-combustibles composed of the same elements, mainly, as our fuels -and, by this direct evolution, escapes the loss of nine-tenths or ninety-five hundredths of the energy drawn upon in our artificial methods of electric light and power generation. Heat production must be similarly economical in the animal machine, as there are no important losses from it, it produces just enough to keep its temperature normal and constant under its covering of non-conducting hair or wool. In all these vital operations, heat and power are always produced and observable. Indications of the generation and use of electricity or some similar energy are detected in all animal machines, and sometimes electricity, also, in large quantity and of high intensity. In some instances the production of light is a result of transformation of energy in these machines, and thus the animal system illustrates the transformation of energies in all known ways, exhibits direct transformations unknown in applied science and engineering, and excels always, and sometimes enormously, in the efficiency with which it effects these transformations and performs its special tasks.

Could the inventor, the man of science, the engineer, compete with nature in these directions, it is evident that the stores of fuel, now so rapidly wasting before the growing demands of civilized races, would last many times longer than now appears probable; the period when the race must bore into the interior of the earth or remove to the tropics to obtain heat and power would be proportionally delayed. A day's work or a dollar would become equal in value to a large multiple of the value to-day measured in heat, light, electricity, or mechanical power, and the human race would be enriched and advantaged inconceivably through the discoveries of science and the ingenuity of the inventor, the mechanic, the engineer. That such outcome of the labors of scientific men is certain, no one can say; that it is probable, no one will deny; that it is possible, every one will admit. The future undoubtedly will display more and greater wonders as the fruit of intelligent scientific investigation, than has the past, than has even the generation just past. Among these marvels, it is safe to predict, will come at least some approximation to nature's methods of production of all the energies.