clothes are necessary to prevent the sudden escape of that heat from the surface of the body+ which the lungs have separated from the atmosphere. What becomes of the nitrogen which was combined with oxygen in atmospheric air? The greatest part of the nitrogen is thrown out of the lungs at every respiration ;‡ and being some which contains that exact portion of caloric which the animal economy requires! It is worthy of remark, that cold-blooded animals, which are not furnished with this breathing apparatus, are so constituted that their temperature changes with every change of the temperature of the surrounding medium. Frogs have been absolutely frozen só as to chip like ice, and yet when carefully and gradually thawed have been completely reanimated. * Clothes keep the body warm in consequence of the air which they infold within them; atmospheric air being a nonconductor of heat. It is on this principle that double windows preserve the warmth of apartments at an equable temperature. In like manner double lids for boilers, formed so as to hold a eheet of air, are found to be very effectual for preserving the heat of the liquor with a very small portion of fuel. On this principle it is, that light spongy substances, such as furs and down, afford the warmest clothing. Hence it is that the carpet which covers the earth in winter, is spread out by Nature with so light a hand, that it might hold an abundance of atmospheric air within its interstices, to preserve the warmth of those innumerable tribes of vegetables which it is destined to protect. We clothe ourselves with wool, because it is a bad conductor of heat and retards its escape from the body. The inhabitants of Russia clothe themselves in fur, because fur is still a worse conductor of heat than wool. Sheep are natives of a temperate climate; but the bear and the ermine of the coldest. The provident care of the Creator is evidently conspicuous in this appointment, and discovers the same undeviating attention to the comfort of all his creatures: hence the clothing of animals in the torrid zone is hair, in the temperate zones wool, in the frigid thick fur. It is not simple nitrogen which is thrown out, but nitrogen gas; and it has been imagined, that when atmospheric air is decomposed by the lungs, part of the disengaged caloric is required for the nitrogen, to preserve it in the form of gas: but what lighter than atmospheric air, it rises into the atmosphere to await fresh combinations.* What provision has nature made for restoring the vast quantity of oxygen which respiration and combustion are perpetually taking from the atmosphere? The leavest of trees and other vegetables give it is a curious fact, that nitrogen gas and carbonic acid gas, both which are thrown off in the act of respiration, have less capacity for caloric than any other gaseous substance. It is a general characteristic of the gases, that they absorb a large portion of caloric to preserve them in a gaseous form. Yet one of these gases has less capacity for caloric than many liquids, and the other (nitrogen gas) less capacity than even ice. Could any thing possibly have been better contrived for the preservation of that portion of caloric, which is necessary to keep up the animal temperature! It may be remarked, that the interval which there is between every inspiration seems to have been designed, to allow time for the nitrogen gas which is thrown out of the lungs to mount in the air above the head, in order that a fresh portion of air might be taken in, and that the same air might not be repeatedly breathed. That this actually entered into the plan of the Divine mind, may be presumed from the levity which has been given to nitrogen gas, and which enables it to rise in atmospheric air. See Additional Notes, No. 53. * It may be observed that, if the specific gravity of the two constituent parts of atmospheric air had been reversed, the nitrogen thrown off by the respiration of men and animals would have perpetually occupied the lower regions of the atmosphere, and produced universal pestilence. Nitrogen gas is very little lighter than atmospheric air, but probably to that little the atmosphere owes the salubrity it possesses. How provident has the Almighty been, in thus foreseeing the operation of those laws which were designed to promote the welfare of every species of animated beings! "A vessel of 1000 cubic inches will contain 315 troy grains of common air; but it will contain 335 of oxygen gas, and only 297 of nitrogen gas." Dr. Black's Lectures, vol. ii. 109. The upper side of the leaf is the organ of respiration; hence some vegetables (as they give out oxygen only in the day) close the upper surfaces of their leaves during the night. The multiplicity of the leaves of trees, &c. indicates the importance of transpiration to a vegetable. out during the day a large portion of oxygen gas, which, uniting with the nitrogen gas thrown off by animal respiration, keeps up the equilibrium, and preserves the salubrity of the atmosphere. Is this perpetual renovation of the atmosphere owing to a fortunate concurrence of circumstances, or is it the effect of design and contrivance? When we recollect the various processes of nature and art, which concur with respiration and combustion in depriving the atmosphere of *To show the production of oxygen gas from the leaves of plants, fill a glass bell with water, introduce leaves under it, and place the bell inverted in a flat dish of water. Expose the apparatus to the rays of the sun, and very pure oxygen gas will be disengaged, which will displace the water in the jar, and occupy its place. In like manner a sprig of mint, corked up with a small portion of carbonic acid air and placed in the light, renders it again capable of supporting life. The plant purifies what the animal had poisoned. Thus, while the vegetable tribes inhale All the oxygen is not given out by plants; part must be retained to form the sugar and acids which are found in vegetables. Mr. Cruickshank has shown by experiment, that oxygen is absolutely necessary for the conversion of mucilage into sugar For more on this subject, see the account of the decomposition of water, chap. iv. of this volume. its oxygen; and that, notwithstanding the atmosphere uniformly contains every where the same proportion of this gaseous substance,* we can attribute the renovation to nothing but design, and perceive in it a proof that the laws of nature must be referred, not to blind chance, but to unerring intelligence combined with, infinite goodness.† (Rollo on Diabetes.) Plants also absorb nitrogen from the atmosphere with avidity; and this is another means of keeping up the standard purity of atmospheric air. * It has been found that the air of the most crowded cities contains as much oxygen gas as that of other places. This was demonstrated by Priestley soon after he became engaged in making experiments with his eudiometer. All kinds of vegetables, when assisted by the rays of the sun, have the power of decomposing water; during which decomposition the hydrogen is absorbed, and goes to the formation of oil and resin in the vegetable; while the oxygen combines with part of the caloric received from the sun, and is given out in the form of oxygen gas; so that this one operation of nature gives nourishment and provides materials of growth to the vegetable world, and, at the same time, renovates that vital principle in the atmosphere which is necessary for the support of the animal creation. Surely nothing short of consummate wisdom could have conceived any thing half so beautiful in design, or extensively and superlatively useful in effect. See Addition al Notes, No. 10 and 18. CHAPTER III. OF CALORIC. WHAT is heat? Heat is the well-known sensation which we per-, ceive on touching any substance whose temperature is superior to that of the human body. What name is given to the matter of heat? * Chemists have agreed to call the matter of heat caloric, in order to distinguish it from the sensation which this matter produces.‡ What are the uses of caloric ?§ Caloric is every where indispensable to the The sensation of heat and cold arises from the tendency which caloric has to diffuse itself equally amongst all substances that come in contact with it. If the hand be put upon a hot body, part of the caloric leaves the hot body and enters thehand this produces the, sensation of heat. On the contrary, If the hand be put upon a cold body, part of the caloric contained in the hand leaves the hand to unite with the cold body: this produces the sensation of cold. Some of the methods of producing artificial cold, may be seen in Mr. Walker's papers in the Philosophical Transactions for 1795 and for 1801, and in Watson's Chemical Essays. In answer to the question. What is the cause of caloric? it may be necessary to state, that philosophers have differed in their opinions on this subject. Some have considered it merely the consequence of a peculiar motion among the particles of bodies, and that it has no existence independent of motion, any more than sound has. Others have supposed that it is really a distinct substance, which exists independent of every other. The latter is the general opinion at present. In order to give precision to chemical language, it was necessary to find a term to distinguish the matter of heat from its effect; for, whenever caloric becomes fixed in a body, it loses its property of affording heat. Nothing can be more evident than that caloric may exist in many substances, without producing any of the effects which arise from the agency of fire. Many of the uses of fire will immediately occur to every |