a small scale may be seen in the common fire engine; but its grandest applications are in water works, where, with its assistance, a steam engine, occupying but one small building, may entirely supersede the necessity of raising water into elevated reservoirs for the supplying extensive districts with water. It is quite astonishing to reflect on the results which are brought about by the simplest applications of the laws of nature; the water works of London are among the most wonderful monuments of human genius and ingenuity.

Another striking illustration of the preceding laws of elastic fluids, is that of the application of high pressure steam as a moving power. It is well known that water at the temperature of 212° passes into an invisible vapour, under the ordinary pressure of the atmosphere (Art. 96); but when the temperature is much higher than this, the steam has an elastic force which the strongest vessels cannot resist. For all temperatures higher than 212o it is called high pressure steam, that is, its elastic force is greater than the elastic force of the atmosphere in its natural state, and when this invisible highly elastic fluid is applied after the manner which modern science has devised, it leads to results which it would take volumes to detail, and which will occupy the energies of generations to come to bring to perfection.

91. Dalton's Laws.-There are some facts connected with elastic fluids so remarkable and curious that they deserve the attentive consideration of every one; hitherto, however, they have not been generally laid before the student of physics. They relate to the mixture of gases with each other, and their diffusion, in defiance of the laws of gravity. The phenomena are briefly as follows; let any two vessels be taken, the one containing hydrogen, and the other carbonic acid gas, and let the hydrogen, which is an extremely light gas, be set above the other, which is an extremely heavy one, and a communication opened between them. Then on the gases being examined after a short

interval, carbonic acid gas will be found in the upper, and hydrogen in the lower vessel, and there will be a complete mixture, or, diffusion through each other. Again, if three or more vessels be set one above the other, each containing a different gas, the same diffusion will take place; there will be a perfect mixture in all the vessels; the gases, whatever be their number, will be equally diffused through each other, so that any portion of any one vessel will contain a portion of all the gases. Again, if a gas be contained in a cracked or porous vessel, the gas will gradually diffuse itself into the air, and the air into the gas, each passing through the cracks or pores at the same time, but in opposite directions.

The consideration of these and many similar phenomena, led Dalton to infer that the particles of one gas, though highly repulsive to each other, exert no repulsive action on the particles of another gas. Thus, one gas is as a vacuum to every other, the particles of one permeate freely the particles of another, without any obstruction except the mechanical action which they may experience from impinging on each other. Thus one gas travels through the interstices of the other, just as a stream of water permeates a sand bed, or air permeates a porous substance, without any active opposition; and though, during the diffusion, the inertia of the particles of another gas may sometimes cause some retardation of the motion, yet when the mixture is complete, or the gases are in a state of rest, the particles act only on those of their own kind.

The evidence for the truth of this theory is of the presumptive kind, and derives its strength from the beautiful explanation which it furnishes of the known phenomena. Any other hypothesis, affording as good an explanation, would be equally entitled to be received as true. But in default of this, if the simplicity of an hypothesis which furnishes a complete explanation of many phenomena, otherwise inexplicable, be evidence of the truth of a physical theory, then has this of Dalton strong claims, if not on

our belief, at least on our attentive consideration. The difficulty of all other explanations arises from the fact, that the phenomena take place in defiance of the known laws of gravity. Carbonic acid gas, which is twenty-two times heavier than hydrogen, remains in equilibrium with this and all other gases, in a state of perfect admixture. But there is one phenomenon which affords direct evidence of the non-action of the particles of one elastic fluid on the particles of the other. If a mixture of alcohol and water be placed under a partially exhausted receiver, the evaporation both of the alcohol and water will proceed rapidly, and only be suspended when the space above becomes charged with vapour, the elastic force of which puts a stop to the formation of any fresh vapour. If now a small quantity of lime be put under the receiver in a cup, the vapour of water combining with this will leave a space for the formation of fresh vapour; in this way may all the water be drawn off, and what is left will be pure alcohol. Thus it appears that the action of the watery vapour alone checks the evaporation of water, the action of the alcohol vapour alone checks the evaporation of the alcohol; withdraw either, and the evaporation proceeds. Thus the action of the particles of the two fluids appears to be entirely independent; each vapour acts only on its own fluid, without any connexion with the particles of the other. This simple experiment alone gives a strong presumption in favour of Dalton's Law.*

* See Principles of Hydrostatics, Arts. 74-77, and Memoirs of Manchester Society, Vol. v.

SECTION IV.

ON THE ATMOSPHERE-ITS WEIGHT-EFFECTS-THEORY OF WINDSLIMITS CONSTITUTION OF ATMOSPHERE.

92. It would be quite superfluous to attempt any proof of the existence of the invisible medium in which we live and breathe; though it does not ordinarily affect our senses as immediately as the solid and liquid matter by which we are surrounded, yet its agency and existence are indicated every where. There are clouds in all climates and tempests in all seas; hence the whole surface of the globe is encompassed by this airy envelope. It forms every where a layer of vast thickness; for in all countries, on the tops of the highest mountains, as well as on the plains, we see the clouds carried along by the wind, and above these clouds we behold the deep colour of the vault of heaven, shewing the height of the atmosphere, just as the colour of the ocean shews the depth of the water. Were it not for the air the heaven would be dark and colourless; it would appear like a black vault, in which the stars would shine with the same brilliancy by day as they do by night. This mighty mass of fluid, which is expanded over the whole globe, and of which the successive layers extend far higher than the highest mountains, is summed up in the term atmosphere. Its constitution is chemically the same at all altitudes, places, and periods, and consists of oxygen and nitrogen, with small quantities of other gases, respecting which however we shall add more hereafter. Besides the molecular force to which it owes its gaseous character, it is subject to gravity; hence it is brought at once under the laws of fluid equilibrium.

93. Pressure of the Atmosphere.-The atmosphere being a fluid acted on by gravity, we must have the same propo

The

sitions respecting its pressure and weight as for a liquid. Hence the pressure is proportional to the depth; but it must be remembered, that we at the surface of the earth are at the bottom of the fluid in question; hence the pressure or weight of the superincumbent column will be greater at the surface of the earth than at any higher levels, and will diminish regularly as we ascend. weight of the atmosphere, which follows so immediately from known laws, was not established till the year 1640, by Galileo; this discovery being followed out by his pupil Torricelli, was applied to account for the ascent of water in the common suction pump; and in the hands of Pascal, was traced out in all its consequences. The instrument by which the weight of the atmospheric column is measured is termed a Barometer. From this, with the assistance of the air-pump, the most distinct and complete proof of the weight of this invisible medium may be derived. The barometer, in its simplest form, consists of a small glass tube, of about 32 inches in length, hermetically closed at one end; the tube being filled with mercury, and a finger placed over the open end to prevent the escape of the mercury, it is inverted over a vessel of mercury, termed the cistern or basin; the finger then being removed, there is free communication betwixt the mercury in the tube and the mercury in the basin, and the former will sink down a little, and stand at a height varying from 28 to 31 inches above the mercury in the basin. The inner diameter of the tube must not be less than one-eighth of an inch, and the mercury must be dry and pure. When a barometer tube has been well filled, the space at the top is the most perfect vacuum with which we are acquainted.

The column of mercury in the tube being thus sustained at a considerable height above the mercury in the basin, contrary to the established law of all fluids in equilibrium standing at the same level, gave rise to the invention of most curious hypotheses for its explanation. When the tube is less than about 29 inches no vacant space is left at