E linear expansion of bars of metal, arrived much nearer to a correct estimate of temperatures above a dull red heat. Daniell calls his instrument the register pyrometer, and describes it as follows: "It consists of two parts, which may be distinguished as the register and the scale. The register is a solid bar of blacklead earthenware, highly baked. In this a hole is drilled, into which a bar of any metal, 6 in. long, may be dropped, and which will then rest upon its solid end. A cylindrical piece of porcelain, called the index, is then placed upon the top of the bar, and confined in its place by a ring or strap of platinum passing round the top of the register, which is partly cut away at the top, and tightened by a wedge of porcelain. When such an arrangement is exposed to a high temperature, it is obvious that the expansion of the metallic bar will force the index forward to the amount of the excess of its expansion over that of the blacklead, and that, when cooled, it will be left at the point of greatest elongation. What is now required is the measurement of the distance which the index has been thrust forward from its first position; and this, though in any case but small, may be effected with great precision by means of the scale. This is independent of the register, and consists of two rules of brass accurately joined together at a right angle by their edges, and fitting square upon two sides of the blacklead bar. At one end of this double rule a small plate of brass projects at a right angle, which may be brought down upon the shoulder of the register, formed by a notch cut away for the reception of the index. A movable arm is attached upon this frame, turning upon its fixed extremity upon a centre, and at its other carrying an arc of a circle, whose radius is Fig. 151 Fig.152 exactly 5 in., accurately divided into degrees and thirds of a degree. Upon this arm at the centre of the circle another lighter arm is made to turn, one end of which carries a nonius with it, which moves upon the face of the arc, and subdivides the former graduation into minutes of a degree; the other end crosses the centre, and terminates in an obtuse steel point, turned inwards at a right angle. "When an observation is to be made, a bar of platinum or malleable iron is placed in the cavity of the register; the index is to be pressed down upon it, and firmly fixed in its place by the platinum strap and porcelain wedge. The scale is then to be applied by carefully adjusting the brass rule to the sides of the register, and fixing it by pressing the cross piece upon the shoulder, and placing the movable arm so that the steel point of the radius may drop into a small cavity made for its reception, and coinciding with the axis of the metallic bar. "The minutes of the degree must then be noted, which the nonius indicates upon the arc. A similar observation must be made after the register has been exposed to the increased temperature which it is designed to measnre, and again cooled, and it will be found that the nonius has been moved forward a certain number of degrees or minutes, as shown at Figs. 151 and 152." Fig. 151 represents the register; A is the bar of black lead; a the cavity for the reception of the metallic bar; ce' is the index, or cylindrical piece of porcelain; d, the platinum band, with its wedge, e. Fig. 152 is the scale by which the expansion is measured: ƒ is the greater rule, upon which the smaller, g, is fixed square. The projecting arc h is also fitted square to the ledge under the platinum band d. D is the arm which carries the graduated arc of the circle E. fixed to the rule f, and movable upon the centre i. C is the lighter bar fixed to the first, and moving upon the centre k. H is the nonius at one of its extremities, and m the steel point at the other. The rule g admits of adjustment on f, so that the arm h may be adjusted to the centre i, in order that at the commencement of an experiment the nonius may rest at the beginning of the scale. The term "nonius," used by Daniell, is only another name for vernier, a contrivance for measuring intervals between the divisions of graduated scales on circular instruments. The scale of this pyrometer is readily connected with that of the thermometer by immersing the register in boiling mercury, whose temperature is as constant as that of boiling water, and has been accurately determined by the thermometer. The amount of expansion for a known number of degrees is thus determined, and the volume of all other expansions may be considered as proportional. The melting-point of cast iron has been thus ascertained to be 2786°, and the highest temperature of a good wind-furnace about 3300°-points which were estimated by Mr. Wedgwood at 20,577° and 32,277 respectively. Mr. Wedgwood, indeed, makes an observation which is calculated to throw suspicion upon the accuracy of his results; for he says, "We see at once how small a portion (of the rays of heat) is concerned in animal and vegetable life, and in the ordinary operations of nature. From freezing to vital heat is barely 1-500th part of the scale-a quantity so inconsiderable relatively to the whole that in the higher stages of ignition ten times as much might be added or taken away without the least difference being discoverable in any of the appearances from which the intensity of fire has hitherto been judged of." Now this, remarks Daniell, "is utterly unlike the gradual progression by which the operations of nature are generally carried on; and the fact is, that a regular transition may be traced from one remarkable point of temperature to another." Thus from the freezing of water, 32°, to vital heat in man is 60°. 60 x 3 180° Boiling water. 60 x 7 420° Melted tin. 60 x 45 2700° Melting cast iron. 60 x 55=3300 Highest heat of wind-furnace. Before the invention of the register pyrometer, the expansion of solids had never been ascertained beyond the temperature of 527°: the following table exhibits the progressive amount of several metals to their point of fusion, as determined by Daniell's pyrometer: Professor Daniell concludes his dissertation by the following passage, which is quite in accordance with those notions which Tyndall has so ably contended for-viz., that heat is a mode of motion:-"The amount of the force which produces these expansions and contractions, measured by any opposing force, that of cohesion, for instance, is enormous. 'Some idea may be formed of it, when it is understood that it is equal to the mechanical force which would be necessary to produce similar effects in stretching or compressing the solids in which they take place. Thus, a bar of iron heated so as to increase its length a quarter of an inch, by this slow and quiet process exerts a power against any obstacle by which it may be attempted to confine it, equal to that which would be required to reduce its length by compression to an equal amount. On withdrawing the heat, it would exert an equal power in returning to its former dimensions." M. Molard used this great moving force to restore the walls of a building to the perpendicular which had been bulged, and the same principle was used at the Cathedral of Armagh. THE EXPANSION OF GASES. We now come to the most expansible bodies-viz., the gases; and, although at first there was considerable doubt whether they all expanded alike, because the experimentalists had neglected to remove the moisture the aqueous vapour-from them, it was finally discovered, not only by Gay-Lussac in Paris, but by our own countryman, the illustrious Dr. Dalton, that all gases expand alike with the same amount of heat, and that the rate of dilatation continues uniform for all temperatures. In discovering the expansibility of liquids it was found that cohesion was not quite overcome, and that there was still a considerable amount of that force which tended to keep the particles in contact. This, however, is not the case with gases; the cohesive power is for the time completely overcome by the motion of heat. Sir H. Davy speaks emphatically upon this motion in his "Chemical Philosophy." "It seems possible to account for all the phenomena of heat, if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest bodies moving with the greatest velocity and through the greatest space; that in fluids and elastic fluids, besides the vibratory motion, which must be conceived greatest in the last, the particles have a motion round their own axes with different velocity, the particles of elastic fluids (gases) moving with the greatest quickness; and that in ethereal substances the particles move round their own axes, and separate from each other, penetrating in right lines through space. Temperature may be conceived to depend upon the velocity of the vibration, increase of capacity in the motion being performed in greater space; and the diminution of temperature during the conversion of solids into fluids or gases may be explained on the idea of the loss of vibratory motion in consequence of the revolution of particles round their axes at the moment when the body becomes fluid or aëriform, or from the loss of rapidity of vibration in consequence of the motion of the particles through space." It has been proved that gases expand by 1-490th of their own volume for every degree of Fahrenheit's scale between the freezing-point, 32°, and the boiling-point of water, 212, and so on at higher or lower temperature, provided the pressure of the air remains the same. If the Centigrade scale is used, the ratio of expansion of any gas will be 1-273rd of its volume for every degree. 490 cubic inches of air at 32° become 491 at 33° From a most careful series of experiments it has been determined that "the coefficient of expansion" of all gases, expressed in decimals, is 0'00,366. These figures are near enough for all ordinary calculations, although it must be observed that, speaking rigidly, this is not exactly the case, except probably with the three permanent gases, oxygen, hydrogen, and nitrogen,—in all the other gases and vapours the expansion being greatest for those which are most readily condensible. M. Regnault has made the most elaborate and careful experiments, and determined that one thousand volumes of certain gases at o° C. or 32° F. (the pressure of the air remaining unchanged) become expanded in the following proportions when heated to 100° C., or 212° F.: It will be apparent that hydrogen expands the least, and, as might be expected, cyanogen, which is liquified with comparative ease, is much higher —viz., 1,387.67. It is, therefore, apparent that if the coefficient of expansion remains the same with all gases, that cyanogen should have been represented by the same figures as those which belong to air-instead of being o'00,387 to 000,367 atmospheric air. The conversion of this property of expansion into power or motion is well described by Tyndall :-"Suppose I have a quantity of air contained in a very tall cylinder (A B, Fig. 153), the transverse section of which is one square inch in area. Let the top, A, of the cylinder be open to the air, and let P be a piston, which, for reasons to be explained immediately, I will suppose to weigh two pounds one ounce, and which moves air-tight and without friction up or down in the cylinder. At the commencement of the experiment let the piston be at the point P of the cylinder, and let the height of the cylinder from its bottom B to the point P be 273 inches, the air underneath the piston being at a temperature of o° C. Then, on heating the air from o' to 1° C., the piston will rise one inch; it will now stand at 274 inches above the bottom. If the temperature be raised two degrees, the piston will stand at 275; if raised three degrees, it will stand at 276; if raised ten degrees, it will stand at 283; if 100 degrees, it will stand at 373 inches above the bottom; finally, if the temperature were raised to 273° C., it is quite manifest that 273 inches would be added to the height of the column; or, in other words, that by heating the air to 273° C. its volume would be doubled. The gas in this experiment executes work. In expanding from P upwards, it has to overcome the downward pressure of the atmosphere, which amounts to 15 lbs. on every square inch, and also the weight of the piston itself, which is 2 lbs. I oz. Hence, the section of the cylinder being one square inch in area, in expanding from P to P' the work done by the gas is equivalent to the raising a weight of 17 lbs. 1 oz., or 273 ounces, to a height of 273 inches. It is just the same as what it would accomplish if the air above P were entirely abolished, and a piston weighing 17 lbs. 1 oz. were placed at P. Р B FIG. 153. "Let us now alter our mode of experiment, and, instead of allowing our gas to expand when heated, let us oppose its expansion by augmenting the pressure upon it; in other words, let us keep its volume constant while it is being heated. Suppose, as before, the initial temperature of the gas to be o C., the pressure upon it, including the weight of the piston P, being as formerly 273 ounces. Let us warm the gas from o° C. to 1 C.; what weight must we add at P in order to keep its volume constant? Exactly one ounce. "But we have supposed the gas at the commencement to be under a pressure of 273 ounces, and the pressure it sustains is the measure of its elastic force; hence, by being heated 1°, the elastic force of the gas has augmented by 1-273rd of what it possessed at o°. If we warm it 2°, two ounces must be added to keep its volume constant; if 3°, three ounces must be added; and if we raise its temperature 273°, we should have to add 273 ounces, that is, we should have to double the original pressure to keep its volume constant. "In the first case marked out, it is shown that by heating the air to 273° C. its volume would be doubled. In the second, that by compressing the air with 273 ounces we may heat it to 273° C., and have, consequently, double the |