PASSION.
The greater space which water occupies, when in the state of ice, has been long observed, and has been made the subject of some curious experiments, particularly by Ma jor Williams, who, during an intense frost at Quebec, actu ally burst a cannon, by the freezing of only a comparatively small quantity of water: (Edin. Trans. vol. ii. p. 23. et seq.) When water is converted into the solid state, it forms spicu lar crystals, which cross each other at a determinate angle; and thus interstices are left, which cause it to occupy mere space. Dr Thomson found, that the specific gravity of ice is to water at 600, as 92 to 100; and hence we may conceive the great force that will be exerted, when water suddenly expands itself, so as to have its gravity diminish ed in the above ratio.
The expansion of solids, which we are now to consider, is much less than that of liquids. The same elevation of temperature, which increases the volume of a gas from 100 to 137 parts, increases that of a liquid only to about 104 or 105 parts, while it adds to a solid not more that of its bulk. A set of experiments on the expansion of metals was performed by Smeaton, when he found, that, in passing from the freezing to the boiling points of water, platina was expanded from 100,000 to 100,087, gold from 100,000 to 100,094 parts, steel to 100,112 parts, copper to 100,170 parts, silver to 100,189 parts, tin to 100,288 parts, lead to 100.287 parts, and zinc to 100,296 parts: (Phil. Trans. 1754, p. 612.) It appears that the densest metals are generally the least expansible ; but this proportion does not univer sally obtain.
The expansion of glass, which is a point of great import ance to ascertain, as affecting the result of many other ex periments, was made the subject of careful examination by Deluc. Supposing its bulk at 32° to be 100,000, at 212° it was found equal to 100,083, an expansion nearly the same with that of platina. From this estimate we learn that glass is expanded about of its bulk by one degree of heat, and the expansion seems to be nearly uniform, for the same degree of heat, through the different parts of the scale. This equal augmentation of bulk, by equal incre .ments of heat, is supposed to prevail generally in solids; but, from the minuteness of the effect, it is less easy to as certain this point, than with respect to gases and liquids.
Several of the metals expand at the time they are con verted from the fluid to the solid form ; and this, as is the case with water, seems to depend upon the occurrence of a kind of crystallization, or regular arrangement of their par ticles. This is said to be particularly the case with respect to iron, and is the reason why we are able to take such ac curate casts, when the melted metal is poured into moulds. It must, however, be observed, that the increase of bulk, which either water or melted metals acquire when they be come solid, is no exception to the general law of expan sion ; for this applies solely to bodies as long as they retain the same state. The only anomaly, therefore, is the expan
sion of water while it remains fluid.
It is upon the expansive power which heat exercises over bodies that the thermometer is constructed ; an instru ment, as its name imports, employed for the purpose of measuring the degree of heat, or temperature of substances, to which it is applied, and which, considered in all its re • lations, may perhaps be regarded as the most useful agent in philosophical researches of which we are in possession. It appears to have been invented by Santorio, the celebrated Italian physician, who devoted so much of his attention to what has been called statical medicine. His instrument dc. pended upon the expansive power of the air, and consisted of a tube, with a bulb of considerable size, the lower end of the tube being left open, and being plunged into a fluid, which was suffered to rise to a certain degree into the tube. As the air in the globe was expanded or contracted by the addition or subtraction of heat, it is obvious that the fluid would sink ur rise in the tube, and thus mark the degree of heat. The Florentine academicians had the merit of con siderably improving the apparatus of Santorio, by substi tuting alcohol for air, and by confining it in a tube nearly similar to the one now in use. Halley is said to have been the first who employed mercury, and Newton made the very imporont improvement of forming a regular scale, by which the observations might be recorded and compared together. It is, however, to Hooke that we are indebted for what may be regarded as a still greater improvement, viz. the formation of fixed points in the scale, by which, at all times and in all cases, thermometrical observations might be compared together, and be, as it were, all referred to one invariable standard. He perceived that water be came solid always at the same temperature, and that, under the same atmospherical pressure, the fluid always becomes converted into vapour at the same degree of heat. Hence it follows, that if we immerse a tube containing mercury first into freezing, and afterwards into boiling water, we obtain two stationary and invatiable degrees or points in the scale, which will bear the same relation to the temperature of other bodies, whatever be the size of the instrument, or un der whatever circumstances the experiment is performed. Having ascertained these points, the numbers which are affixed to them, or the number of degrees into which we di vide the interval between them, is entirely arbitrary, and comparatively unimportant.