Cast iron, antimony, and bismuth, form exceptions to the general law of expansion, as they actually in crease in bulk in passing from the solid to the fluid state. Mr Mushet has established this fact for cast iron, by nu merous experiments, which prove that it is most dense when fluid, and that it acquires its greateset volume in passing from the fluid to the solid state. The contraction of argillaceous bodies, by an increase of temperature, will be described in our account of 1,Vedgwood's THER MOMETER.
On the Exfiansion of Liquids.
We have already remarked that all fluid bodies are also expanded by an increase of temperature. The com mon method of measuring these expansions is to enclose them in a glass tube, and to observe the different heights to which they rise when subjected to different tempera tures. These heights diminished by the expansion of the glass tube, will afford an accurate measure of the expansion of the included fluids. In this way M. De Luc made a number of experiments, which we have given in the following Table, the degrees in the column being those of Reaumur's thermometer.
By examining the results in the preceding Table, it will appear that the expansibilities increase rapidly near the boiling point, and that near the freezing point there are considerable irregularities. If we divide the scale of the thermometer between the freezing and the boiling point, and between 0° and 80° of Reaumur, or 32° and 212° of Fahrenheit, into two equal parts, viz. from 0° to and from 40° to 80° of Reaumur, or fi on) 32° to 122°, and 122° to 212° of Fahrenheit, Deltic found that fluids expand more in the higher di vision than in the lower, and he obtained the following results : Ratio of expansion in the higher Fluids. division to that in the lower.
Quicksilver, 15 to 14.0 Olive oil, 15 13.4 Linseed oil, 15 13.4 Chamomile oil, 15 13,0 Water saturated with salt, 15 11,6 Alcohol, 15 10.9 Water, 15 4.7 The results for water contained in the last column of the Table, exhibit a very remarkable deviation from the general law observed by all the other fluids in the Table, as it possesses the remarkable and anomalous property of having its maximum density at 40°, and consequently of contracting when its temperature is either diminished or increased. This property of water, first observed by the Florentine Academicians in 1667, seems to have been first accurately examined by Dr Croune towards the end of the 17th century. Dr Hook ascribes the phe nomenon to the contraction of the vessel; but it. has been shewn, in the most satisfactory manner, by the experi ments of Mairan, De Luc, Dr Ifope, Mr Dalton, and Sir Charles Blagden, that the phenomenon is not the re sult of any illusion. Sir Charles Blagden has proved, that if water be kept free of all agitation so as to pre vent it from being frozen at 32°, and is cooled down to 21° or 22', the expansion still continues under an in creasing ratio. Mr Dalton fixed the maximum density of water at 421° of Fahrenheit; he. found that the ex pansion was scarcely perceptible from 41° to 44°, and that from 41° to 32° the expansion was about part of the whole expansion from 421° to 212°. He confirmed Deluc's observation, that the expansion for any number of degrees, either above or below 42,1°, was the same; but he afterwards thought that it was greatest below 42°. He also verified the observation of Sir Charles Blagden, that the expansion continues below 32°, and he succeeded in cooling it so fat' down in a tube, that the water had expanded and risen to the same height as if it had been heated to 75°. Hence, it must have been
cooled down to 10°. As soon as it was frozen, it sprung up to 128°.
These remarkable appearances were regarded by se veral philosophers as so anomalous, that they were pro bably occasioned by some hidden source of error in the experiments; and even Mr Dalton himself was induced to believe, that the anomaly exhibited by water was only apparent, and arose from the contraction of the vessel in which the experiment was made. In order to inves tigate this point, he repeated the experiments in earth en ware and metallic vessels, and compared the results with those made in glass. In all these trials, the point of maximum density, instead of being the same, varied with each substance, as will appear from the following Table : Notwithstanding the accurate experiments of M. Le fcvre Gineau, who proved the maximum density of water to be 39°.2, the results obtained by Mr Dalton threw a considerable degree of uncertainty over the subject, and chemists were divided in their opinions, till the subject was investigated by Dr Hope in a new manner, which was not liable to any error arising from the contraction of the containing vessels. It occurred to this learned chemist, that if water expanded by a diminution of its temperature below 40°, the expansion might be render ed visible by the change of place, which would happen in a column of water when cooling from to or when heating from 32° to 40°, as the water, which had a temperature of 32°, would necessarily rise in both cases towards the surface. He therefore filled a glass jar, 81 inches deep and 41 broad, with water at the tempera ture of 32°, and suspended in it two thermometers, one having its bulb about half an inch from the bottom, and the other about half an inch from the sot face. The vessel being placed in a room having the temperature of 60°, it was found that as the temperature rose to 38°, the thermometer at the surface was always one degree lower than the one at the bottom; a sufficient proof that the density of the water increased as its temperature rose above 32°. In order to obtain a still more striking re sult, Dr Hope employed a jar 21 inches high, and four in diameter, and he adjusted at the middle of its height a bason of tinned iron, by filling which, either with hot water, or a frigorific mixture, he could apply heat or cold to the middle portion of the fluid column. A ther mometer being fixed at the top and bottom of the jar as before, the jar was filled with water at 32°, and water of the temperature of 68° was poured into the bason of tinned iron. The lower thermometer rose from 32° to nearly 37° before the upper thermometer indicated the smallest increase of temperature, the warm current having moved downwards in consequence of the con traction or increase of density of the water. Dr Hope reversed this experiment by filling the bason with a fri gorific mixture, while the water in the jar had a tem perature of 39°. The upper thermometer descended to 33° in the course of two hours, while the lower one suffered almost no change, the current of cooled water ascending to the top of the jar. By thus varying his experiments in a very beautiful and skilful manner, Dr Hope decided the question of the maximum density of water, and is entitled to the honour of having been the first who really established the existence of this singular anomaly.