36 



CHEMISTRY. 



Eln*nu onence 32, and the boiling point 212. This is the 

 .' thermometer used in Britain. It is the one always 



?^ n **^T' used in this article, except when some other is ex- 

 pressly mentioned. In the thermometer of Delisle, 

 the space between the two points is divided into 150, 

 but the graduation begins at {he boiling point, which 

 is marked 0. The freezing point is marked 150. 



As mercury does not expand equably, the thermo- 

 meter does not give us an exact measure of the in- 

 crease of heat. Mr Dalton has endeavoured to prove 

 that mercury expands as the square of the tempera- 

 ture, reckoning from its freezing point. This opi- 

 nion has induced him to construct a new thermome- 

 ter, graduated according to that principle. If this 

 opinion be correct, the common degrees are too large 

 near the bottom of the scale, and too small towards 

 the upper part of it. 122, or half way between 

 frcc/.ing and boiling, corresponds according to the 

 new graduation with 1 10 of the old. 



KtcepiioDt The exceptions to expansion by heat are of two 



rp rxpan- kj n ds. 1. Those liquids which have a maximum of 

 density corresponding with a certain temperature, 

 and which of consequence expand whether they be 

 heated or cooled beyond that temperature. 



2. Certain liquids which become solid by cooling, 

 and expand during the solidification. 



n Water. Water is the only liquid at present known belong- 

 ing to the first class. Its greatest density is at the 

 temperature of 4O, or a little below it. If it be 

 heated above that temperature it expands, and it ex- 

 pands equally if it be cooled below it. A vast num- 

 ber of experiments have been made upon this point, 

 and there appears no doubt of the matter of fact. 

 Mr Dalton has lately endeavoured to show, that 36 

 is the degree at which the density of water is a maxi- 

 mum, and his observations appear satisfactory. No 

 satisfactory explanation of the cause of this singular 

 anomaly has yet been offered. 



r. Other The second class of bodies is numerous. Water 



fcxjies. expands with great force when it freezes, and is con- 

 verted into ice. The specific gravity of ice is 

 0.92, that of water at 6O being 1 .00. Hence, ice 

 is lighter than even boiling hot water. It always, 

 therefore, swims on the surface of the water. A si- 

 milar expansion is observable during the crystalliza- 

 tion of most of the salts. Among the metals there 

 are three which expand in the act of congealing ; 

 these are cast-iron, bismuth, and antimony. All the 

 rest seem to contract instead of expanding. Sulphur 

 appears also to expand when it congeals. This ex- 

 pansion in these bodies must be ascribed to a new ar- 

 rangement, which their integrant particles assume. 

 It would lead one to suppose a kind of polarity in 

 these integrant particles, otherwise it is difficult to 

 conceive why they tend to expansion with so much 

 force. Honey, oils, and most metals contract when 

 they become solid. Sulphuric acid also appears to 

 contract. 



2. Changes in the Slate of Bodies. 



Changes in All substances in nature, as far as we know them, 

 Hi* uate of occur in one or other of the three states, that of so- 

 I** 1 "**- lids, of liquids, and of elastic fluids. In a vast num- 

 ber of cases, the same substance is capable of assu- 

 ming each of these states in succession. Thus tul- 



phur is usually solid, but at 218 it becomes a li- Elrmfnt* 

 quid, and at 570 it boils, and is converted into an of 

 elastic fluid. Water ii a liquid, but at 32 it freezes ^] 

 into a solid, and at 212 it boils into an elastic fluid. ""V" 



All solids (a very few excepted) may be convert- 

 ed into liquids, by heating them sufficiently ; and al- 

 most all liquids, by cooling them sufficiently, may be 

 converted into solid*. The law then is, that solids 

 by heat are converted into liquids and elastic fluids ; 

 while elastic fluids and liquids by cold are brought 

 into the state of solids. 



1. When solids are converted into liquids, the Freezing 

 change in some cases takes place at once, without any explained, 

 perceptible interval between solidity and liquidity. 

 In other cases, the solid passes slowly through all 

 the intermediate degrees of softness, till at last it be- 

 comes a complete liquid. The melting of ice is an 

 example of the first kind, that of wax and tallow of 

 the second. This change takes place at a particular 

 temperature, which is easily ascertained in the first 

 class, but not so easily in the second. If the sub- 

 stance at the usual temperature of the atmosphere be 

 liquid, this point is called the freezing point ; but if 

 it be usually solid, it is called the welling point. 

 Thus 32 is the freezing point of water, and 4-76 

 the melting point of bismuth. 



Though 32 be the freezing point of water, it may Freezing 

 be cooled down considerably below that point with- of water, 

 out freezing. In thermometer tubes, we have cooled 

 it down to 7, and in a wine glass to 20. When 

 agitated or touched with a bit of ice, it freezes very 

 suddenly. 



The freezing point of water is lowered by dissol- 

 ving different salts in it. Thus water saturated with 

 common salt freezes at 4, with sal ammoniac at 8, 

 with Rochelle salt at 21, and with nitre at 26. 

 When the proportion of the same salt dissolved in 

 water is varied, it follows from the experiments of 

 Sir Charles Blagden, that the freezing point is al- 

 ways proportional to the quantity of the salt. 



The nitric and sulphunc acids vary remarkably in of aci<!s 

 their freezing points, according to circumstances. 

 When much diluted with water, the weakest part 

 freezes, while a strong portion remains liquid. When 

 very much diluted, the whole freezes, and the free- 

 zing point is lower according to the proportion of 

 acid present. The strong acids themselves undergo 

 congelation, and each has a particular strength at 

 which its congelation is the easiest. If it be stronger 

 or weaker, more cold is necessary to congeal it. 

 Sulphuric acid of the specific gravity 1.780 freezes 

 at 46. But if it be diluted with a little water, it 

 requires a cold of 45 ; the strongest sulphuric acid 

 freezes at 1. The strongest nitric acid freezes at 

 4-5.5. When considerably weaker it freezes at 

 2, and when still weaker at 27.7. 



We are indebted to Dr Black for the first satis- 

 factory explanation of the change of solids into li- 

 quids by heat. According to him, solids, in order 

 to liquify, combine with a quantity of heat which 

 enters into them, and remains, in them without increa- 

 sing their temperature. Hence he called it latent Latent 

 heal. Liquids congeal by giving out this latent heat. heat. 

 This opinion is established oy simple but satisfactory 

 experiments, and he ascertained that the latent heat 



