CHEMISTRY. 



C'hemniry- 



Pcrnunon 

 oecMions 



IKMI. 



K.lcmenu be heated red hot by striking it with a hammer, aud 

 the parks emitted by flint and steel are well known. 



This evolution of heat appears to be the come- 

 qiience of the permanent or temporary condensation 

 of the bodies struck. Iron and most metals become 

 specifically heavier when hammered. Now conden- 

 sation always evolves heat. When air is condensed, 

 it gives oul a considerable quantity of heat, sufficient 

 to set tire to tinder. When muriatic acid gas is pas- 

 sed through water, it is condensed, and the water be- 

 comes hot. On the other hand, when air is rarefied, 

 it becomes suddenly much colder. 



It is not difficult to see why condensation evolves 

 heat. The particles being forced nearer each other, 

 the repulsive force of the neat is increased, and a por- 

 lion in consequence is driven off. The specific calo- 

 ric of bodies is diminished by condensation. Now the 

 specific caloric can scarcely be conceived to diminish 

 without the body giving out heat. 



A part of the heat which follows percussion, is of- 

 ten owing to another cause. By the percussion, the 

 heat of the body is raised so high, that combustion 

 commences, and this occasions a still farther increase 

 of the heat. It is in this way that sparks are produ- 

 ced when flint and steel are struck. The sparks are 

 small pieces of the steel which have taken fire and 

 melted during their passage through the air. 



4. Friction. 



Friction. Heat is not only evolved by percussion, but also 



by friction. And not only by the friction of hard 

 bodies, but even of soft bodies, as when the hand is 

 rubbed against the sleeve of the coat. No heat has 

 ever been observed from the friction of liquids. 



The heat evolved by friction seems to be owing to 

 the same cause as that by percussion ; namely, a con- 

 densation of the substances rubbed. This condensa- 

 tion is, in some cases, permanent ; but when the bo- 

 dies rubbed are soft, it can only be momentary. 



The heat evolved by friction is sometimes very 

 considerable. Thus Count Rumford boiled water 

 by the heat evolved by rubbing a steel borer against 

 a cylinder of gun-metal. Probably in this case the 

 density of the metal was a little increased. A very 

 small increase would account for the whole heat 

 evolved. 



5. Mixture. 



In a great number of cases, a change of tempera- 

 ture takes place when bodies combine chemically with 

 each other. Sometimes the compound becomes cold- 

 er than before, and sometimes hotter. 



When Glauber's salt in crystals pounded is dissol- 

 ved in water, a considerable degree of cold is produ- 

 ced ; and the cold is still more intense, if the salt be 

 dissolved in muriatic acid. If muriate of lime, in pow- 

 der, and dry snow be mixed together, so great a de- 

 gree of cold is produced, that mercury may be frozen 

 if it be surrounded by such a mixture. Potash and 

 snow produce an equally great cold. When nitric 

 acid or sulphuric acid is poured upon snow, the snow 

 dissolves, and an intense cold is produced. 



On the other hand, when sulphuric acid and water 

 Are mixed, so great a heat is evolved, that the liquid 



Mixture. 



Change of 

 tempera- 

 ture by 

 mixture 

 rwing to 

 water. 



is considerably hotter than boiling water. Heat also 

 is produced, when nitric ncid and water, or water and 

 ale. hoi, are mixed together. Heat also is produced 

 if Glauber salt, in a state of efflorescence, is dissolved 

 in witer. at is produced by dissolving 



quick-lime in sulphuric acid. 



In most of these cases of change of temperature, 

 water is either one of the substances combined, or it 

 forms an essential constituent of one of them. The 

 heat or the cold produced, depends often on this con- 

 stituent. Thus Glauber's salt, containing its water 

 of crystallization, produces cold when dissolved; 

 while the same salt, deprived of its water of crystalli- 

 zation, produces heat. 



If the new compound be more fluid than the two 

 constituents of it, the temperature sinks-, if it be less 

 fluid, the temperature rises. Thus, when snow and 

 common salt are mixed, they gradually melt and as- 

 sume the form of a liquid, and the temperature sinks 

 to zero. Solid water cannot become liquid, without 

 combining with a quantity of heat ; and the same rule 

 applies to all solid bodies which become liquid. Hence 

 the cold evolved iu these cases. The water of cry- 

 stallization in Glauber's salt is solid : it becomes liquid 

 when the salt is dissolved. Hence the cold produ- 

 ced. When the same salt, free from its water of 

 crystallization, is thrown into water, it first combines 

 willi a portion of thejwater, and renders it solid. Hence 

 the heat evolved. Dr Black's doctrine of latent heat 

 affords a satisfactory explanation of these phenomena. 



When the density of two liquids united is greater Or to the 

 than the mean, heat is evolved, because the specific specific 

 caloric of the new compound is less than that of the hcal 

 constituents. This was first observed by Dr Irvine, 

 and it accounts for the heat evolved when water is 

 mixed with sulphuric acid, nitric acid, or alcohol. 



Thus it appears, that the changes of temperature 

 produced by mixture, are either occasioned by the 

 change of stale which the water undergoes, or by a 

 diminution of specific caloric, in consequence of the 

 new combination. 



BOOK II. 

 OF COMPOUND BODIES. 



COMPOUND bodies are substances composed of twa of com . 

 or more simple substances uniled together. They pound bo 

 amount to several thousands ; but the present state d ' et - 

 of the science docs not permit us to give an account 

 of them all under their proper heads. 



Compound bodies are of two kinds. Some are T WO clan- 

 formed by the combination of two or more simple of com. 

 substances with each other ; while others are formed 

 by the union of two or more compound bodies with 

 each other. To the first class belong phosphoric 

 acid, composed of phosphorus and oxygen ; and am- 

 muniii, composed of azote and hydrogen. To the 

 second class belongs phosphate of ammonia, composed 

 of phosphoric acid and ammonia. The first of these 

 sets we shall call primary eompOMKU ; to the second 

 we shall give the name of secondary compovndt. 



