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about; a fall of temperature is less frequent. (Vid. Heat.) Some comtly binations are attended with a slight development of electricity. (Vid.

Electricity) lis

B. Time in which combination takes place. a. With the same two bodies, the rapidity of combination is increased by diminishing the quantity of a solid body in proportion to that of a liquid, or of a gaseous body in proportion to that of a liquid or a solid; by diminishing the cohesion of a solid by heating, or the elasticity of a gas by cooling and compression; and lastly, by comminuting a solid body and increasing the number of points of contact by agitation and friction.

A salt placed at the bottom of a quantity of water dissolves very slowly when at rest, because the heavy film of saline solution which is gradually forming remains above the salt and prevents its contact with the rest of the water: the same salt placed at the upper part of the water in a muslin bag or a filter dissolves very rapidly, because the solution as it is formed sinks to the bottom and allows the rest of the water to come in contact with the salt Ammoniacal gas directed upon the surface of water is very slowly absorbed, because the new compound (solution of ammonia) is lighter than water, and therefore forms a layer on the surface preventing the further contact of the water and gas; but if the gas is directed through a tube to the bottom of the water, the absorption

takes place very quickly. Hydrochloric acid gas, on the contrary, is ol

rapidly absorbed when directed on the surface of water because its solution in that liquid being heavier than water sinks to the bottom, and fresh water comes to the surface. A metal slowly cooled after fusion dissolves more rapidly in acids than it would if it had been hammered.

6. With different bodies, the rapidity of combination is greater, in proportion as their affinity is greater, their cohesion less, their difference of specific gravity smaller, their diffusion through one another more easy, and the fluidity of the new compound more complete. The combination of solids with fluids takes place niuch more slowly in consequence of the offreater cohesion of the former than that of fluids with fluids. Liquids f different specific gravity combine slowly when at rest and disposed one bove the other in layers, but quickly when shaken. Gases combine post quickly of all, because they diffuse themselves through each other pontaneously by adhesion (page 20). If the new compound is solid at

rdinary temperatures, it places itself between the new bodies and hinders Hat heir further combination ; e.g., zinc and sulphur.

C. Proportions in which bodies combine.
This forms the subject matter of Stoichiometry, or the Doctrine of
Chemical Proportions or Chemical Equivalents.

Ponderable bodies generally combine in definite proportions, which come out with greater distinctness as the affinity between the combining Vsubstances is stronger. With respect to the proportion in which two bodies combine, the following cases present themselves :

a. Two bodies may be mixed in any proportion whatever, and in no

rase does the mixlure present any peculiar properties : e.g., Water and non alcohol, alcohol and ether, ether and volatile oils.

b. One body A may take up any quantity whatever of another body B; but B, after having combined with a certain quantity of A, takes up no more of it.-B is then said to be saturated with A; the point of satu

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ration is attained ; a saturated combination or solution has been formed. 1 part of linseed-oil may be dissolved in 40, 1000 parts or any greater quantity of alcohol; but when 30 parts of alcohol have taken up 'l part of linseed-oil, any greater quantity remains undissolved and forms a milky liquid on agitation. 10 parts of common salt mixed with any quantity of water greater than 27 pts. will form a clear solution; but if common salt be added by small portions at a time to 27 parts of water, the first 10 pts. will dissolve completely, but any further quantity will remain undissolved. Similar relations are exhibited by water, alcohol and ether towards many salts and other solid bodies, and likewise towards gases. Water and ether agitated together in equal quantities separate when left at rest into two layers; the lower consists of water saturated with to ether, which may be replaced by any quantity of water whatever; the upper is ether holding a very small quantity of water in solution, and miscible with ether in all proportions.

In most of these cases the point of saturation varies with the temperature and external pressure. Most solid bodies dissolve more abundantly in fluids the more the temperature is raised, probably on account of diminished cohesion ; but as exceptions to this law we find that lime and some of its salts dissolve more abundantly in cold than iv warm water, and 10 pts. of common salt saturate 27 of water at all temperatures. Under increased pressure, liquids will dissolve larger quantities of gaseous bodies; moreover Perkins found (Ann. Ch. Phys. 23, 410, also Schw. 39, 361) that a milky mixture of alcohol with a larger quantity of bergamot oil than it can dissolve at the ordinary pressure of the air, became perfectly limpid from solution of the oil under a pressure of 1100 atmospheres.

c. Two bodies combine in one or a small number only of definite proportions, subject to no variation from temperature or outward pressure.

This law, the most important of all, holds good in all cases in which the more powerful affinities are concerned. It implies a mutual saturation of A with B, and B with A.

a. The two bodies A and B combine in one proportion only. In this case the same relative quantities ensure the saturation of A with B, ai of B with A.

Chlorine and hydrogen combine only in the proportion by weight 35.4:1; zinc and sulphur only as 32:2 : 16.

B. The two bodies combine in 2 definite proportions only: A is satı rated with B at one of these proportions, and B with A at the other.

Six parts of carbon combine with 8 pts. of oxygen to form carboni oxide, with 16 to form carbonic acid ; in carbonic oxide the oxygen saturated with carbon, in carbonic acid, the carbon is saturated within oxygen; for 8 oxygen will not take up more than 6 carbon, nor 6 carbon more than 16 oxygen; moreover between carbonic oxide and carboni acid there exists no intermediate combination containing more than 8 and less than 16 of oxygen united with 6 of carbon.

It is true that carbonio lewise oxide and carbonic acid gases may be mixed in any proportion whatever tween and thus a gas obtained in which 6 parts of carbon are present in connectioiubine with more than 8 and less than 16 of oxygen: but this is no chemica nice compound, but a mere mixture of gases, from which potash will remov the carbonic acid and leave the carbonic oxide behind. Similarly 35:44 chlorine with 101.4 mercury form corrosive sublimate, and with 202.81 mercury they form calomel: a substance, which for every 35.4 pts. ont i chlorine contained more than 101•4 and less than 202.8 mercury, would

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be a mixture of corrosive sublimate and calomel, from which alcohol would dissolve the former and leave the latter.

%. The two bodies combine in 3, 4, or 5 distinct proportions. In this case the combination of A with the largest quantity of B, gives one point of saturation, and that of B with the largest quantity of A, the other; between these two points of saturation are situated 1, 2, or 3 intermediate combinations. But here, as in the former case, there is no gradual transition from the minimum to the maximum, but a sudden passage from one characteristic combination to another.

48 molybdenum with 8 oxygen form molybdous oxide, with 16 oxygen, molybdic oxide, and with 24 ox, molybdic acid. 16 sulphur with 8 oxygen form hyposulphurous acid ; with 16 ox. sulphurous acid with 20 ox, hyposulphuric acid; and with 24 ox. sulphuric acid. 14 nitrogen with 8 oxygen form nitrous oxide; with 16, nitric oxide ; with 24, nitrous acid; with 32, peroxide of nitrogen or hyponitric acid; and with 40 of oxygen, nitric acid. Many intermediate compounds may be regarded as combinations of two saturated compounds in definite proportions. Thus, 103-8 lead form with 8 oxygen (the smallest possible quantity) the yellow, with 16 oxygen (the greatest) the brown oxide of lead : between these is found the red oxide, which contains 103.8 lead with 10% oxygen, or (multiplying by 3) 311•4 lead with 32 oxygen, and may be regarded as a compound of yellow oxide, 2 (103.8 lead + 8 oxygen) and brown oxide (103:8 lead + 16 oxygen). Moreover, the red oxide is decomposed by acetic acid, which dissolves out the yellow oxide, leaving the brown. Similarly, magnetic iron ore may be regarded as a compound of protoxide and peroxide of iron.

These more intimate and definitely proportioned compounds considered under c, are subject to the two following important laws.

First LAW RELATING TO THE SAME TWO BODIES. Suppose two bodies A and B to be capable of uniting in several proportions; then if the smallest quantity of B which can combine with a given quantity of A, be multiplied either by 11, or by l?, or by 2, or by 21, or by 3, 4, 5, or any bicher whole number, the products will give the other quantities of B, off nich may combine with the before mentioned given quantity of A.

erzelius.) Thus 6 carbon combine with 8 and 2.8 oxygen; 16 sulphur th 8, 2.8, 2.8 and 3.8 oxygen; 14 nitrogen with 8, 2.8, 3.8, 4.8 and

8 oxygen; 103.8 lead with 8, 11.8 and 2.8 oxygen. This law affords a Weck on the results of experiment: thus if experiment had indicated that

carbon unite with 8 oxygen to form carbonic oxide, and with 15.5 ygen to form carbonic acid, it might have been suspected, since 15.5 is

t one of the multiples of 8 by 11, 12, 2, 2, 3 ...., that the comsition either of carbonic oxide, or of carbonic acid, or of both, had not en correctly determined by experiment.

Second LAW, RELATING TO DIFFERENT BODIES. From the proportion which A combines with B on the one hand and with C on the other, may kewise be calculated the proportion in which combination may take place

tween B and C. If, for example, experiment shows that 1 part of A W mbines with 3 parts of B and with 8 parts of C, then B and C must mbine either in the proportion of 3 B to 8 C, or in some other proporbn in which the 3 B are multiplied by one of the following numbers, 14,

2, 2, 1, 3, 4, 5, &c., or the 8 C by one of the same numbers, or the 3 B one and the 8 C by another number of the same series. The same law Ids good in the case of any number of bodies, so that if 1 A will combine ith 3 B, 8 C, 10 D, 12 E, &c., then B will combine with C, D and

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E either in the proportion of 3 : 8, 3 : 10, 3: 12, or else in proportions obtained by multiplying one or each of these numbers by some factor taken from the above-mentioned series. Taking sulphur for the body denoted by A, we find that 16 sulphur with 103.8 lead form sulphuret of lead; with 24 oxygen, sulphuric acid; with 1 hydrogen, hydrosulphuric acid; with 3 carbon, bisulphuret of carbon; and with 13.6 iron, ironpyrites. Now 103.8 lead combine, not with 24 oxygen, but with 8 oxy. gen to form yellow oxide of lead; the 103.8 lead must therefore be multiplied by 3 to give the proportion in which lead and oxygen are combined in the yellow oxide. Oxygen and hydrogen combine, not in the ratio of 24:1, but of 8 : 1 or 24:3; the 1 hydrogen must therefore be multiplied by 3.–24 oxygen combine not with 3 carbon, but with 18 carbon in carbonic oxide: the 3 carbon has therefore to be multiplied by 6.—24 oxygen combine not with 13:6, but with 81:6 iron to form protoxide of iron, the latter number being equal to 6 times 13.6.

From these two laws it follows that to every simple substance there belongs a certain relative weight, according to which it combines with given relative weights of other simple substances, only that in many cases this relative weight requires to be multiplied by some number of the series already mentioned. This determinate relative weight of a body is by those who admit the atomic theory, called the Atomic Weight; by those, on the other hand, who either reject this theory altogether, or regard it as not sufficiently established,—the Combining Weight, Chemical Weight, Chemical Equivalent, Combining Proportion, Équivalent Proportion, or Equivalent Number, Stoichiometrical Proportion, or Stoichiometrical Number.

The origin of these two laws is most satisfactorily explained by the atomic theory (which we shall bereafter develop more completely), according to which every simple substance consists of very small invisible particles called atoms, these atoms being of uniform weight and volume in each individual substance, while the atoms of different substances may be of different weight and volume. It is assumed that in chemical combination the heterogeneous atoms lay themselves close together, and so form compound atoms, which, when collected into a mass, constitute the new compound: further, that the atoms have a tendency to unite in silber ple numerical proportions: e.g., 1 atom of A with 1, 2, 3, or more atoms of B; or 2 atoms of A with 3 or 5 atoms of B, or 3 atoms of A with 11.4 atoms of B. It is only in organic compounds that more complex prop tions occur.

If we now examine the preceding examples according to this vie we may assume that the absolute weight of an atom of carbon is be that of an atom of oxygen = 6:8, and that 1 At. carbon combines either with 1 At. oxygen to form carbonic oxide, or with 2 At. oxygen to form carbonic acid. It will then follow that in carbonic oxide eve 1 6 parts by weight of carbon are combined with 8 parts of oxyge and in carbonic acid, every 6 parts of carbon with 16 of oxygen. W may also with great probability assume that the atomic weight of sulph is twice as high as that of oxygen, and therefore 16, if that of oxyg be taken = 8. Since now, according to experiment, 16 parts of sulphur can combine with 8, 16, 20 or 24 parts of oxygen, it follows that 1 A sulphur combines with 1, 2, 2} and 3 At. oxygen; and since h: 1 atoms are inadmissible, the combination of 16 sulphur with 20 oxygen i (= 32 : 40) may be regarded as consisting of 2 At. sulphur combined with 5 At. oxygen.--If the atomic weight of nitrogen be taken equis to 14, it will be found that 1 At. nitrogen can combine with 1, 2, 3, 4 1

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5 At. oxygen.-The atomic weight of lead being assumed equal to 103.8, the yellow oxide of lead must be supposed to contain 1 At. lead with 1 At. oxygen, the brown oxide 1 lead with 2 oxygen, and the red oxide 3 lead with 4 oxygen. Thus it is explained why the smallest quantity of B with which A will combine, must be multiplied by 13, 14, 2, 2, 3, 4, and other whole numbers, to find the other proportions. For 1 At. of A takes up sometimes 1, sometimes 2, sometimes more atoms of B, and hence the augmentation proceeds according to whole numbers: or 2 At. of A unite with 3 or 5 of B: hence arises multiplication by 11 or 2; or 3 At. A with 4 At. B, whence multiplication by 1j.

With respect to the second law, we have the following. If experiment shows that 1 part of A combines with 3 parts of B, and with 8 parts of C, then on the supposition that in these compounds 1 At. of A exists in connexion with 1 At. of B, and with 1 At. of C, it follows that the atomic weights of A, B, C, = 1:3:8; since, however, in these combinations, 1 At. of A may be combined with 2, 3, 4, or any greater number of atoms of B or C, or 2 At. of A with 3 or 5 At. of B or C, or 3 At. of A with 4 At. of B or C, &c. &c.; or finally, since B and C must not be supposed always to combine in equal numbers of atoms,-it will often be necessary, in determining the equivalent numbers according to which B and C combine, to multiply the 3 parts of B or the 8 parts of C, or both of them, by one of the numbers in the series 1$, 19, 2, 2, 3, 4, 5....

Atomic Weights of Simple Substances. Of the absolute weight of atoms we can know nothing, excepting that they must be extremely small. It is only the relative weight of the atoms of different bodies that can be determined with any degree of prohability from the proportions by weight according to which the bodies combine. This relative atomic weight may be discovered by assuming arbitrarily a particular number to represent the atomic weight of any one substance, and then determining the atomic weights of the other bodies according to the proportions by weight in which they combine. Some chemists, following Dalton, put the atomic weight of hydrogen, because it s the smallest, = 1; but the greater number agree with Berzelius in wsuming oxygen = 100. The former method is to be preferred, because it gives simpler numbers and thereby favours the retention of them in the memory and facilitates calculation. The atomic weights of many other bedies appear to be simple multiples of that of hydrogen, and consequently en hydrogen is taken = 1, they are represented by whole numbers; 9. carbon 6, oxygen 8, nitrogen 14, sulphur 16, &c. These numbers cewise contain one digit less than the others: thus, oxygen 8 instead of 00, carbon 6 instead of 75, nitrogen 14 instead of 175, sulphur 16 inead of 200, &c. This simplicity, moreover, obtains particularly with ward to those substances of which the innumerable organic compounds

formed. . In favour of the other method it is indeed advanced that gen is of all the elementary bodies that which forms the widest range kodi

compounds, and consequently that calculation must be facilitated boden that element is expressed by such round numbers as 100, 200, 300,

, 500, &c., but the numbers 8, 16, 24, 32, 40, are more quickly knitten, and encumber the addition so much the less as the other eledints of the compound are at the same time expressed by simpler num

**. Moreover hydrogen occurs in a great number of inorganic compoljade, especially in the form of water; and in organic compounds it

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