compounds are such as easily dissociate, and these elements form the ions. The majority of organic compounds containing these halogens are either incapable of being dissociated or dissociate at high temperatures only, and then only in some cases is the dissociation such that the halogens chlorine, bromine, and iodine appear as ions. In complete agreement with these facts, the chlorine, bromine, and iodine of such compounds either do not react at all with silver nitrate or only slightly; many other compounds of these and other elements behave in a similar manner. The chlorine of chlorates and perchlorates in which. the metals are the cations and the radicals CIO, and C10 the anions, does not in solution give any silver chloride, but forms first silver chlorate and perchlorate, from which the chloride can be produced by their decomposition. The sulphates with the anion SO, in the ordinary course of things give rise to sulphates with the same 'anion,' and many other salts and similar compounds behave in the same way. The compounds may decompose in other ways if the manner of the dissociation, and consequently the nature of the ions, be changed by heat or by the action of other bodies.

3

If by the study of a series of compounds capable of undergoing dissociation the ions contained in them are known with any degree of certainty, the majority of the reactions of these compounds may be predicted, for the combinations and changes always result from the union of the ions with those of the other active bodies. These facts afford an explanation of the principle known as the 'conservation of the type'—a rule which has been recognised for a considerable length of time, and which states that the bodies produced in any given reaction belong to the same types as those from which they are formed; in other words, they represent compounds analogous to those from which they are produced. An acid and a salt yield usually by their mutual reaction a salt and an acid, thus:

HCl + AgNO2 = AgCl + H‡ NO。.

A sulphate and a nitrate act upon one another to form by exchange of metals another sulphate and nitrate:

K2SO1 + Ba 2NO2 = Ba‡SO, + 2K ‡ NO,

Again, a hydroxide and a salt form another salt and hydroxide:

Ba(OH)2 + Mg(NO,), = Mg(OH)2 + Ba | 2NO,,

and so on. Changes of this kind take place in all probability even when the final result is different from what this rule would lead one to expect; the instability of one or other of the compounds formed may lead to the formation of new substances.. Thus copper iodide should be formed by the action of potassium iodide on copper sulphate:

2KI+ Cu SO, K2 SO, + Cu 12.

= 4

2

4

But cuprous iodide and iodine are formed by reason of the instability of cupric iodide :

2Cu | I2 = Cu2 | I2 + I2.

The action of potassium hydroxide on silver nitrate affords another example of a similar kind; the product should be silver hydroxide and potassium nitrate:

KOH+ AgNO2 = K‡ NO2 + Ag OH

but the silver hydroxide dissociates into silver oxide and water, thus:

2Ag OH = Ag2O + HOH.

Numerous other examples might be given in which the 'type" is not maintained.

For the commencement of the reaction it would appear to be sufficient if one of the reacting substances is capable of dissociation, although the other is entirely incapable of being dissociated. Thus benzene and many other hydrocarbons do not undergo dissociation at the ordinary temperature; yet when brought in contact with nitric acid, which is easily dissociated, the ions of the acid act energetically and the hydrocarbon is nitrated, thus:

RATES OF CHEMICAL CHANGE

189

When neither of the substances dissociate, then as a rule no reaction takes place, or a rise in temperature is needed to start the reaction, which aids or simply induces dissociation.

Free oxygen, O2, does not appear to be easily dissociated, for the oxidation of most bodies by its aid can only be effected at high temperatures. It is, however, dissociated by electricity, and ozone produced, which probably has the formula O,, and which itself is extremely easily dissociated, and, as is well known, acts as a powerful oxidising agent.

It is very remarkable that many substances, such as phosphorus, are less easily oxidised by pure oxygen than by air, in which it is mixed with a considerable proportion of nitrogen, or even by oxygen diluted by reduction of pressure. As phosphorus when slowly oxidised is luminous in the dark, these facts may be easily observed.

In pure oxygen at 20° C. and under a pressure of 760 mm. no light is given out; the phosphorus becomes gradually luminous as the pressure is reduced, and is very distinctly so when the pressure has fallen to 150 mm., or to about of an atmosphere. This remarkable phenomenon is probably in part due to the fact that the dissociation of the oxygen particles is favoured by the dilution.

§ 107. Rates of Chemical Change.-Every chemical action requires a certain length of time for its completion. The time required is, however, very different, varying with the nature of the reacting substances, with their amounts, and the conditions under which they are brought in contact. This subject has hitherto been thoroughly investigated only in comparatively few cases. In most cases the conditions are so complex that it is difficult to separate and estimate their various influences. Numerous observations show, however, that the rapidity of a chemical action is influenced by the quality, the quantity, the mass of the reacting bodies; also by their state of aggregation, as well as that of the products; further, by temperature and pressure, and by the presence of bodies taking no active part in the action, such as solvents and diluents, &c. The influence of mass, solubility, and volatility was submitted to a thorough investigation by Claude Louis Berthollet more than a hundred years ago; but only in recent years have his endeavours

obtained their just recognition, and the work been resumed and extended by the aid of more modern methods.

§ 108. Simple Decomposition. The simplest case is that in which, with several active substances present, one only of these undergoes a change. Such a case we have in the inversion of cane sugar under the influence of a dilute acid,' whereby it is converted into a mixture of dextrose (grape sugar) and levulose (fruit sugar), which rotates the plane of polarised light in the opposite direction to that in which it is rotated by cane-sugar solutions. This decomposition, represented by the equation

has been carefully investigated by Wilhelmy, and more recently by Ostwald. If a given quantity of sugar dissolved in water be mixed with a definite amount of an acid, capable of producing the inversion, then in every interval of time an amount of the sugar is inverted which is proportional to the amount of sugar still remaining unchanged. If A be the quantity of sugar originally present, and x the quantity of sugar inverted during the time, t, of mixing, then the amount de inverted in the infinitely short interval of time dt is proportional, the amount (A) remaining unaltered. In this manner we obtain the differential equation:

in which K represents a constant, or in this case at least an invariable quantity. By integration the following expression is obtained for the amount x inverted in the time t

-log, (A-x) = K. t + constant.

Reckoning t from the moment of mixing, when t = 0, soalso a becomes o; consequently the integration constant is

1 We may neglect the part played by the water in this reaction, as also.

that of the acid, the proportion of which remains unchanged.

in which e represents the base of natural system of logarithms, viz. e 2.71828.

The general correctness of these equations has been proved

to such an extent, that the quantity log

A

A-x

may be calculated from the value of x, determined experimentally, and divided by the corresponding values of t. The values of

have thus been found to be constant, as is required by theory. The quantity of sugar, therefore, inverted every moment is proportional to the amount of unaltered sugar present; and of this equal portions are always inverted in the same time.

The invariable quantity K is not absolutely constant, but varies with the nature as well as the amount of the acid used for inversion, and also with the proportion of sugar contained in a given volume of the solution, consequently with the concentration of the solution. An alteration in the mass of the acid is of much greater influence than a change in the quantity of the sugar. According to Ostwald's experiments, by increasing the sugar to ten times the amount whilst the hydrochloric acid remains constant, the value of K is only increased by half its original value. The increase in the proportion of acid, with the sugar remaining constant, produces a different effect, according to whether the acid is strong and easily dissociated, or weak and one which does not easily dissociate.

With the strong acids, such as nitric, hydrochloric, and hydrobromic acids, the inversion is approximately proportional to the acid, but decreases with the dilution to a somewhat greater extent than would correspond to the amount of the dilution. In the case of the weaker organic acids-formic, acetic, propionic, butyric, and succinic acids-the inversion takes