M O L M O L 623 limiting value for the internal kinetic energy J of a molecule of AS. If a molecule of AB, by encounters with other molecules or with the wall of the vessel containing the gas, acquires a greater amount of internal kinetic energy than this limit, it at once breaks up into A and B, so that in the gaseous mixture there are no molecules of AB having more internal kinetic energy than the limit. Further, if two molecules, one of A and one of B, meet one another with such a velocity and with such an amount of internal kinetic energy that together the internal kinetic energy is less than the limit, they will unite to form a molecule of AB. Thus the molecules with great internal kinetic energy will be separate molecules of A and B ; those with small internal kinetic energy will mostly be united as AB. This hypothesis has been to a considerable extent worked out and applied by Pfaundler and by Naumann, and the deductions from it agree fairly well with the results of experiment ; but in some points the theory has not been fully developed, and in some it does not seem altogether to accord with observed facts. Some of these difficulties have been mentioned above. We know enough of the nature of dissociation to see that it belongs to the class of balanced chemical actions, in which a chemical change is reversible, and equili brium is kept up, with constant external conditions, by the two opposite chemical changes taking place to an equal extent in a given time. We can see that all such cases are explicable by the statistical method, but we cannot apply this method mathematically until we know more of the intimate nature of the molecules and of the way in which they act upon one another. In this discussion of dissocia tion we have looked specially at the cases in which A, B, and AB are all gaseous, because it was the question of anomalous vapour densities which led us to treat of the subject. Dissociation also occurs where one or two of the substances are solid or liquid. We now see with what restrictions the method of vapour density is applicable to the determination of molecular weight, and we can understand more fully the example given in the article CHEMISTRY, vol. v. p. 469. It is there shown that acetic acid vapour does not conform to the laws of Boyle and Charles until the temperature is raised to about 250, at the ordinary barometric pressure. At and above that temperature the vapour density corresponds to the formula C. 2 H 4 2 . At lower temperatures the density corresponds to a higher molecular weight. Now Playfair and Wanklyn determined the vapour density at much lower temperatures than the ordinary boil ing point of acetic acid, by greatly diminishing the pressure of the acetic acid vapour. This they accomplished by mixing it with a large quantity of hydrogen, so that the pressure due to acetic acid vapour formed only a small fraction of the total pressure. The vapour density of acetic acid at the low temperatures at which they worked was found to correspond very nearly with the formula C 4 H 8 4 , and, by comparing this result with what has been said (p. 6 20) of the chemical evidence as to the molecular weight of acetic acid, we may reasonably conclude that the molecule of acetic acid at low temperatures is C 4 H 8 4 , and that as the temperature is raised it undergoes dissociation, each molecule dividing into two of C 2 H 4 0. 2 . This is then a case where A and B are equal, and A A divides into A + A. Another instance of the same kind is probably to be found in peroxide of nitrogen (CHEMISTRY, p. 513), where N 2 4 divides into NO.j + NCXj. Similarly, sulphur vapour has, at temperatures below 500 C. , a density corresponding to the formula S 6 . This dissociates as the temperature rises until, about 1000 C., the density corresponds to the formula S 2 (CHEMISTRY, p. 498). We have now seen that chemistry receives great assistance in the determination of molecular weight from physics, but this assistance is almost entirely confined to the case of gases, or of substances which can be volatilized. The phenomena of the diffusion of liquids show us that there also there are independently moving particles ; but the laws of liquid-diffusion have not been sufficiently gener alized to give us much help in the determination of the relative masses of these particles. In liquids it is probable that the par ticles are very near each other, and that their shape and their mutual action, as well as their mass and the temperature, deter mine their rate of motion. In solids we have no independently travelling particles, and it is perhaps scarcely correct to speak of a molecular structure of solids at all. Solids are no doubt composed of atoms, and those atoms are evidently arranged in what may be called a tactical order. When the solid is fused or dissolved or volatilized, it breaks into molecules, each repetition of the pattern, if we may use the expres sion, being ready to become an independent thing under favourable circumstances. But, while these potential molecules of solids can not perhaps be properly called molecules in a physical sense, 2 for chemical purposes we may call them so, for they are the smallest portions of the substance which fully represent it chemically, and, as we have seen, this is the chemical molecule, the quantity which should be represented by the formula. (A. C. B. ) MOLESKIN is a stout heavy cotton fabric of leathery consistence woven as a satin twill on a strong warp. It is finished generally either as a bleached white or as a slaty drab colour, but occasionally it is printed in imitation of tweed patterns. Being an exceedingly durable and econo mical texture, it was formerly much more worn by working- men, especially outdoor labourers, than is now the case. It is also used for gun-cases, carriage-covers, and several pur poses in which a fabric capable of resisting rough usage is desirable. MOLESWORTH, SIR WILLIAM (1810-1855), the eighth baronet, was born in London, 23d May 1810, and succeeded to the extensive family estates in Devon and Cornwall in 1823. On the passing of the Reform Act of 1832 he was re turned to parliament, though only twenty-two years old, for the eastern division of the county of Cornwall, to support the ministry of Lord Grey. For some time he took little part in the debates of the House of Commons; but in April 1835 he founded, in conjunction with Mr. Roebuck, the London Revieiv, as an organ of the politicians known to the world as " Philosophic Radicals." After the publication of two volumes he purchased the Westminster Review, and for some time the united magazines were edited by him and J. S. Mill. From 1837 to 1841 Sir William Molesworth sat for the borough of Leeds, and during those years acquired con siderable influence in the House of Commons by his speeches and by his tact in presiding over the select committee on Transportation. From 1841 to 1845 he remained in private life, occupying his leisure time in editing the works in Latin and English of Thomas Hobbes of Malmesbury, a recreation which cost him no less than 6000. In the latter year he 1 By internal kinetic energy is meant the kinetic energy of motion of the parts of the molecule relatively to one another, in contradis tinction to the kinetic energy of motion of the molecule as a whole. was returned for the borough of Southwark, and retained that seat until his death. On his return to parliament he devoted special attention to the condition of the colonies, and delivered many speeches in favour of a reduction in colonial expenditure and on their better administration. His arguments on these questions changed the opinions of the members of the House of Commons ; and the criticisms of the daily press, aided by the printing of his speeches, led to the gradual acceptance of his views by the electors at large. It was not, however, until many years afterwards that he was allowed full opportunity for working out the difficult problems connected with the government of Great Britain. Office was conferred upon him in December 1852 by Lord Aberdeen, but it was the minor post of directing the public improvements and crown lands of his own country, and the chief work by which his name was brought into prominence at this time was the construction of the new Westminster Bridge. At last, in July 1855, he was called to preside over the Colonial Office, but unfortunately its duties were no sooner entrusted to his care than he was cut off by death (22d October 1855), to the universal regret of his countrymen, for he had lived down the animosities of his youth, and had attracted to himself the sympathies of all thoughtful men. The influence which his views had acquired, and still retain, may be judged from the fact that in 1878 the delegates of the Transvaal Government put forward, as the chief argument for the withdrawal of the English from the Transvaal, the substance of his speech on the abandonment of the Orange River Territory in 1854. A full pedigree of the Molesworth family is printed in Sir John Maclean s Trigg Minor, vol. i. ; the titles of his speeches and works 2 It may be urged that the cleavage of crystals indicates that they possess a molecular structure, but a tactical or pattern-like arrangement of atoms may easily be supposed to present planes of easier separation,
without the assumption of really independent molecules.