Volume 1

Watts' dictionary of chemistry / revised and entirely rewritten by H. Forster Morley and M.M. Pattison Muir ; assisted by eminent contributors.

  • Watts, Henry, 1815-1884.
Date:
1888-1894
Catalogue details

Licence: Public Domain Mark

Credit: Watts' dictionary of chemistry / revised and entirely rewritten by H. Forster Morley and M.M. Pattison Muir ; assisted by eminent contributors. Source: Wellcome Collection.

    777/800 (page 751)
    Konowalow (B. 17, 1360 ; 18, 2808), when determining the vapour density of tertiary amylacetate, found that the dissociation-pheno- mena exhibited by the vapour of this body are influenced in a sti iking manner by the presence of many finely divided substances such as silica, magnesia, calcium sulphate, &c. The effects varied with the chemical, as well as with the physical, characters of the substances placed in contact with the vapour. To such a slight extent have these contact actions been studied that it seems as yet impossible to interpret them in the same way as those chemical actions which are here termed cyclical. Faraday’s theory for the action of platinum in bringing about the union of oxygen and hydrogen by a mere con- densation of the gases would seem to be the most reasonable explanation in the face of the facts at present known. Examples of catalytic actions are known among liquids, which may be explained on the theory of cyclical change; such are the evolu- tion of oxygen from a solution of a hypochlorite when warmed with cobaltous or manganic oxide, and the decomposition of hydrogen per- oxide by manganese dioxide, finely divided platinum or silver, or by oxide of silver. The last case is remarkable, for here oxygen is evolved both from the silver oxide and from the hydrogen peroxide; to explain this, and a number of analogous reactions, Brodie (T. 140, 759) assumed that atoms of the same body may have an attraction for each other or be in a state of polarisation. Brodie expressed the H— — reaction of Ag20 with H202 thus, H._,00 + Ag20 = —b H20 + 00 + Ag2. The change of cane sugar and water into glucose, and of ethereal salts and water into acids and alcohols, in the presence of acids which themselves remain unchanged, are other instances of catalytic action probably of a cyclical character (see Affinity, pp. 71 et seq.). If potassic chlorate is heated alone it melts at about 345°C. and on increasing the temperature to about 370°C. oxygen begins to be produced. Many substances in a fine state of division when mixed with this salt cause an evolution of oxygen much below the temperature at which the chlor- ate decomposes when heated alone, and without the salt entering into a state of fusion. The substances which facilitate this decomposition do not themselves appear to undergo any chemi- cal change. It is probable that the theory pro- pounded by Mercer (B. A. 1842. 32) to explain analogous chemical changes is the true one, viz. that the material which facilitates the decompo- sition has a tendency to pass into a higher state of oxidation, and that an unstable compound is formed, but is decomposed at the temperature of the experiment. On this hypothesis the potas- sium chlorate is regarded as being decomposed at the lower temperature by the double effect of heat and the affinity of the contact substance— as Mn02—for the oxygen of the chlorate. Heated by itself, potassium chlorate passes through an intermediate stage in its decomposition with the formation of perchlorate ; this intermediate stage is represented according to some chemists by the equation 2KC!103 = KC10, + KC1 + 02, but it appears to be more correctly expressed by the equation 10KC103 = 6KC10., + 4KC1-i-302 (Teed, G. N. 52, 248). If, however, manganese dioxide is heated with the chlorate, no perchlorate is formed; this fact may be explained and used as an argument in favour of Mercer’s view, by sup- posing that KC103 when decomposed by itself forms KC1 and 03, and that the nascent ozone oxidises a second molecule of chlorate to per- chlorate, whereas in the presence of Mn02 a higher but very unstable oxide of manganese is formed, and is almost simultaneously decom- posed. The oxides which most markedly facilitate the decomposition of potassium chlorate are as a rule those the metals of which form several oxides. It is a well-known fact that the oxygen prepared from KC103, either by heating the salt alone or mixed with Mn02, liberates iodine from an alkaline iodide ; this is usually considered to be due to a trace of free chlorine; it may, how- ever, be occasioned by a little ozone that escapes decomposition by the MnO., or the KC103. Contact chemical action, whatever be its true cause, plays a highly important part in several industrial operations, as the inversion of cane sugar, the conversion of starchy matters into glucose, the decolorisation of sugar solutions by charcoal, and probably in the purification of waters by filtration through porous media. For- merly the great industrial processes of fermenta- tion in the formation of alcoholic liquors were referred to this cause, but it seems now certain that such changes are phenomena connected with organic life and not with those of unorgan- ised matter. (For a theoretical consideration of catalysis see Mendel6eff, B. 19, 456.) A consideration of the facts that are known relating to chemical change shows that in the study of the subject not only must the kind or quality, and the mass, of the reacting matter, be taken into account, but attention must also be given to the intrinsic forces that come into play, as well as to the notion of molecular or atomic motions. It is not at present so much the relative values of these forces that one desires to know, in whatever way they may be measured, as the circumstances under which the forces act, or are modified in their action. It has been seen that all atomic structures are possessed of rela- tive degrees of stability, as is shown when sub- mitted to the action of physical agencies, or when they play an active part in chemical sys- tems, this stability being due to the interaction of the affinities holding the structure together. These forces or affinities offer different degrees of resistance to the action of different agencies, and it would seem to be only by the study of such influences that a rational conception of the nature of chemical action will be arrived at. That the ultimate constituents of matter— the atoms or molecules—are in continual motion, the interdiffusion of gases, and of salts in solu- tion, seems to prove; and the fact that, in a chemical system undergoing change, such change is more or less gradual, taking fractions of seconds or whole years to be accomplished, seems to offer conclusive proof that the atomic consti- tuents are in a continual motion of interdiffusion. But whether or not the change going on in a chemical system is brought about by simple