CHEMISTRY. 



103 



the same physiological reactions. To this rule 

 he has found only two exceptions nitrogen 

 and potassium. When the same element forms 

 two classes of salts belonging to distinct iso- 

 morpbous groups, the phy&iological action of 

 the salts in each class is different, but agrees 

 with that of other substances of the group 

 to which it belongs. In substances of the 

 same isomorphous group the physiological ac- 

 tion increases in intensity with the atomic 

 weights of the elements present. A speciali- 

 zation of the reactions tor each isomorphous 

 group is also observed. Thus the compounds 

 of the alkali-metals act upon the internal 

 ganglia of the heart ; the compounds of phos- 

 phorous, arsenic, and antimony act upon the 

 splanchnic ganglia ; the substances of the mag- 

 nesian group act upon the center for vomiting; 

 and in an analogous manner for other groups, 

 it is by modifying the action of some nerve- 

 center that their physiological action is shown. 

 Isomorphous substances are found to give rise 

 to the same physiological reactions when they 

 have homologous spectra, but when in an 

 isomorphous group there are found elements 

 whose spectra do not resemble the spectra of 

 other elements of the group, these elements 

 with anomalous spectra give rise also to anoma- 

 lous physiological reactions. The exceptional 

 character of the action of nitrogen and potas- 

 s slum is indicated by the exceptional character 

 of their spectra, which are homologous with 

 no other; and nitrogen, with a very complex 

 spectrum, reveals its presence by its action on 

 the nerve-centers, and is sharply distinguished 

 from all the other elements of the same group. 

 M. Konovaloff, in treating of contact-action 

 phenomena, has thrown out the suggestion 

 that the bombardment of the molecules on the 

 solid matter might cause a transformation in 

 part of their kinetic energy into intra-molecu- 

 lar work, tending to bring about dissociation of 

 the molecules. A. Irving has investigated that 

 class of results which are observed when 

 under the influence of heated spongy platinum, 

 or platinized asbestos, or other porous or finely- 

 divided bodies, combinations are brought about 

 with the intervention of contact-action at much 

 lower temperatures than are required without it. 

 While we admit that the kinetic energy of a gas 

 is proportional to its absolute temperature, and 

 that the total resistance of a gas is the sum of 

 the energies of motion of its constituent mole- 

 cules, it does not follow that the energy of mo- 

 tion is the same in all the molecules. The tem- 

 perature indicated by the thermometer must 

 rather be regarded as the mean of the tempera- 

 tures (or energies of translation) of the mole- 

 cules. Further, the atoms themselves must be 

 regarded as carriers of dynamical energy, and 

 there must be variations in atom temperatures 

 along with variations in the energy of translation 

 ot the molecules. " Dissociation temperature," 

 so far as individual molecules are concerned, 

 may be taken to represent simply a sufficient 

 increase in the vibratory motions of the atoms 



to carry them beyond the range of the attrac- 

 tion of affinity which holds them together in 

 the molecule. Since both the energy of trans- 

 lation of the molecule and the atom-tempera- 

 ture may vary within wide limits, it follows that 

 in a given mass of gas there must be some mole- 

 cules which require a smaller accession of heat 

 from without the system than is required by 

 those which are in the condition of the mean 

 dynamical intensity of the system, in order 

 that they may be dissociated. This is what is 

 known as the initial temperature of dissocia- 

 tion. If the mean temperature of the system 

 be maintained constant above that of the ini- 

 tial temperature of dissociation, a certain per- 

 centage of the gas will be dissociated. For the 

 same temperature, other things being equal, 

 the same percentage of the gas is dissociated, 

 but not the same identical molecules. New 

 molecules are continually undergoing dissocia- 

 tion, while some of the previously dissociated 

 atoms, owing to diminution in the intensity of 

 their vibratory motion, enter again into molec- 

 ular union, so that for complete dissociation 

 either the removal from the system of the iso- 

 lated atoms, or an enormous elevation of tem- 

 perature of the whole system is required. 

 When we have the intervention of a porous or 

 finely-divided solid, the enormous increase of 

 the solid surface must lead to a corresponding 

 increase of the number of collisions in a given 

 time between the individual molecules and that 

 surface ; and consequently internal work may 

 be, and probably is, done among the atoms of 

 the molecules by increasing the intensity of 

 their vibratory action ; and so far as the mole- 

 cules thus affected are concerned, a smaller ac- 

 cession of heat derived from without the sys- 

 tem would most likely be necessary to bring 

 about initial dissociation. 



Landholt has made some researches on the 

 velocity of movement of the molecules of fluids 

 and solids. He found that fluids which react 

 immediately upon one another in certain de- 

 grees of concentration require more space 

 for the process when dilute, in proportion to 

 the degree of dilution. In his test experiment 

 with a mixture of sulphurous and hydriodic 

 acids resulting in the separation of iodine, 

 other conditions being the same, the time in 

 which the moment of reaction would take 

 place could be foretold by the clock. A mixt- 

 ure of one molecular part of the sulphur acid 

 to 20,000 molecular parts of water and two of 

 the iodine acid to the same quantity of water, 

 with starch added, remained colorless for twen- 

 ty seconds, when the change to deep blue took 

 place at once. When the dilution was doubled, 

 two minutes, when tripled, seven minutes 

 elapsed before the sign of the reaction was given. 

 If, instead of water the dilution was effected 

 with a liquid of greater viscosity glycerin, 

 for example, was employed as a diluent the 

 time required for the reaction was prolonged, 

 and the exhibition of the effect, or the appear- 

 ance of the blue color, was gradual. 



