Let us now imagine what degree of transparency of air is admitted by its molecular constituents, viz.
In 1831, from a study of the specific heats of compounds, he formulated "Neumann's law," which expressed in modern language runs: "The molecular heat of a compound is equal to the sum of the atomic heats of its constituents."
The boiling and freezing-point determinations of the molecular weight in solution indicate the formula S8.
This oxide exists in two forms. The aform is readily fusible and melts at 14.8° C. It corresponds to the simple molecular complex S03.
In such experiments the molecular energy of a gas is converted into work only in virtue of the molecules being separated into classes in which their velocities are different, and these classes then allowed to act upon one another through the intervention of a suitable heat-engine.
It corresponds to the molecular complex (S03)2.
Hantzsch (Ber., 1901, 34, p. 3337) has shown that in the action of alcohols on diazonium salts an increase in the molecular weight of the alcohol and an accumulation of negative groups in the aromatic nucleus lead to a diminution in the yield of the ether produced and to the production of a secondary reaction, resulting in the formation of a certain amount of an aromatic hydrocarbon.
In 1894 and 1895, Fischer, in a remarkable series of papers on the influence of molecular structure upon the action of the enzyme, showed that various species of yeast behave very differently towards solutions of sugars.
The alkyl derivatives may be obtained by heating phenol with one molecular proportion of a caustic alkali and of an alkyl iodide.
There is some doubt as to the molecular formula of fulminic acid.
The extent of the area affected and of the variation in the turgor depends upon many circumstances, but we have no doubt that in the process of modifying its own permeability by some molecular change we have the counterpart of muscular contractibility.
He held that every fermentation consisted of molecular motion which is transmitted from a substance in a state of chemical motion - that is, of decomposition - to other substances, the elements of which are loosely held together.
Deviation from this rule indicates molecular dissociation or association.
Isomerism, or the existence of two or more chemically different substances having identical molecular weights, is adequately shown; and, most important of all, once the structure is determined, the synthesis of the compound is but a matter of time.
In place of the relative molecular weights, attention was concentrated on relative atomic or equivalent weights.
Recent researches have shown that the law originally proposed by Kopp - " That the specific volume of a liquid compound (molecular volume) at its boiling-point is equal to the sum of the specific volumes of its constituents (atomic volumes), and that every element has a definite atomic value in its compounds " - is by no means exact, for isomers have different specific volumes, and the volume for an increment of CH 2 in different homologous series is by no means constant; for example, the difference among the esters of the fatty acids is about 57, whereas for the aliphatic aldehydes it is 49.
We may therefore conclude that the molecular volume depends more upon the internal structure of the molecule than its empirical content.
The molecular volume is additive in certain cases, in particular of analogous compounds of simple constitution.
Regnault confirmed Neumann's observations, and showed that the molecular heat depended on the number of atoms present, equiatomic compounds having the same molecular heat.
Kopp systematized the earlier observations, and, having made many others, he was able to show that the molecular heat was an additive property, i.e.
He introduced the idea of comparing the refractivity of equimolecular quantities of different substances by multiplying the function (n-1)/d by the molecular weight (M) of the substance, and investigated the relations of chemical grouping to refractivity.
Compounds having the same composition, have equal molecular refractions, and that equal differences in composition are associated with equal differences in refractive power.
The molecular weight determinations of W.
With Sydney Young and others he investigated the critical state and properties of liquids and the relationship between their vapour pressures and temperature, and with John Shields he applied measurements of the surface tension of liquids to the determination of their molecular complexity.
Chemistry and physics, however, meet on common ground in a well-defined branch of science, named physical chemistry, which is primarily concerned with the correlation of physical properties and chemical composition, and, more generally, with the elucidation of natural phenomena on the molecular theory.
Gerhardt found that reactions could be best followed if one assumed the molecular weight of an element or compound to be that weight which occupied the same volume as two unit weights of hydrogen, and this assumption led him to double the equivalents accepted by Gmelin, making H= 1, 0 =16, and C = 12, thereby agreeing with Berzelius, and also to halve the values given by Berzelius to many metals.
Laurent generally agreed, except when the theory compelled the adoption of formulae containing fractions of atoms; in such cases he regarded the molecular weight as the weight occupying a volume equal to four unit weights of hydrogen.
From a study of the free elements Cannizzaro showed that an element may have more than one molecular weight; for example, the molecular weight of sulphur varied with the temperature.
Thus, the symbols 14 2 and P4 indicate that the molecules of hydrogen and phosphorus respectively contain 2 and 4 atoms. Since, according to the molecular theory, in all cases of chemical change the action is between molecules, such symbols as these ought always to be employed.
When chemical change is expressed with the aid of molecular formulae not only is the distribution of weight represented, but by the mere inspection of the symbols it is possible to deduce from the law of gaseous combination mentioned above, the relative volumes which the agents and resultants occupy in the state of gas if measured at the same temperature and under the same pressure.
Formulae which merely express the relative number of atoms of the different elements present in a compound are termed empirical formulae, and the formulae of all compounds whose molecular weights are undetermined are necessarily empirical.
The first set provides evidence as to the molecular weight of a substance: these are termed " colligative properties."
In any attempts to gain an insight into the relations between the physical properties and chemical composition of substances, the fact must never be ignored that a comparison can only be made when the particular property under consideration is determined under strictly comparable conditions, in other words, when the molecular states of the substances experimented upon are identical.
When this is done, such densities are measures of the molecular weights of the substances in question.
It is found that isomers have nearly the same critical volume, and that equal differences in molecular content occasion equal differences in critical volume.
Kopp, begun in 1842, on the molecular volumes, the volume occupied by one gramme molecular weight of a substance, of liquids measured at their boiling-point under atmospheric pressure, brought to light a series of additive relations which, in the case of carbon compounds, render it possible to predict, in some measure, the cornposition of the substance.
In practice it is generally more convenient to determine the density, the molecular volume being then obtained by dividing the molecular weight of the substance by the density.
These values hold fairly well when compared with the experimental values determined from other compounds, and also with the molecular volumes of the elements themselves.
Schroeder the silver salts of the fatty acids exhibit additive relations; an increase in the molecule of CH2 causes an increase in the molecular volume of about 15'3.
From the ratio Cp/C„ conclusions may be drawn as to the molecular condition of the gas.
456) has given the formula Cp=6.5--aT, where a is a constant depending on the complexity of the molecule, as an expression for the molecular heat at constant pressure at any temperature T (reckoned on the absolute scale).
We now proceed to discuss molecular heats of compounds, that is, the product of the molecular weight into the specific heat.