RELATION OF VAPOUR DENSITY TO MOLECULAR FORMULE. 29 and 2. H‚¤ ̧ ̧. The first of these formulæ indicates the amount of tartaric acid equivalent to the quantity of hydrochloric acid represented by the formula HCl, or of nitric acid expressed by the formula HNO3; yet it is not the true representation of the molecule of tartaric acid, as a further examination of its salts will show. The acid is, indeed, dibasic, requiring two atoms of a metallic monad to neutralize it, so that the second formula is the least by which its molecule can be indicated. Supposing that each of the following tartrates be sufficiently dried to expel its water of crystallization, the results of analysis may be expressed by the following formulæ, in which the proportions of carbon and oxygen are alike :— Tartaric acid (tartrate of hydrogen) Potassio-sodic tartrate ннен KH,H, 6 Of the two atoms of basic hydrogen in the first of these four formulæ, one atom only is displaced by a metallic basyl in cream of tartar, but both atoms are displaced by potassium in the normal potassic tartrate, whilst in potassio-sodic tartrate one atom is displaced by sodium, the other by potassium. The determination of the molecular formula of a body demands a careful investigation of a number of its compounds and derivatives, and frequently calls forth all the sagacity of the chemical investigator. The study of the physical properties of a compound often affords great assistance in the determination of its molecular formula, and amongst these physical properties the connexion between the vapour density and the molecular constitution is the most important. (1054) Relation of Vapour Density to Molecular Formulæ.— All bodies in the aeriform condition, whether simple or compound, if compared at equal temperatures increase by an equal fraction of their volume, for equal increments of temperature (134); in other words, they have the same coefficient of dilatation when heated; and in like manner all gases and vapours undergo equal amounts of compression for equal increments of pressure, if the comparison be made under similar circumstances (27). It appears therefore that the elastic force of all gases, whether simple or compound, is equal and uniform; and hence it has been concluded that under similar circumstances of temperature and pressure all gases and vapours contain an equal number of molecules. This is commonly known as Ampère's hypothesis of the constitution of gases. 30 RELATION OF VAPOUR DENSITY TO MOLECULAR FORMULA. * Assuming then that equal bulks of vapours contain an equal number of their constituent molecules, it is easy, if the vapour density of any compound be known, to calculate its molecular formula; whilst from the molecular formula it is equally easy to calculate the vapour density of the body. In order to calculate the vapour density of any compound from its molecular formula, it is assumed, in accordance with the results of observation, with few exceptions, that the volume of the molecule of the body in the state of vapour is double that of hydrogen; or, that it corresponds to H,; H, being the representative of the molecule of hydrogen when in the free state. Now, according to Regnault's experiments, the weight of a given bulk of atmospheric air is 1447 times that of an equal bulk of hydrogen under similar circumstances of temperature and pressure; consequently, doubling this for H, (the molecule of free hydrogen), we obtain 2x1447=0'0691 as the vapour density, or specific gravity, of hydrogen. ls7 And in like manner by dividing the molecular weight m, of any organic compound, by 2 × 14'47 (or 28'94), we obtain the vapour density d of the compound; 4 = d. m 28.94 For example, the molecular formula of marsh gas is CH, and its molecular weight, or the sum of the atomic weights of its component elements, is 16. Now=0'55, and this number agrees with Thomson's experimental determination of the density of the gas. 16 On the other hand, if the vapour density of a compound be known, and if its percentage composition have been determined, the accuracy of the molecular formula of the body may be checked by reversing the foregoing process; that is to say, by multiplying the vapour density by 28'94, we obtain a very close approximation to the molecular weight, since dx 28'94 m. The numbers obtained by this calculation are never absolutely correct, owing to unavoidable experimental errors in ascertaining the vapour density; but the errors do not affect the value of the result in controlling the molecular formula; the atomic constitution of the molecule may be safely inferred from the calculated result; and thus the accuracy of the formula deduced from the analytical operations may be checked. For example, the experimental determination of the specific gravity of the vapour of ether shows it to be 2:586. Now, 2586 Among these exceptions are chlorous anhydride and some of the oxides of nitrogen, as well as many of the salts of ammonia-such as the hydrochlorate, hydrocyanate, and hydrobromate. The anomalies which they present will be considered hereafter. ON MOLECULAR WEIGHTS AND FORMULE. 10 31 × 28'94 74 26. The simplest formula deducible from the analysis of ether is He, and the corresponding molecular weight (=74) agrees as closely as can be desired with the result of the calculation from the vapour density. Important as this subject is, it would, however, be out of place here to enter further into the methods of checking the correctness of an analysis in its various parts. For information upon this point the reader is referred to Liebig's Handbook of Organic Analysis. (1055) Distinction between Atomic Weight and Molecular Weight.—It is obvious that if the atom of oxygen be taken as =16, the smallest particle of water which can exist must be represented by the formula H20=18: and in like manner the formula for sulphuretted hydrogen expressing its atom must be H.S=34; consequently, the formulæ H, and HS constitute the atomic as well as the molecular formulæ of these bodies. Although, therefore, the atomic weight of an element or of a compound may sometimes coincide with the molecular weight, the two expressions are by no means necessarily synonymous, but are to be carefully distinguished from each other: the atomic weights being the numbers which represent the relative weights of the atoms of the different elements, when referred to some arbitrary but recognised standard, such as the atom of hydrogen=1; whilst the molecular weights, whether of the elements or of compounds, represent the relative weights of the molecules; and the molecular weight, when once the magnitude of the molecule is agreed upon, is determined by the sum of the weights of all the atoms which enter into the formation of the molecule itself. The molecular formula of a compound body may frequently be ascertained by considerations analogous to those adduced in the cases of water and of sulphuretted hydrogen, when the density of its vapour cannot be ascertained by experiment. (See p. 47.) (1056) On the Use of Molecular Formula.-Allusion has already been made (1054) to the facility which a knowledge of the law that the densities of the vapours are exactly in the ratio of their molecular weights gives to the calculation of the vapour density of a body from its molecular formula; and, conversely, to the valuable control which the vapour density affords to the calculated molecular formula. Although the use of molecular formulæ in our equations sometimes renders them less simple in appearance, yet the simplicity introduced into our calculations of vapour volume by the use of such molecular formulæ in our ordinary equations for the representation of chemical changes, 32 ON THE USE OF MOLECULAR FORMULE. affords a strong argument in favour of their uniform employment: for by such a method the gaseous volume of each constituent is at once placed before the eye. If we continue to assume as the unit of volume that of hydrogen, H, with the atomic weight 1, the molecule of hydrogen, or H,, will represent 2 volumes. Now the gaseous volume occupied by each molecule of any compound will (with the exceptions already specified) also occupy 2 volumes. For example, the formulæ H, and Cl, each represent a molecule of hydrogen and chlorine respectively, whilst HCl represents a molecule of hydrochloric acid; and we see by the equation H2+Cl2=2HCl, that a molecule of hydrogen and one of chlorine unite without condensation to form 2 molecules of hydrochloric acid; whilst, when oxygen and hydrogen unite to form water, from the equation 2+2 H2=2 H2, it is manifest that I molecule of oxygen and 2 molecules of hydrogen unite to form 2 molecules of steam, and that the 3 volumes which the gases occupied in their separate condition become condensed into 2 when they have united. So, again, it is clear from the equation employed, that a similar condensation occurs when carbonic oxide and oxygen unite to form carbonic anhydride; for 2+2 0=2€0,, 1 molecule of oxygen and 2 of carbonic oxide yielding 2 molecules of carbonic anhydride, 3 volumes again becoming 2 by combination. Again, when marsh gas is detonated with oxygen, the formula at once gives the volume of oxygen required, and the gaseous products which are obtained : I volume of marsh gas and 2 volumes of oxygen yielding 1 volume of carbonic anhydride and 2 volumes of steam. If olefiant gas be detonated with oxygen we find 3 volumes. of oxygen and 1 volume of olefiant gas furnish 2 volumes of carbonic anhydride and 2 of steam, as indicated by the molecular formulæ, The formula which express the composition of salts cannot in strictness be regarded as molecular formulæ, or formulæ of uniform vapour volumes: they indicate the relative proportions of their constituents in the solid form, and have no reference to gaseous volume. Some further discussion of the relation between the atom or molecule of elementary and compound bodies will be found at par. 1061. § II.-CLASSIFICATION OF ORGANIC COMPOUNDS. (1057) General Principles of Classification.—The classification of the immense variety of compounds that are presented to the chemist, either naturally by the organs of the living plant or animal, or that are derivable from the bodies so obtained, by the employment of chemical reagents, may be effected upon two principles. The first of these, which rests upon analogies of function (according to which bodies are arranged under the head of acids, bases, &c.), might seem to be the more natural, but the second, which is based on the chemical relations of the different compounds, is the truly philosophical system. The first method of arrangement was the only practicable one in the infant state of the science, and it still presents certain advantages to the student in the early stage of his career; but the second will eventually supersede the former, as it greatly facilitates the important study of the true analogies of the different compounds with each other. Since, however, the true relations of a great number of bodies to each other have been as yet only imperfectly traced, the most devoted admirer of a strictly chemical classification is obliged to make a large appendix of unclassified bodies, and to bring into the more systematically arranged portion of his subject many substances which have but a questionable title to the place accorded to them; moreover, a rigid adherence to such a principle of classification would lead to the separation of bodies which in their applications and in general properties are closely allied. We shall, therefore, here, as in other portions of the present work, be guided rather by considerations of convenience than by the rigid requirements of system; and shall endeavour to select such a method of arrangement as shall most facilitate the progress of the student. In the present work, the various compounds of organic chemistry will be examined in the following order: 1. Sugar, starch, and ligneous fibre. : 4. Certain organic acids of vegetable origin. 6. Essential oils and resins. 7. Colouring matters. 8. Products of destructive distillation. 9. Cyanogen and its derivatives. 10. Nitrogenized principles of plants and animals. 11. Certain products peculiar to animals. |