MOLECULES. 683 MOLECULES. (I.ODOOlt .54,000,OULI.OOO,(X)0,(X)0 0.000,000,000,000,000,000,000,0106 milli- gram, in a similar manner, the absolute weight of a moleiiilL' of any other j;as or vapor may l)e read- ijv calculated, by diviilinj; the weight of one cubic millimeter by 54,000.000.000,000,000. MOLECUI,.R WEIGHTS. Tile practical value of the above results is, at least for the present, not nearly so great as that of the purely relative molecular weights constantly used in scientific work. For it is on our knowledge of these relative molecular weights that the modern theories concerning the struc- ture and the mutual relations of chemical com- pounds, especially the numerous comjiounds of carbon, are based. Vapor-Density Method. The molecular weights of ga.ses, or of substances that can be obtained in the state of vapor, can be readily determined by ascertaining the density of the gas or vapor. The principle involved is as fol- lows: The density (D) — i.e. the weight of a cer- tain volume of gas — is obviously the product of the number (h) of molecules contained in that volume, and the weight ( M ) of each molecule : D = mM, whence D M = "• Xow, according to Avogadro's rule, equal vol- umes of all gases and vapors contain the same number of molecules, provided the temperature and pressure are the same ; in other words, the number (») contained in a given volume of gas does not vary with the nature of the latter; it is the same for nil gases. For any two different gases we shall therefore have L D, _ n„ _ whence i.e. the molecular Heights of gases are propor- tional to their densities. Evidently, so that if D, and M, stand for the density and weight of a molecule of hydrogen (the lightest known gas), the weight of a molecule, say, of oxv'gen. may be found by ascertaining its density referred to hydrogen ( i.e. the weight of any volume divided by the weight of the same volume of hydrogen at the same pressure and tempera- ture) and multiplying that density by the weight of a molecule of hydrogen. Now, if in- stead of multiplying the densities of gases by the true weiglit of a molecule of hydrogen, we mul- tiply them by any arbitrary number, we obtain, instead of the true weights of the molecules of gases, a series of other numbers whose values depend on that of our arbitrary number for hydrogen. But, if we adhere to the same ar- bitrary number, our niunbers for the various gases will bear the same ratios to one another as the true weights of their molecules. And since in studying substances and reactions we need know not the absolute, but the relative weights of molecules, we may assign to our Vol. XIII.— m. standard gas. hydrogen, any arbitrary number whatever. For reasons e.plaiiii'd in the article Atomic WEiniiTs, the molecular weight of hydrogen is assumed to lie 2. Therefore, to determine the molecular weight of a volatile compound, all a chemist has to do is to ascertain the weight of a given vohiinc of vapor, to divide that weight by the weight of an equal volume of hydrogen gas at the same pres- sure and temperature, and to multiply the vapor density thus obtained by 2. Water vapor is found to be !) times as heavy as hydrogen gas; hence, its vajior densit.v is said to be !): nuilti- plying 9 by 2, the molecular weight of water is seen to be 18. Tiie va])or density of chemical compounds is usually determined with the aid of one of the following ajiparatus: A. Dumas's Apparalu.s consists of a light glass flask (of about 2,50 cubic centimeters capacity), whose neck is bent and drawn out into a long and narrow jjoint. After carefully weighing the flask, a few grams of the substance to be ex- amined are introduced into it ; it is then im- mer.sed in a bath whose temperature is constant and somewhat higher than the boiling-tempera- ture of the substance in the flask. The substance soon begins to boil, and its vapors drive all the air out of the flask. When the substance has com- pletely evaporated and the flask contains noth- ing but its vapor, the open end is carefully sealed off with the aid of a blowpipe. On cool- ing, the flask is cleaned and again weighed. Fi- nally, its end is broken off under water, which rushes into the flask owing to the low pres- sure within ; and when the latter is complete- ly filled, it is weighed a third and last time, together with the end that has been broken off. From the three weighings, the weight and the volume of the vaporized substance in the ap- paratus become known. Dividing this weight by that of an eipial volume of hydrogen at the temperature of the bath and the barometric pres- sure under which the determination has been carried out, we get the vapor-density, and from this, by multiidying by 2. the molecular weight of the substance examined. Dumas's apparatus can be employed only when a considerable amount of substance is available. B. Victor Mc;/cr's Apiiaratiis consists of a wide glass tube (a), the lower end of which is closed, and the ujiper joined on to a long glass tube (h) of smaller diameter. The narrow tube is provided, near its iqijier end. with a side-tube (c). During a determination this apparatus is placed in a long test-tube-like vessel ((/) contain- ing some liquid boiling at a higher temperature than the substance to be exjierimented upon. A little of the siibstance is carefully weighed in a small glass capsule: the rubber stopper closing the top end of the apparatus is for an instant removed, and the weighed capsule is dropped in. The subs(:ince quickly evaporates, driving out through the side tube c a volume of air equal to the volume of its own vapor. The air is col- DBMAS S APPAB.VTCS.
