B. Victor .Voter's .Ipparatus consists of a wide glass tube (a), the lower end of which is closed, and the upper joined on to a long glass tube (b) of smaller diameter. The narrow tube is provided. near its upper end, with a side-tube (r). During a determination this apparatus is placed in a long test-tube-like vessel I r11 contain ing some liquid boiling at a higher temperature than the substance to he experimented upon. A little of the substance is carefully weighed in a small glass capsule: the rubber stopper (dosing the top end of the apparatus is for an instant removed, and the weighed capsule is dropped in. The substance quiekly driving out through the side tube c a volume of air equal to the volume of its own vapor. The air is col keted in a graduated tube. and thus the volume of the vaporized substance known. Di viding the weight of the substance taken by the weight of an equal volume of hydrogen, at the teniperature of the vaporized substance in the apparatus and at the barometrie pressure under which the determination has been Carried out, we get the vapor-density. and from this. by multiply ing by 2, the molecular weight of the substance. Victor apparatus yields precise results and requires but a very small quantity of the substance experimented upon. The latter cir cmastanee forms an ingortant advantage: for the preparation of large quantities of a substance often involves great loss of time and is some times very difficult.
Alrriton or I)ETERNI I N.%TION OF 1110LECEL.%It AVF.161urs 1)ittEcr on INDIRECT AIF:.%SURE NI ENT OF TIIE /SMOTIC l'Itt:SSURE OE '40LI rioNs. In the gaseous state a small mass of substanee occupies a InIrgc. V(d11111(• of spaCP. and the properties pecu liar to the gaseous state are due to the fact that the molecules of a gas are separated from one an other by considerable distances. Rut a little sugar dissolved in mulch water is also distributed throughout a large volume: the of su gar are likewise separated from One another by considerable distances. and this is why the prop erties of sitgar in (Mille solution are very muck like %dolt they would he if the sugar existed in the gaseous state. Thus, within the volume of a solution. (as well as any other substance) has been proved to exert a pressure equal to that which it would exert were it and in closed within a vessel whose volume is equal to that of the solution. The pressure of a substance
in solution is called osmot ie pressure. \\lien equal weights of IlitTerent gases arc confined within equal volumes, the pressures are inversely proportional to the moleeular weights of the gases. Thus. if equal quantities by weight of oxygen and hydrogen were inclosed within vessels, say, of one liter capacity, the pressure in the vessel containing hydrogen would be 16 times as great is the pressure in the vessel containing oxygen: while, on the contrary, the molecular weight of hydrogen (2) is the molecular weight of oxygen (32). The reason is obviously this: the lighter the molecules, the greater must lie their number, to constitute a given weight of gas: but the greater the number of molecules, the greater the pressure of the gas: henee, the lighter the molecules, the greater the pressure of a given mass of gas. Similarly. in the ease of equal weights of different substances in solution, the osmotic pressures are inversely proportional to the moleoular weights. The relative molecular weights of soluble substances may therefore he fount(' by preparing solutions having equal vol umes and holding (vial weights of the dissolved substances. then measuring the osmotic pressures of the solutions. Sec Soil ON.
But as the direct measurement of osmotic pres sure is exceedingly difficult, indirect methods of measurement are usually employed by the in vestigator. \\len a substance enters some sol vent. say water, it changes the characteristic freezing and boiling temperatures of the solvent: the freezing-point is lowered, the boiling-point is raised. Now, the depression of the freezing-point and the elevation of the boiling-point have been shown to be proportional to the osinotie pres sure of the solution. Therefore. instead of carry ing out a direct determination of osmotic pres sure. the chemist measures the ehange of freezing or boiling temperature caused by dissolving a given weight of substance. and calculates the molecular weight of the latter on the principle that the change caused by a substance is in versely proportional to its moleeular weight. The laboratory methods commonly employed in de termining the depression of freezing-point and the rise of boiling-point are described in the