CHEMISTRY. 429 



Over these two sets of determinations, which differ in mean by nearly 

 0.2, some controversy has arisen. For details, see Thorpe and Laurie 

 (Berichte, xx, 3036, and Jouru. Chem. Soc, December, 1887), and Kriiss, 

 (Berichte, xx, 23(35, and xxi, 128). Meanwhile Mallet has i)ublished a 

 preliminary note, without particulars, concerning a determination which 

 he has made. He gets a direct ratio between gold and hydrogen, giv- 

 ing for gold a value somewhat higher than that obtained by Thorpe and 

 Laurie. (Chem. News, N^o. 1452.) 



The deferminafion of molecular iceights. — Early in 1888 Victor Meyer 

 called attention to Raoult's method for ascertaining molecular weights, 

 pronouncing it to be the most important advance in that field of work 

 since the announcement of Dulong and Petit's law. The method, in 

 princii)le, is as follows: When any substance is dissolved, the freezing 

 l^oint of the solvent is lowered ; and that depression of freezing point is 

 directly related to the molecular weight of the body in solution. If we 

 divide the observed depression of freezing point for each substance ex- 

 amined by the weight of the substance dissolved in 100 grammes of 

 water, we get a coefificient of depression which gives, when multiplied 

 by the molecular weight, a constant quantity. The latter constant is 

 independent of concentration, and has approximately uniform value for 

 all similarly constituted bodies in any given solvent. It is called the 

 moJecular depression ; ?i\\(\. (W&QVB for different solvents; but in glacial 

 acetic acid it seems to have one value for all substances. Water, acetic 

 acid, and benzene seem to be the only solvents of general applicability. 

 If now it is desired to know the molecular weight of a soluble body to 

 which vapor-density methods are inapplicable, we have only to deter- 

 mine its coefficient of depression and compute from that the sought for 

 value. The method, as a physical process, is not sharply accurate; but 

 it is close enough to decide between the lowest empirical formula assign- 

 able to a substance and the various multiples thereof. Although the 

 original investigation was published by Raoult in 1883, it attracted 

 little attention until Meyer's paper appeared; but since then it has 

 been widely noted, and the method extensively applied. (See Meyer, 

 Berichte, xxi, 536, and Auwers, ihid.^ xxi, 701.) Auwers also describes 

 a convenient form of apparatus, and so too does Beckmann. (Zeit. 

 Phys. Chem., ii, 638.) 



In illustration of the use of Raoult's method, the following researches 

 out of many may be cited : 



Ramsay, working with an acetic acid solution, finds the molecular 

 weight of liquid nitric jieroxide to be 92, at a temperature of about 

 160. Hence its formula isN204, and it seems, furthermore, to undergo 

 uo dissociation upon dilution. (Journ. Chem. Soc, Liii, 621.) 



Brown and Morris, using aqueous solutions, get depression values for 

 dextrose, cane sugar, maltose, lactose, and arabinose corresponding to 

 their accepted molecular weights. For ratlin ose the simplest empiri- 

 cal formula, CisHszOie . 5H2O, corresponds to the results obtained by 

 Raoult's method. (Journ. Chem. Soc, Liii, 610.) 



