Chapter VII

.), and designated by M, the less, under other similar conditions, will be the depression _d_, and therefore if the concentration of a solution (the amount by weight of substance dissolved per 100 parts of water) be designated by _p_, then the fraction M_d_/_p_ or the molecular depression for a given class of substances will be a constant quantity; for example, in the case of methyl alcohol in water 17·3, for acetone about 18·0, for sugar about 18·5. (3) In general the molecular depression for substances whose solutions do not conduct an electric current is about 18·5, while for acids, salts, and such like substances whose solutions do conduct electricity, it is _i_ times greater; for instance, for HCl, KI, HNO_{3}, KHO, &c., about 36 (_i_ is nearly 2), for borax about 66, and so on where _i_ varies in the same manner as it does in the case of the osmotic pressure of solutions (Note 19). (4) Different solvents (water, acetic acid, benzene, &c.) have each their corresponding constants of molecular depression (which have a certain remote connection with their molecular weight); for example, for acetic acid the molecular depression is about 39 and not 19 (as it is for water), for benzene 49, for methyl alcohol about 17, &c. (5) If the molecular weight M of a substance be unknown, then in the case of non-conductors of electricity or for a given group, it may be found by determining the depression, _d_, for a given concentration, _p_; for example, in the case of peroxide of hydrogen, which is a non-conductor of electricity, the molecular weight, M, was found to be nearly 34, _i.e._ equal to H_{2}O_{2}.

Similar results have also been found for the fall in the vapour tension of solutions (Note 51), and for the rise of their boiling points (hence these data may also serve for determining the molecular weight of a substance in solution, as is shortly described in