IOWA ACADEMY OP SCIENCES. 
15 
2 
^“100 L 
in which T = the absolute temperature, L = the heat of fusion 
of the solvent. 
For Raoult’s second law a similar dependence of the constant 
upon the temperature and the heat of vaporization was estab- 
lished. To Van’t Hoff belongs, therefore, the signal service of 
bringing the empirical laws of Coppet and Raoult into close con- 
nection with the general principles of thermodynamics, from 
which they are derived as necessary consequences. 
This important step greatly stimulated research. Many 
investigators began to till the field upon which the sod had been 
so successfully turned ; new and convenient methods were 
devised for the accurate determination of melting and boiling 
points. In fact, a new technique was developed. An incidental 
result of this activity is that mercurial thermometers capable of 
reading to so’^oo of a degree centigrade have become articles of 
commerce. The only compounds whose molecular weights can 
not be determined by the new methods are the very few which 
resist solution in all solvents. 
It is interesting to note that the numerous molecular weight 
determinations thus made show that the great majority of sub- 
stances have the same molecular weights in the liquid as in the 
gaseous form, but few being more complex, and that most sub- 
stances not capable of vaporization without decomposition pos- 
sess, in fact, when in solution, the relatively simple constitution 
which had x>i*eviously been assigned to them on insufficient 
chemical grounds alone. Among the many other valuable 
results attained may be mentioned the conclusion that the per- 
manganates are salts of the simple acid H Mn O4 and not of the 
more complex molecule Hg Mng Os and that chromic acid 
exists in solution as dichromic acid H2 Crs O 7 . The further 
conclusion, that most of the true metallic elements resemble 
mercury in that they consist of monatomic molecules, and differ 
in this respect from the non-metallic elements whose molecules 
are complex, has recently been confirmed, in several cases, by 
vapor density and other independent determinations. The most 
important theoretical development due to Van’t Hoff is the close 
analogy shown by him to exist between a gas, on the one hand, 
and a solution on the other. We may summarize the points of 
analogy as follows; 
