270 
DE. HEEMANK KOPP OX THE EELATIOX BETWEEX BOIUXG-POIXT 
There can be no doubt that in different series of homologous compounds the difference 
of boiling-point corresponding to the elementary difference x Cg may vary ; that 
frequently it is found to be ^xl9°, but sometimes more, sometimes less. As to these 
inequalities of differences, they are sure to be governed by a more general law, which 
will probably be recognized when a sufficient number of boiling-points shall have been 
determined under a pressure different from the ordinary pressm-e of the atmosphere ; 
for it cannot by any means be taken for granted that two substances will exhibit the 
same difference of boiling-point, whatever may be the pressure. Let S and S, be the 
boiling-points of two liquids under the ordinary pressme, and s and Sj the boiling-points 
under another pressure. Then the assumption of S — Si=s — would involve the infer- 
ence S— s=Si — s^; i. e. that the boiling-points must undergo precisely the same change 
for the same change of pressure. But this conclusion, the so-called law of Daltox, is 
by no means correct ; and it may be legitimately held that the homologous substances 
which under the ordinary pressure do not exhibit the most ffequently observed difference 
of boiling-point (19°) for the elementary difference C 2 H 2 , would certainly show that 
difference under a different pressure. 
Sufficient data are still wanting for the appreciation of the mode in which differences 
in boiling-points are affected by changes of pressure. But the extent of these alterations 
may in a measure be inferred from the fact that the difference in boiling-points of 
alcohol and ether, which, under the ordinary pressure, amounts to about 44°, from what 
is known with respect to the vapour-tension of these two substances at different tempera- 
tures, would, under the pressure of half an atmosphere, be above 45°, and below 40° 
under a pressure of three atmospheres. 
From the observations at present at our disposal, it may be affirmed as a general rule, 
that in homologous compounds belonging to the same series, the differences in boiling- 
points are proportional to the differences of the formulae. Exceptions obtain only in 
cases in which terms of a particular group are rather difficult to prepare, or when the 
substances boil at a very high temperature, at which the observations now at oiu com- 
mand are for the most part uncertain. Again, it may be affirmed that the difference 
in boiling-points corresponding to the elementary difference C 3 H 2 , is in a great many 
series =19°; in some series more, in some series less. 
There are a few quite exceptional cases in which the higher terms of a homologous 
series boil at a lower temperature than the inferior terms. A case in point exists accord- 
ing to WuRTZ in the glycols. 
Table XXII. 
Ethyl-glycol 
Propyl-glycol 
Butyl-glycol 
Amyl-glycol 
C. H, 0, 
Cc H, 0 , 
Ca H.oO, 
197—197*5; Atkinson, 193°. 
188—189 
183—184 
177 
