THE PHYSICAL PROPERTIES OF METHYL- ALCOHOL. 513 



end, we determined the vapour density of our alcohol by means of an apparatus 

 of our own invention, the description of which Ave prefer to reserve for an 

 appendix to this memoir; suffice it to point to Plate XXXIII. fig. 1, and 

 to state that it is constructed on the Gay-Lussac principle, in such a manner 

 as to avoid the uncertainties in the variable density of the suspended mercury 

 column, and that the vapour volume in it is measured under very nearly the 

 prevailing atmospheric pressure at about 100° C. Three determinations made 

 with from 90 to 100 milligrammes of alcohol gave the values 16'17, 1627, 

 16*22 ; hydrogen = 1 ; i.e., a little more than the theoretical number 1600. * 



We could not help noticing that our three numbers are a little above the 

 value demanded by CH 4 ; yet as the excesses lie almost within the limits of 

 unavoidable errors, we preferred to accept our numbers as showing that our 

 preparation was, at the worst, a fair approximation to the ideal substance. 

 We now regret that we did not endeavour to attain a higher degree of precision 

 in our vapour-density determinations by a suitable modification of our apparatus, 

 and adopt the exact vapour density at a high enough temperature as the final 

 test of purity. At the time it appeared to us better to try, and, if possible, 

 prove, the purity of our alcohol by a series of chemico-physical tests, which all 

 agreed in this, that the given alcohol was subjected to some chemical process of 

 fractionation, and the two fractions were compared with each other, and the 

 mother substance, in regard to some exactly measurable physical property. 



To give a better idea of what we actually did, let us quote a few special 

 cases. 



I. 300 c.c. of a certain alcohol (I.) were " tortured " with 90 grammes of dry 

 caustic potash, and the resulting mass was distilled by means of a water-bath 

 as long as anything came over. The distillate, amounting to 120 c.c, was put 

 aside as " A." 



From the residue of alcoholate and hydrate of potash the alcohol was 

 regenerated by distillation with water as fraction "B," which, according to its 

 volume and specific gravity, contained about 142 grammes of absolute alcohol. 

 Each of the two fractions was dehydrated completely by repeated distillation 

 over baryta-lime until its specific gravity became constant. The final specific 

 gravities, determined by the Westphal balance, and reduced to 0° C. by the 

 same formula, were — that of A = - 8138, that of B = 08142, i.e., the two frac- 

 tions were practically identical. 



* Kramer and Grodzkx (Berichte der Deutschen Chem. Oes., 1876, p. 1928) determined the vapour 

 densities of synthetically prepared mixtures of methyl-alcohol with acetone or dimethyl-acetal, and 

 arrived at the curious result that the densities of the mixtures differed from the calculated numbers. I 

 have recalculated the numbers from their own data, and arrived at values which agree quite closely 

 with those demanded by theory, i.e., the assumption that the several vapours mix without contraction 

 or expansion. Anybody who cares can easily satisfy himself that I am right. I am glad to avail 

 myself of this opportunity for disinterring a piece of meritorious work which got lost by an unfortunate 

 lapsus calami in the construction of a formula. — Y\ 7 . D. 



