514 PROFESSOR DITTMAR AND MR C. A. FAWSITT ON 



Even absolute identity of specific gravity of course does not prove chemical 

 identity. What goes a great deal further, as was shown by Regnault, is equality 

 of vapour tension at a series of temperatures ; and as a proof of chemical purity 

 independence of the vapour tension, at a given temperature, of the volume of 

 vapour produced from a given weight of substance. This method we proposed 

 to ourselves to chiefly rely on, and we accordingly employed it pretty exten- 

 sively. The apparatus we used will be fully described in a later section ; mean- 

 while Plate XXXII L fig. 2 may be referred to as giving a sufficient idea of its 

 construction and of the way it is used. 



To test a given alcohol, we either charged one limb of the apparatus with 

 only one or two drops, the other with some 2 c.c. of the preparation, established 

 a convenient temperature (by means of a water-bath) and external pressure, 

 and took the difference of level between the mercury menisci in the two com- 

 municating tubes ; or else we subjected the given alcohol to some kind of 

 chemical fractionation, and compared nearly equal volumes of the two fractions 

 in regard to their tensions at a selection of temperatures. 



Being anxious to avoid everything that might disturb the proximate com- 

 position of a specimen, we at first expressly refrained from boiling off the 

 absorbed air from the samples to be shut up in the tensiometer ; but we soon 

 found that we thus introduced an error which is in general far greater than we 

 had anticipated. After having recognised this error of judgment, we made an 

 attempt at correction for the absorbed air by measuring the tension of a given 

 specimen at two (or more) widely different volumes. Our vapour density 

 apparatus (suitably modified) lent itself well for this purpose. From 1*9-2 c.c. 

 of a certain alcohol, which we supposed to be very pure, were introduced into 

 the tube over mercury ; a fixed -upon temperature was established by means of 

 a water-bath, and the vapour tension then determined at three different volumes. 



We did not succeed in maintaining absolute equality of temperature at the 

 three different volumes, but had sufficient data from previous experiments for 

 determining the necessary coefficient a/Va^ f° r reducing the several observa- 

 tions to a standard temperature. Two such series of experiments on the same 

 specimen gave the following results : — 



First Series : * = 16°-0 C. 





Volume of Vapour 

 in c.c. 



Observed Tension in 

 mm. of Mercury. 



(1) 



16-8 



. 



97-05 



(2) 



66-8 





8310 



(3) 



1290 

 Second Series : t — ll°0 C. 





81-18 



(1) 



8-5 



. 



9574 



(2) 



83-2 



. 



62-50 



(3) 



1299 



. 



6008 



