RESEARCHES ON THE VOLATILE HYDROCARBONS. 1&9 



in many cases so considerable, as to make it appear almost useless to attempt further 

 generalizations upon those unreliable data. It may be hoped, however, that the supe- 

 rior means which my process furnishes for separating mixtures of liquids, will lead to 

 the accumulation of reliable facts of sufficient number and variety for a profitable 

 review of this question in its different bearings, which, from its importance, it clearly 

 merits. 



The frequent inaccuracy of the determinations of boiling-points, upon which Kopp 

 has justly laid so much stress, may, I think, be more reasonably attributed, at least in 

 a majority of cases, to a want of purity in the substances themselves, rather than to a 



- 



neglect of the precautions and corrections which he recommends to be observed in 

 such determinations; although errors as great as those mentioned by Kopp* mny 

 doubtless occur, and in the particular instances which lie had in mind may have 

 occurred from the cause which he assigned for them. It should be borne in mind, 

 however, that these errors, in the case of an impure substance, may be compensating 

 errors ; or, on the other hand, they may go to increase that which would arise from 

 impurity. 



That the conditions under which my results have been obtained may be clearly 

 understood, and hence the value of these results fairly estimated, in comparison with 

 those of others, I shall endeavor, as I proceed with these researches, to specify, in 

 sufficient detail, the processes which I have employed. Having, in the memoir pre- 

 viously referred to, described the process by which the hydrocarbons were separated, 

 the special object of this paper only requires, in this regard, that I should add a 

 description of the method employed in determining the boiling-points of these bodies, 

 which has already been partially given in the foregoing memoir, when treating of the 

 boiling-point of benzole. 



Of the Method of determining Boiling-points. — I use for this purpose a small tubulated 

 glass retort, and usually operate on about 150 c. c. of the liquid. The thermometer 

 extends into the liquid, even nearly to the bottom of the retort, taking care that the 

 bulb shall not come in contact with the glass, but remain free in the liquid. To pre- 

 vent abnormal elevation of temperature from adhesion to the glass, — which I have 

 observed in some instances, when operating on impure hydrocarbons, to amount to 

 several degrees, — I introduce pieces of sodium, instead of platinum, as it seems to 

 serve at least as well for this purpose, and at the same time tends to preserve the 

 purity of the material. Sodium has also this advantage over platinum for hydrocar- 



• « Bestimmung des Siedepunkfc." PoggendorTs Annalen, 1847, LXXII. 38. 



