RESEARCHES ON THE VOLATILE IIYDItOCAKDOXS. ] f . 1 



ghall slightly incline towards the body of the retort. If nea ssarv. some picrei of i<-*», 

 which will adhere firmly to the cloth, may be laid along the neck to insure ooiii]»]t*t<- 

 condensation of the vapors during ebullition. While the retort is in this position, 

 ebullition is continued for considerable time, until it ceases to have any affect on tin 

 height of the mercury in the thermometer. The lamp being now removed for tin 

 moment, the neck of the retort is turned down, and quickly connect < d with a Liebig 

 condenser. The lamp being now replaced, the distillation is commenced. 80 soon as 

 the mercury in the thermometer shall have become constant, which will now occupy 

 but a few seconds, the temperatures by the retort thermometer and the side ther- 

 mometer are carefully noted, and also the time at which these observations are made. 

 During the distillation, which is continued nearly to dryn« <. the readings of the thcr- 

 mometers and of the watch are noted at regular intervals, or so often as any appre- 

 ciable variation of the retort thermometer shall have taken place. The average of the 

 several observations, or of those corresponding to the longer intervals of time, apply* 



the corrections for atmospheric pressure and for the upper column of mercury, 

 according to Kopp* is taken for the true boiling-point. I have generally obtained 

 the hydrocarbons so pure that the whole quantity operated upon would distil within 

 the range of 1° of temperature, and not unfrequently within 0°.5. In a few cases, 

 however, when the quantity of material at command would not permit of the attain- 

 ment of so high a degree of purity, the distillation would range over two or three 

 degrees ; in such cases I have generally taken the average of the temperature- corre- 

 ponding to the longest interval of time, as probably representing more nearly the 







used was calibrated, and 



true boiling-point of the body. In stating my results, however, I shall give the limits 

 of temperature within which the distillation was effected. The thermometers em- 

 ployed in the determinations were the best that I could obtain from Fast re of Paris; 



for the temperatures below 100° the 



divided into fifths of a degree. The determinations above 100° were all made with 



one thermometer. 



The method just described differs in some respects from that of Kopp. He objects 

 to the practice of taking boiling-points with the thermometer bulb immersed in the 

 liquid,f on the ground that the thermometer in this condition hardly ever indicates a 



* Poggendorff's Annalen, 18 17, LXXII. 38. 



t 



t man die Kugel des Thermometers in die siedende Fliissigkeit lauehen, ~o zeigt fast nie das Ins.ru- 



Quccksilberfadens to in stetter hiipfeiidcr od 



zittcnder Bewegung ; der auf diese Art gefundene Siedepunkt karm nur 



lire re Grade 



Thermometers 



befi 



vol. ix. 25 



