146 Dr. Kane on the Composition of certain Essential Oils. 



I must not be understood as stating positively this formula to represent 

 the truth. 



ADDITIONAL REMARKS. 



There is a peculiarity in the method of ebullition of these oils which renders 

 it very difficult to fix upon a certain fixed temperature as the boiling point, but 

 rather compels vis to consider the oil as boiling within a limit of temperature, some- 

 times extending to ten degrees of Fahrenheit. Thus, in taking the boiling point 

 of an oil in a tube, a thermometer being immersed therein to some distance above 

 the bulb, the oil will enter into full ebullition apparently at 355°, and the tem- 

 perature of the thermometer, on continuing the boiling for five or ten minutes, 

 will gradually rise to 360° or 365°, and will not then stop so completely, but that 

 an ebullition continued for five or six minutes more, may produce a further rise 

 of a couple of degrees. If the oil be allowed to cool, and be then again heated, the 

 same phenomenon will be repeated, and so, as often as may be wished ; but the 

 most colourless oil, when thus frequently heated, gradually becomes brown, and 

 then there is a permanent elevation of the boiling point, arising from decom- 

 position. 



I attribute this phenomenon to an unequal distribution of heat through the 

 mass, and to the heat being supplied by the spirit lamp too rapidly to be carried 

 off from the oil by the vapour formed at the limited surface of contact of the oil 

 with the air in the tube. This is supported by the fact, that by moderating the 

 heat the boiling point may be kept constant ; but, by a suitable heat, it may be 

 kept constant at any degree, between the limits already alluded to. This is the 

 reason why the boiling points of the oils analyzed are generally given within a 

 limit of a few degrees. Some cases where the boiling points were almost really 

 constant, I attribute to a closer approach to absolute purity in the oil. 



A great deficiency exists in analytical results obtained under circum- 

 stances such as those described in the present paper, from the total want of a 

 control over their exactitude ; and one or two words on the nature of these con- 

 trolling results, with reference to obtaining such in our experiments, may be here 

 of use. There are four modes of control — 1st, by synthesis, which is the most 

 complete, but which in organic chemistry is attainable only in some very few 



