79 



as 300° and more. The hydrocarbon-oils are, as a rule, more 

 volatile than those containing oxygen, and the volatility seems to 

 diminish with the increasing percentage of the latter. 



The qiiantitative estimation of volatile oils is very diificult and 

 can only be done approximately. When the oil has been obtained 

 by distillation and spontaneous separation from the aqueous 

 distillate, that portion of the oil which remains dissolved in the 

 water, has also to be accounted for [irrespective of the fact, that 

 the proportionate quantity of volatile oils is subject in the same 

 species to much fluctuation according to the soil, climate and local 

 influences. — F. v. M.], and which is calculated as constituting 

 one-tenth per cent, of the weight of the whole aqvieous distillate, 

 when the oil was a hydi'ocarbon ; and one-fifth per cent, when an 

 oxygenised oil. 



A thorough analysis of volatile oils requires not only a vast 

 quantity of material but also much time and circumspection. 

 After cüstillation the oil is first freed from any water by agitating 

 occasionally and for several days with fused chloride of calcium. 

 It is then poured off" and examined in regard to reaction with 

 litmus-paper, specific gravity, boiling point, and its behaviour in 

 low temperatui'e. 



The determination of the boiling-point is carried on in a retort, 

 not more than half filled with the oil, and through the tubulus of 

 which a thermometer, indicating at least 300° above zero, is 

 passed so as to touch with its bulb the surface of the oil, while 

 the neck of the retort is adapted to a Goebel's or Liebig's 

 condensor. As crude volatile oils are commonly mixtures of 

 different oils, in most instances the boiling point gradually rises 

 during the operation. A difference of only a few degrees is not 

 regarded as of much consequence, but as soon as the boiling-point 

 rises 10° or 20° and remains constant there, the receiver is 

 changed and the first distillate poured into a glass-phial with 

 a well closing glass stopper. Meanwhile, the distillation is 

 carried on, but not quite to dryness, to prevent contamination 

 with empyreumatic products, and each portion of the distillate 

 differing by ten or more degrees, is collected separately. These 

 different oils are then examined separately in regard to their 

 physical properties (specific gravity, colour, smell, taste, fluidity, 

 vapour-density, rotatory power, &c.), and chemical constitution. In 

 cases, where material enough is at hand, the same process of 

 fractional distillation is advantageously rej^eated with each 

 se])arate portion, to secure greater precision of results. 



Some volatile oils are combinations of an acid and a neutral 

 body, and often se2)arate into these different parts during distilla- 

 tion, the neutral portion (mostly a hydro-carbon or an ether) being* 

 more volatile and passing over first, while the acid oil (always 

 oxygenised) distils much later. 



