Dr. Kane on the Composition of certain Essential Oils. 447 



I attribute this phainomenou 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 analysed 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 ap- 

 proach to absolute purity in the oil. 



A great deficiency exists in analytical results obtained 

 under circumstances 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 controlling re- 

 sults, 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 cases. Of these 

 few, the synthesis of urea, and that of acetic acid, may be 

 taken as illustrations. 2nd, The method next in complete- 

 ness is the breaking up of the body into other compounds 

 whose composition is already known ; as, formal into formic 

 acid and methylic alcohol ; acetal into acetic acid and vinic 

 alcohol : acetic acid into carbonic acid and acetone. 3rd, The 

 determination of the atomic weight of the body, by the com- 

 pounds into which it enters with other well-known substances. 

 Thus the composition of oxalic aether controls the analysis of 

 sulphuric aether, and the salts of the vegetable alkaloids give 

 the only means of verifying the composition of the base itself. 

 The fourth method of control is limited to such bodies as pass 

 into vapour without being decomposed, and then the density of 

 the vapour should stand in some simple relation to the sum of 

 the densities of the constituents, taken in the atomic propor- 

 tions given by analysis. Thus the discussion as to whether 

 naphthaline was represented by the formula Cg Hg or Cg H3 

 was decided by the vapour possessing a] density immediately 

 following from the former, but inconsistent with the latter. 

 That, however, we must not insist on very simple relations, is 

 shown by the complex numbers found for some of the inor- 

 ganic compounds by Mitscherlich and Dumas. 



Now in examining the composition of the oils, we are de- 

 barred from efficiently exerting any of these methods of con- 

 trol: — 1st, we cannot generate them by synthesis ; 2nd, we do 

 not as yet know their chemical nature sufficiently to break them 



