ELECTRICAL RESISTANCES OF FIXED AND VOLATILE OILS. 17 9 
Table of Oils, 
Arranged in order according as they contract the less refrangible, and expand 
the more refrangible spaces (irrationalities of dispersion ). 
Oil of 
Cassia. 
Oil of Nutmeg. 
59 
Bitter Almonds. 
99 
Peppermint. 
59 
Aniseed. 
55 
Castor. 
59 
Sassafras. 
99 
Nut. 
59 
Fennel. 
99 
Olive. 
59 
Cloves. 
55 
Sweet Almonds. 
99 
Turpentine. 
Alcohol. 
59 
Caraway. 
Although bromine and iodine exert on some of the essential oils chemically 
characteristic effects, it does not appear certain to what extent the action may 
be modified by the addition of small quantities of other oils ; consequently the 
chemical phenomena, as well as a knowledge of their specific gravities and 
boiling-points, cannot be considered as offering any assistance to the detection 
of accidental or intentional impurities when existing in small quantities. 
The process which I have to submit is one which has given considerable 
satisfaction in all the experiments which I have made, and was suggested by a 
discovery due to M. Rousseau, quoted by De la Rive,* “ that olive oil when 
mixed with -^th part its volume of oil of poppies, increased the number of 
vibrations of a magnetic needle in a given time, when the same was included 
or made to form part of a voltaic circuit.” This isolated fact would be of ser¬ 
vice for the determination of the purity of olive oil, if oil of poppies were the 
only sophisticating ingredient. 
I thought it useful to extend the observation to the effects produced by other 
oils when mixed with oil of olive, and to ascertain how far the process might be 
applied as a test for the commercial and chemical valuation of oils generally. 
For this purpose I had first to measure the resistances offered by a column 
of each of the oils experimented on, having in each case the same length and 
sectional area. 
From the low resistances possessed by the volatile oils, the apparatus used by 
M. Becquerel for ascertaining the resistances of liquids might be employed,! 
but from the high resistances offered by the fixed oils I have designed a modifi¬ 
cation. 
I must here acknowledge the obligation I am under to W. Hooper, Esq., for 
the use of Sir William Thomson’s delicate astatic reflecting galvanometers, and 
a battery which possesses remarkable constancy, viz. that of Daniell as modi¬ 
fied by Minotte. 
With such a galvanometer as used for these tests, the deflections obtained 
are strictly proportional to the resistances, and by means of noting the deflec¬ 
tion produced through a constant resistance, by a standard current, it is easy to 
compare the results obtained at different times and under different conditions. 
The standard current represents a known relation to the full electro-motive 
power employed.^ 
* ‘Treatise on Electricity’ (translated by Walker). 
f Since writing this I find the following note in e Paris’s Pharmacologia ’ (1833), under 
the article “ Olive Oil”:—“ M. Rousseau has discovered the curious fact that of all the oils, 
both vegetable and animal, olive oil most feebb r conducts electricity. It may be stated, that 
as a medium it acts 675 times more feebly than the others. Two drops of oil of beechmast, or 
of poppy seeds, poured into ten grammes of olive oil, renders the needle four times more 
sensible. This difference, therefore, furnished M. Rousseau, by means of his diagometer, a 
test for determining adulterations with precision .”—Journal de Pliarmacie , t. ix. p. 587. 
£ See De la Rive’s Treatise, vol. ii. 
N 2 
