272 
DR. MARCET ON THE IMMEDIATE PRINCIPLES OF 
dark stain is left on the previously polished platina ; this stain finally disappears, 
leaving no residue. They were found to be capable of combining- with hydrated 
oxide of lead, the compound being insoluble in ether. The salt of lead, previously 
washed with ether in order to remove the oleate of lead, should any he present, and 
then decomposed by sulphuric acid, yielded to the ether an acid solution, which, 
after being thoroughly washed with water in order to separate the free sulphuric acid, 
and then evaporated spontaneously, deposited a colourless crystalline substance, 
This substance was dried under the air-pump over sulphuric acid, then fused on the 
water-bath and introduced into a capillary tube, when on cooling it again assumed 
the crystalline state. Its fusing-point was found to be 52° Centigrade, a temperature 
inferior, it is true, to that of 60° required for the fusion of margaric acid ; the differ- 
ence I believe to be owing to a trace of oleic acid, which remained mixed with 
the crystals. The small quantity of the substance left after the above manipulation, 
prevented me from having recourse to repeated crystallizations, the only process by 
which the fatty acids can be obtained perfectly pure. 
The characters of the fatty acid which I have just described so completely agree 
with those of margaric acid, that no doubt remains in my mind as to the complete 
identity of the two substances *. 
As an additional proof of its presence, a sample of pure margaric acid, which I 
happened to have prepared on a previous occasion, was dissolved in hot alcohol and 
lime-water added to the solution, a compound of lime and margaric acid preci- 
pitated and was collected upon a filter ; the precipitate, suspended in alcohol, was 
decomposed and dissolved by hydrochloric acid, aided by the application of heat, 
and on cooling, the margaric acid was obtained crystallized. I afterwards ascertained 
that, margaric acid was endowed with the singular property of crystallizing from its 
solution in alcohol, when the liquid is heated with just enough water to make it become 
turbid when cold. This circumstance is also a proof that the above fatty acid obtained 
from human evacuations is not stearic acid, as a sample of pure stearic acid treated 
by the same process could not be made to crystallize. The solution retained its 
muddy appearance, the addition of a large amount of water causing an amorphous 
indistinct precipitate. 
It is not possible to state whether a small quantity of the margaric acid obtained 
by the above process does not exist in the form of a compound as a constituent of 
human faeces, but from the very acid reaction of the alcoholic extract of excre- 
ments, it appears most probable that the whole of this fatty acid exists in the free 
state in human faeces as one of their immediate principles. I hope, at all events, to 
have proved satisfactorily that margaric acid is to be found, though not constantly, 
in the human faeces; and from the circumstance of my never having detected mar- 
garine as one of their constituents, I may be allowed to conclude that a process similar 
* According to Lehmann, margaric acid has occasionally been detected in solid excrements by means of 
the microscope. — Physiological Chemistry, vi. p. 108. 
