§ 322 .] MELTING-POINT—IDENTIFICATION. 265 
given in text-books, because the latter melting-points are not determined 
in the same way. The usual method of determining melting-points is 
to place a very small quantity in a thin glass tube closed at one end ; the 
tube should be almost capillary. The tube is fastened to a thermometer 
by means of platinum wire, and then the bulb of the .thermometer, 
with its attached tube, is immersed in strong sulphuric acid or paraffin, 
contained in a flask. The thermometer should be suspended midway 
in the liquid and heat carefully applied, so as to raise the temperature 
gradually and equably. It will be found that rapidly raising the heat gives 
a different melting-point from that which is obtained by slowly raising the 
heat. During the process careful watching is necessary: most substances 
change in hue before they actually melt. A constant melting-point, how¬ 
ever often a substance is purified by recrystallisation, is a sign of purity. 
§ 322. Identification by Organic Analysis. —In a few cases (and in 
a few only) the analyst may have sufficient material at hand to make 
an organic analysis, either as a means of identification or to confirm 
other tests. By the vacuum process described in Foods , in which 
carbon and nitrogen are determined by measuring the gases evolved 
by burning the organic substance in as complete a vacuum as can be 
obtained, very minute quantities of a substance can be dealt with, and 
the carbon and nitrogen determined with fair accuracy. It is found in 
practice that the carbon determinations appear more reliable than those 
of the nitrogen, and there are obvious reasons why this should be so. 
Theoretically, with the improved gas-measuring appliances, it is 
possible to measure a c.c. of gas ; but few chemists would care to create 
a formula on less than 10 c.c. of C0 2 . Now, since 10 c.c. of C0 2 is equal 
to 6*33 mgrms. of carbon, and alkaloids average at least half their weight 
of carbon, it follows that 12 mgrms. of alkaloid represent about the small¬ 
est quantity with which a reliable single combustion can be made. 
The following determinations may also be of service occasionally in 
identifying the alkaloids. 
Oxygen .—The majority of the alkaloids contain oxygen, but there 
is no oxygen in coniine, methylconiine, y-coniceine, nicotine, nicotinine, 
nicoteine, nicotelline, sparteine, lupinidine, curarine, conessine, aribine, 
adenine, and hymenodictine. 
Methoxyl groups may be determined by Zeisel’s method, which 
consists in boiling from 0-2 to 0-3 gramme of the substance with 10 c.c. 
of hydriodic acid of sp. gr. 1-68, and passing the methyl iodide formed 
into a solution of silver nitrate. From the weight of silver iodide formed, 
the number of methoxyl groups may be calculated ; one molecule of 
silver iodide = one methoxyl group. There is one methoxyl group 
in quinine and codeine, two in hydrastine and brucine, three in narcotine, 
four in papaverine and aconitine, and six in pseudaconitine. 
Nitrogen .—By a modification of Zeisel’s method the methyl groups 
