The detection of spent or partially exhausted leaves in teas is not a simple problem. The chemical methods all depend upon the proportion of certain of the soluble constituents which remains in the tea leaves. A large number of analyses of genuine teas made by different chemists show a very wide range in the percentage composition of teas; hence the difficulty in obtaining a basis for comparison. A genuine tea may contain as little as 10 per cent tannin or even less, and as high as 25 per cent. It is evident from this that the percentage of tannin can not be depended upon for the detection of the admixture of spent leaves. A large quantity of such exhausted leaves Mould be required to reduce a tea containing a high percentage of tannin to the average percentage. The same may be said, in a measure, of each of the soluble constituents of the leaves. Hence, we see the difficulty in securing data of value on this point from a determination of the soluble matters. The only estimation of this kind that is of value, and its value it must be said is but relative, is the percentage of extract matter. A large number of analyses by different authorities shows a range in the extract matter of from 26 to 50 per cent of the air-dried tea. This extract is not the total soluble matter of the prepared leaf, but is the soluble matter extracted under certain conditions which are fully described on page 892.

One of the most valuable series of analyses of genuine teas that have been made is that of Mr. Joseph F. Geisler.1 This chemist obtained the following averages of extract matter:

Formulae have been given, notably by Allen, for the calculation of the proportion of spent leaves from the amount of extract matter. Little reliance can be placed on the results of such calculations, owing to the sources of error already named. If a tea contains less extract matter than the minima given by Geisler, the kind of tea being taken into account, it may be placed on the doubtful list, and if all other conditions agree it may be considered to have been adulterated by the addition of spent leaves. In investigating this form of adulteration the appearance of the leaf must largely determine the chemist's decision. If the leaves are very much broken, frayed, or partly unrolled, there is evidence of spent tea. For the examination, the suspected sample is soaked in water and the softened leaves are unrolled and examined on a glass plate. The following averages of the soluble constituents of genuine teas are given for comparative purposes. The analyses of Geisler are given above, so far as they relate to the extract.

1 Joseph. F. Geisler in Am. Grocer, October 23, 1884; also in Prescott's Organic Analysis, pp. 505 to 512.

Table showing the maxima, minima, and mean percentages of the principal soluble constituents of tea.

The differences between the highest and lowest percentages of each constituent are so marked that little possibility of detecting other than exceedingly gross adulteration, by the admixture of spent leaves, is afforded by these analytical data.

The total ash of genuine teas, according to various authorities, ranges from a little below 5 to nearly 7 per cent, the iron from .08 to .17 per cent, and the silica from .14 to .80.