_d. Ash insoluble in acid._--The ash insoluble in water is boiled with dilute hydrochloric acid, and the residue separated by filtration, washed, ignited, and weighed. In pure tea, the remaining ash ranges between 0·3 and 0·8 per cent.; in faced tea, or in tea adulterated by the addition of sand, etc., it may reach the proportion of 2 to 5 per cent. Fragments of silica and brickdust are occasionally found in the ash insoluble in acid. _The Extract._--Two grammes of the _carefully sampled_ tea are boiled with water until all soluble matter is dissolved, more water being added from time to time to prevent the solution becoming too concentrated. The operation may also be conducted in a flask connected with an ascending Liebig’s condenser. In either case, the infusion obtained is poured upon a tared filter, and the remaining insoluble leaf repeatedly washed with hot water so long as the filtered liquor shows a colour. The filtrate is now diluted to a volume of 200 c.c., and of this 50 c.c. are taken and evaporated in a weighed dish until the weight of the extract remains constant. Genuine tea affords from 32 to 50 per cent. of extract, according to its age and quality; in spent tea the proportion of extract will naturally be greatly reduced. Mr. Allen employs the formula below for determining the percentage of spent tea E in a sample, R representing the percentage of extract found. E = ((32 - R)100) / 30. In order to test the practical value of this equation, a sample of black tea was mixed with 50 per cent. of spent tea-leaves, and a determination made of the extract afforded. The calculated proportion of spent tea was 44 per cent., instead of 50 per cent. It should be added, however, that the tea taken subsequently proved to be of a very superior quality, yielding an extract of 40 per cent. _Gum (Dextrine)._--The proportion of gum contained in genuine tea is usually inconsiderable. Its separation is effected by treating the concentrated extract with alcohol, allowing the mixture to stand at rest for a few hours, and collecting the precipitated gum upon a tared filter, and carefully drying and weighing it. As a certain amount of mineral matter is generally present in the precipitate, this should afterwards be incinerated and a deduction made for the ash thus obtained. A more satisfactory method is to treat the separated dextrine with very dilute sulphuric acid, and estimate the amount of glucose formed by means of Fehling’s solution (see p. 37); 100 parts of glucose are equivalent to 90 parts of dextrine.