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different characters. Specimen A was a fine crystal obtained from Mr. D. Howard. Specimen B was a part of fine crystals made at Millwall. Specimen C had been obtained by recrystallising B in the laboratory. All the crystals were freed from any adhering moisture with blotting-paper, and then powdered. Of A, 3 grams, and of B and C 1 gram were taken. Specimens B and C were placed in the same desiccator; A in another, standing by the side. Both desiccators contained fresh oil of vitriol, sp. gr. 1.845. In the following table will be found a selection from numerous successive weighings of the three specimens. The experiment commenced on April 21st, 1880:TABLE I.-Gradual Loss of Water by three Specimens of Citric Acid Crystals in Dry Air.

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The theoretical amount of water in citric acid crystals is 8.57 per

cent.

The very different degree of tenacity with which water is held in different citric acid crystals is probably connected with some difference in the structure of the crystals, determined by the conditions prevailing during crystallisation. It is clear from the above results that when the molecular structure has been weakened by the loss of some water, the crystal parts with a considerable portion of the remainder with comparative ease; the last portions of water are, however, lost with great slowness.

Determinations of Citric Acid in Lemon and other Juices.

The value of lemon juice has usually been determined from its acidity. Besides the free acid a certain amount of combined organic acid is also present (Chem. Soc. J., 1875, 931). The whole of the free and combined acid in lemon juice is not, however, citric acid. The

method employed in the Millwall Laboratory for determining the amount of citric acid present, or rather the amount of organic acid capable of yielding an insoluble calcium salt, has been already described (ibid., 934). The juice is exactly neutralised with soda, an excess of calcium chloride added, and the calcium citrate precipitated by boiling in a salt or glycerin bath. The precipitate is collected and washed with hot water. The filtrate and washings are neutralised with dilute ammonia, and concentrated in the bath to a small bulk; the second precipitate thus obtained is also collected and washed. Mr. Grosjean was in the habit of concentrating yet a second time, and collecting a third precipitate, if any. The precipitates are finally ignited, and the citric acid originally present calculated from the neutralising power of the resulting carbonate of calcium. Working in this way, with 1 gram of citric acid, Mr. Grosjean obtained 99.6 per cent. of the acid taken.

In the following table will be found determinations of free acid, and of precipitable acid, made by Mr. Grosjean in commercial samples of concentrated lemon and other juices; the acid found is in all cases calculated as citric acid :—

TABLE II,-Determinations of free and precipitable Acid in concentrated Lemon and other Juices.

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It appears from the above figures that the precipitable acid in commercial concentrated Lemon juice is, on an average, very nearly equal

in quantity to the free acid present; in 65 analyses, representing 895 pipes, the precipitable acid averages 99.2 per cent. of the free. There is, however, occasionally a somewhat considerable range of variation, as will be seen from the figures showing the highest and lowest proportion of precipitable acid found in the 65 samples. In two exceptional samples of juice, A and B, representing together 27 pipes, the proportion of precipitable acid is much lower; and in one sample, representing 25 pipes, not mentioned in the table as it seemed to lie intermediate between lemon juice and bergamot, the precipitable acid was only 81.1 per cent. of the free. There can be no doubt, therefore, that the method of precipitation ought to be adopted in all valuations of commercial lemon juice.

The analyses of concentrated Bergamot juice are much fewer; the proportion of precipitable to free acid appears very similar to that in lemon juice.

There are only two analyses of concentrated Lime juice; both show a distinctly low proportion of precipitable acid, a result confirmatory of the analyses already published (Chem. Soc. J., 1875, 936).

The two specimens of concentrated Orange juice show, as we should expect, a very small amount of free acid. This acid is also to a considerable extent not citric, the acid precipitable as calcium salt bearing a smaller proportion to the free acid than in any of the other juices analysed.

The figures in the table clearly show that specific gravity is no guide to the value of juice. All lemon juice is brought as nearly as possible to the standard gravity of 1:24 for the purposes of sale.

Influence of Heat on Solutions of Tartaric Acid.

That crystallised tartaric acid when fused at 170° is converted into metatartaric, and at a higher temperature, or by longer heating, into ditartaric acid, is well known. According to Gmelin, the change into metatartaric acid may be effected in the presence of a small quantity of water; 2 or 3 parts to 60 of acid. Some of my own experiments with the tartaric liquors of the factory (Chem. Soc. J., 1875, 988) had made it probable that the same change may to some extent occur when solutions of tartaric acid are concentrated in the ordinary way over steam, as it was found that two such liquors, when diluted and boiled for three hours, yielded 3.36 and 3.68 per cent. more of tartaric acid than before boiling, Mr. Grosjean has now amply proved that solutions of tartaric acid, concentrated on a waterbath, may undergo conversion into metatartaric acid to a very considerable extent. He has also shown that in a dilute solution, the reverse change, even in the cold, will slowly take place.

I. 40 grams of tartaric acid were dissolved in water, concentrated

in a beaker over a water-bath till a crust formed on the surface, and maintained four hours in this condition; the temperature of the tartaric solution was 93°. When cold, the whole was redissolved and brought to a litre. The acidity was now 97.9 per cent. of the original; while the tartaric acid, found by precipitation with citrate of potas sium, was only 74.6 per cent. of the tartaric acid taken. On boiling a portion of the diluted solution for two hours, the acidity was raised to 99.9 per cent., and the tartaric acid also to 99.9 per cent. of the original acid, showing that a complete reversal of the reaction had been effected.

The remainder of the diluted solution which had not been boiled was mixed with a little phenol, and the amount of tartaric acid determined from time to time with the following results:

:

Aug. 24, 1881, tartaric acid 74-6 per cent. of original acid.

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The experiment now ceased for want of material.

II. 80 grams of tartaric acid were treated as before, the heating being continued 3 hours after a crust had formed; the temperature of the solution was 92°. When cold, the beaker was weighed. Assuming that it still contained 80 grams of tartaric acid, the water present was 24.75 grams. Redissolved and diluted to one litre, the acidity was found to be 97.7 per cent. of the original tartaric acid. Phenol was added, and the proportion of tartaric acid determined from time to time in the unboiled solution, with the following results:

Sept. 23, 1881, tartaric acid 84.3 per cent. of original acid.

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It will be noticed that the reconversion in the cold took place at first with some rapidity, but afterwards at a diminishing rate. In neither experiment is it absolutely completed, although in the last experiment the observations extended over 137 days.

With tartaric liquors containing a considerable amount of sulphuric acid, Mr. Grosjean found a smaller production of metatartaric acid on heating. The following experiment, save for the presence of sulphuric acid, is a duplicate of Experiment II.

III. 80 grams of tartaric acid were dissolved in water, with 16 grams of oil of vitriol; the solution was heated in a beaker on the water

bath till a crust formed; heating was then maintained for 3 hours; temperature, 92-94°. When redissolved the acidity had diminished to 97.6 per cent., and the tartaric acid to 90-2 per cent. of the tartaric acid taken.

Mr. Grosjean has further confirmed my earlier results respecting the presence of metatartaric acid in factory liquors. In one liquor he obtained an increase of 64 per cent. of tartaric acid by dilution and boiling; while in another old liquor the increase following this treatment was 7.6 per cent. In a third instance the metatartaric acid did not amount to more than 1 per cent. of the total tartaric acid. Ditartaric acid was not found in any case.

Influence of Sulphuric Acid on the Crystallisation of Tartaric Acid.

The presence of much free sulphuric acid considerably diminishes the solubility of tartaric acid at ordinary temperatures. When to a saturated aqueous solution of tartaric acid half its volume of oil of vitriol is added, and the mixture cooled and stirred, an abundant separation of tartaric acid takes place. There is also apparently a greater difference between the solubility at high and low temperatures when sulphuric acid is present. A hot saturated solution of tartaric acid in water will deposit about 50 per cent. of its tartaric acid on standing in the cold a few days. A hot solution of tartaric acid in 1 volume of water, and 14 volume of brown oil of vitriol (sp. gr. 1-713), deposited 70 per cent. of its tartaric acid on standing. The mother-liquor, amounting to 260 c.c., contained only 30 grams of tartaric acid. The free sulphuric acid, always present in crude tartaric liquors, though in other ways injurious, thus allows a greater proportion of the acid to be obtained by a single crystallisation. The above observations were made in 1881.

XLIV.-Note on a Basic Ammonio-copper Sulphate.

By SPENCER UMFREVILLE PICKERING, B.A. Oxon, Chemical Lecturer at Bedford College.

IN a paper on the basic sulphates of copper (Chem. News, 47, 182), the author stated that on dilution of a solution of ammonio-copper sulphate a basic precipitate is thrown down, containing only traces of ammonia. This holds good for solutions of the strength there named (0.02165 grams CuO per c.c.); with stronger solutions, however, the results are different.

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