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Eleventh Month... Fine. 2. Cloudy. 3. Fine. 4, 5. Cloudy and fine. 6–8. Fine. 9. Foggy. 10. Cloudy. -11. Morning, foggy. 12. Rain at night: day,' fine.' 13, 14. Cloudy. 15. Morning, fine: night, rainy. 16. Rainy. 17. Cloudy and fine.

18. Cloudy. 19. Cloudy: night rainy. 20. Cloudy. 21. Fine. 22. Cloudy. 23. Cloudy and fine. 24. Fine. 25. Fine. 26. Cloudy. * 27. Cloudy and fine. 28. Rainy. 29. Showers. 30. Showers: overcast.

RESULTS.

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Winds : N, I; NE, 4; S, 1 ; SW, 17 ; W, 3; NW, 2; Var. 2.

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Barometer : Mean height

For the month..

29.860 inches.

29.860

SI)

For the lunar period, ending the 6th
For 13 days, ending the 8th (moon north).
For 14 days, ending the 22d (moon south).

... 30.048

INDI

29-824

46-6500

Thermometer: Mean height

For the month,.
For the lunar period ...
For 31 days, the sun in Scorpio ..

50.616

50.000

Evaporation. .

1.88 in.

Rain.

3.46

thias

mained

then,

Laboratory, Stratford, Twelfth Month, 21, 1822.

R. HOWARD

31:રાદ,

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ARTICLE I.
Experiments and Observations on Indigo, and on certain Sub-
slances which are produced from it by Means of Sulphuric
Acid. By Mr. Walter Crum.

(To the Editor of the Annals of Philosophy.)
SIR,

Glasgow, Jan. 1, 1823. INDIGO may be obtained in a state of tolerable purity by the ordinary process of agitating the yellow liquid, which forms the dyer's blue vat, with common air, till the deoxidized indigo which is there held in solution by lime-water is revived ; and then digesting the precipitate in dilute muriatic acid, to remove a little iron, and carbonate of lime, which it contains. In this state, it is known by the name of precipitated indigo, and must be distinguished from the powder so called by Bergman, a substance to which I shall refer more particularly in the sequel. It generally contains a small portion of sulphate of lime, and, as Dr. Thomson observed, a little resin, which is removable by alcohol,

It has been long known, that when indigo is heated, it sublimes. The first mention of this fact that I have noticed is contained in a work on calico printing, published in 1789 by

a pattern drawer in London, who also gives a method for collecting the sublimate. “The curious may sublime indigo, and thereby procure flowers, as with zinc, sulphur, &c. For

on a small scale, it may be done in a common flask New Series, vol. V.

O'Brien,

experiments

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over a common fire, defending the flask from the contact of the

To prepare these flowers, Chevreul directs* that eight grains of common indigo in powder, in a covered crucible, should be placed upon burning coals; in which case the colouring matter will crystallize in the middle part of the crucible, from which it is to be detached with a feather when cold. I have never been able to obtain any quantity of it in this way; part of it is always discoloured, and the whole of it is condensed upon the ashes, which are in the state of a fine powder, and from which it cannot be detached without contamination.

I succeeded, however, by another method to procure as much as I needed for my experiments, perfectly pure. I used the covers of two platina crucibles, nearly three inches in diaineter, of such a form, that, when placed with their concave sides inwards, they were about three-eighths of an inch distaut in the middle. I placed thinly about the centre of the lower one ten grains of precipitated indigo, not in powder, but in small lumps of about a grain in weight; then, having put on the cover, I applied the flame of a spirit-lamp beneath the indigo. In a short time, this substance partially decomposed, begins to melt

, and the purple vapour to be disengaged, which is known by the hissing noise that accompanies it. The heat is continued till this noise nearly ceases, when the lamp is withdrawn, and the apparatus allowed to cool. Then, on removing the cover, the sublimed indigo will be found planted upon its inner surface, with sometimes a few long needles upon the bottom of the apparatus, which are easily removed from the button of coaly matter that remains. In this way I have generally obtained 18 to 20 per cent. of the indigo employed; a small quantity unavoidably escapes, but I am persuaded that very little more can, by any means of this kind, be obtained. I have made several attempts, by using different forms of apparatus, to prepare this substance on a greater scale ; but have found none that answers so well as that which I have described. When the cover is kept cool, as by means of a wet cloth, none of the indigo condenses upon it. The interior of the apparatus being then comparatively cold, the vapour is condensed before it reaches the cover, falling back, or rather crystallizing upon the cinder. The same effect is pros duced when ashes are formed instead of a fused coal; and pure indigo. when pounded, and common indigo in any shape, always leave a quantity of loose matter.

Those who cannot readily procure precipitated indigo for the purpose of subliming, may find it convenient to combine with my method, one lately given in the Journal de Pharmacie, by MM. Le Royer and Dumas. It consists in spreading about 30 grains of common indigo in coarse powder upon an open silver capsule,

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and applying the heat of a spirit-lamp till all the sublimed indigo is formed upon the surface of the ashes. On repeating this process, I find that ten parts of common indigo yield one of sublimed indigo, which is, however, far from being pure, Ву resublimation in my apparatus, it is again reduced one-half from the loss of its impurities, and some vapour,

I shall here describe what else takes place during the sublimation. Ten grains of precipitated indigo in the apparatus I have described yielded

1.88 grain of sublimed indigo.
6.44 grains of cinder remained, and consequently
1.68 grain of volatile matter escaped.

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Thirteen grains of the same indigo, kept a quarter of an hour at a strong red heat in a small platina crucible, firmly, though not exactly closed, left 7.9 grains of cinder, which is equal to 61 per cent.

In order to ascertain what gases were evolved during this destruction of the indigo, I introduced 5•28 grains into a small glass tube connected with a mercurial trough. On applying the heat of a spirit-lamp, the vapour of indigo was formed, and condensed in the colder part of the tube, but was at last destroyed by repeated applications of heat. A quantity of water appeared in the tube, and 0.96 cubic inch of gas was found in the receiver. On removing the tube, it was found to have lost in weight 0.71 grain, or 13.5 per cent. of the indigo employed. The water that was formed had a disagreeable burnt ammoniacal smell. I found, on analyzing the gas in the receiver (making allowance for the common air of the apparatus, and a small portion of gas remaining in the tube), that it consisted for 100 of indigo of Carbonic acid ...

2.8
Carburetted hydrogen and carbonic oxide. 0.8
Azote.

1.9
The difference between the sum of these and the
loss 13.o was water, with a little ammonia. ..., 8.0

13.5
Sublimed Indigo.
Indigo sublimes in long flat needles, which readily split when
they are bruised into four-sided prisms.

Viewed at a particular angle, they have the most brilliant and intense copper colour; but when lying in heaps, they have a rich chesnut-brown colour; one that would be produced by mixing a very little yellow with a bright but deep reddish purple. Besides these needles, this substance is found in the form of

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plates, much broader than the needles, and extremely thin; twisted sometimes almost into tubes. These appear to the naked eye perfectly opaque. I was not a little pleased, however, with their unexpected appearance, when seen through the microscope. Viewed obliquely, they appear still opaque, and copper coloured like the needles; but when held perpendicularly to the rays of light, they are seen to be transparent, and of a beautiful blue colour, exactly similar to a dilute solution of indigo which has been acted upon by sulphuric acid. Their intensity varieș, according to the thickness of the plate, from a blue just distinguishable from white, to one almost black. The bronze colour which these crystals assume when in heaps is obviously a mixture of the copper colour with this blue.

The vapour of indigo is transparent, and of a most beautiful reddish-violet colour, resembling a good deal the vapour of iodine, but sufficiently distinguished from it by the shade of red. The sublimation takes place at the heat of about 550° Fahrenheit; for the vapour rises at a heat lower than that of melting lead, and requires more than the , melting heat of bismuth. Upon the rough bright surface of the lead, I observed some of the crystals melt while the vapour was rising; but I have in no other circumstances remarked any thing like fusion, till the substance was decomposed. Hence the melting point of indigo, its point of volatilization, and that' at which it is decomposed, are remarkably near each other.

The specific gravity of sublimed indigo is 1.35.

These crystals sublime when heated in open vessels, leaving no residue. In close vessels, the vapour is at first reddishviolet, as in the open air ; but as the heat advances, it acquires a tirge of scarlet; and before it is entirely deconiposed, becomes deep scarlet, and then orange coloured : a quantity of charcoal is at the same time deposited.

Action of Oils.-Of the essential oils, oil of turpentine dissolves at its boiling heat as much indigo as gives it the fine violet colour of the

vapour of indigo, but a slight reduction of temperature is sufficient to precipitate the whole of it. The fixeil oils and fatty substances, as they may be heated to a higher degree, exert a more powerful action upon indigo. None of them that I have tried act upon it at the heat of boiling water ; but when the heat is increased, they gradually dissolve it, acquiring the colour of its vapour much more deep than oil of turpentine does. If the solution be cooled at this period, the indigo precipitates blue. As the heat is continued, mure of the indigo is dissolved, but the colour of the solution begins to change; it gradually inclines to crimson, and has then begun to be destroyed. It is now green when cooled. Heated still more; the solution is of a strong crimson; then becomes orange; and at last, when entirely decomposed, it is yellow, which colour it retains wlien cold.

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