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The pieces C and c, applied. Distance froin C to c, 500,5

divisions. The word Fortin above.

Reading of Reading of

Divisions to

Corr. for Length of the Tempe- (the microm. The microm. Differ- be deducted Length of the temperature Mètre, the metre rature. at 39,4 of at the brass

from 39,4

Metre. in decimals being at 32', and the scale. pieces.

Jinches.

of an inch. the scale at 62o.

ence.

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The pieces D and d, applied. Distance from D to d, 456,7

divisions. The word FORTIN above.

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The following is the manner in which the correction for temperature was obtained. The expansion of platina according to the experiments of BORDA and others, is ,00000476 parts of its length for one degree of FAHRENHEIT, and as this is the expansion used by the French in adjusting the length of their mètre, it must be employed on the present occasion. The mètre being taken at 32°, the expansion for the difference between this and the temperature of measurement, must be subtracted from the apparent length of the mètre. The English standard temperature is 62°, therefore if the temperature of measurement be under this, the expansion of the scale for such difference of temperature must be deducted from the length of the mètre before obtained. These two corrections are combined in the column entitled “ correction for temperature.” Sir G. SHUCKBURGH's standard scale is of cast brass, and as I could not conveniently determine its actual expansion with that degree of accuracy that would have satisfied me, I have taken for it, the mean result of two experiments made on plate brass, which gave me an expansion of ,0000101 parts of its length for one degree of FAHRENHEIT. The mean of most of the experiments made on the expansion of brass gives ,0000104, and had Iemployed this last number instead of my own, the difference in the length of the mètre would have been utterly inconsiderable.

Supposing then both mètres to be of equal authority, we have for the length of the mètre à traits 39,37076, and for that of the mètre d bouts 39,37081 inches; the'mean of which, 39,37079, may be taken for the length of the mètre in inches of Sir G. SHUCKBURGH's standard scale when each is brought to its proper temperature.

* The length of the mètre compared with Bird's parliamentary standard is 39,37062 inches.

London, November 1817.

VI. A few facts relative to the colouring matters of some vege

tables. By James Smithson, Esq. F. R. S.

Read December 18, 1817.

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BEGAN, a great many years ago, some researches on the colouring matters of vegetables. From the enquiry being to be prosecuted only at a particular season of the year,

the great delicacy of the experiments, and the great care required in them, and consequently the trouble with which they were attended, very little was done. I have now no idea of pursuing the subject.

In destroying lately the memorandums of the experiments which had been made, a few scattered facts were met with which seemed deserving of being preserved. They are here offered, in hopes that they will induce some other person to give extension to an investigation interesting to chemistry and to the art of dying.

Turnsol. M. FOURCROY has advanced, somewhere, that turnsol is essentially of a red colour; and that it is made blue by an addition of carbonate of soda to it; and he says that he has extracted this salt from the turnsol of the shops.

If turnsol contained carbonate of soda, its infusions should precipitate earths and metals from acids.

I did not find an infusion of turnsol in water to have the least effect on solutions of muriate of lime, nitrate of lead, muriate of platina, or oxalate of potash.

Its tinctures, or infusions, consequently, contain neither any alkali, nor any lime ; nor probably any acid, either loose or combined. This is unfavourable to the opinion of urine being employed in the preparation of turnsol.

I put a little sulphuric acid into a tincture of turnsol, then added chalk, and heated; and the blue colour was restored. It appears, therefore, that the natural colour of turnsol is not

, red, but blue, since it is such when neither disengaged acid or alkali is present.

No addition of chalk brought the cold liquor back to a blue colour; the carbonic acid absorbed by it, during the effervescence of the carbonate of lime, being sufficient to keep it red.

Some turnsol was put into distilled vinegar. An effervescence arose; and after some time the acid was become neutralized. On examining the mixture with a glass, there were seen, at the bottom of the vessel, a multitude of grains like sand. It was found on trial that these grains were carbonate of lime; probably of slightly calcined Carrara marble.

When turnsol is treated with water till this no longer acquires any colour whatever, the remaining insoluble matter is nearly as blue as at first.

Acids made this blue insoluble matter red, but did not extract any red tincture. ,

Carbonate of soda did not affect it.

If the vegetable part of this blue residuum is burned away, or it is washed off with water, a portion of smalt is obtained.

On exhaling, on a water bath, a tincture of turnsol, the colouring matter is left in a dry state.

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This matter heated in a platina spoon over a candle, tumefied considerably, as much as starch does, became black and smoked, but did not readily inflame, nor did it burn away till the blowpipe was applied. It then burned pretty readily, leaving a large quantity of a white saline matter. This saline matter saturated by nitric acid afforded crystals of nitrate of potash, and some minute crystals like hydrous sulphate of lime.

Is this potash merely that portion of this matter which exists in all vegetable substances ? or is the colouring matter of turnsol a compound, analogous to ulmin, of a vegetable principle and potash ? Its low combustibility gives some sanction to this idea.

Of the colouring matter of the violet. The violet is well known to be coloured by a blue matter which acids change to red; and alkalies and their carbonates first to green and then to yellow.

This same matter is the tinging principle of many other vegetables : of some, in its blue state; of others, made red by an acid.

If the petals of the red rose are triturated with a little water and carbonate of lime, a blue liquor is obtained. Alkalis, and soluble carbonates of alkalis, render this blue liquor green; and acids restore its red colour.

The colouring matter of the violet exists in the petals of red clover, the red tips of those of the common daisy of the fields, of the blue hyacinth, the holly hock, lavender, in the inner leaves of the artichoke, and in numerous other flowers. It likewise, made red by an acid, colours the skin of several

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