Experimental Researches in Chemistry and Physics/Analysis of native caustic lime

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Experimental Researches in Chemistry and Physics by Michael Faraday
Analysis of native caustic lime

Analysis of Native Caustic Lime[1].

On the native Caustic Lime of Tuscany.

The interesting communication of Dr. Giovacchino Taddei respecting his discovery of caustic lime in the water of the ancient bath of Santa Gonda, in August 1815, induced me to visit the spot. The following is the result of my researches:—

The bath is situated in a laguna in the corner of a field near the high road to Pisa, which divides the plain called La Catena from the mountains of Cigoli and San. Miniato. The soil is a mixture of clay, calcareous earth, siliceous sand, and vegetable matter. There are two sources of water; one issues from the bottom of the laguna, and the other from the side. The first is hot, raising the thermometer of Reaumur to 35¼°. It is so saturated with lime, that upon cooling the water, it deposits a considerable quantity. It contains also muriate of lime and muriate of soda. The upper spring contains a little carbonic acid gas, some sulphuretted hydrogen, and some sulphate of soda. The following is the manner in which the caustic lime is formed in this bath. The lower spring yields a quantity of lime, but as this spring does not rise freely, but oozes through the bottom of the bath, the lime forms a stratum at the bottom of the lagune; which stratum, absorbing the carbonic acid gas of the water above, passes to the state of a carbonate, and thus forms a defence to the lime, which is continually depositing itself underneath, and prevents it losing its causticity. In fact, the caustic lime is found enclosed between the stratum of the carbonate of lime and the clayey bottom of the laguna.

Signor Taddei found the masses of caustic lime so large, that he could not get them out but by breaking them into pieces. He, however, succeeded in removing the whole of it: and I, having visited the spot two months after, found small incrustations of the same substance newly formed.

Analysis of the native Caustic Lime. By Mr. FARADAY, Assistant in the Laboratory of the Royal Institution.

This substance came to England in a bottle filled up with water, the atmospherical air being perfectly excluded.

It is almost entirely soluble in muriatic acid without effervescence, leaving nothing but a few light flocculi. The solution, when tested, was found to contain lime and iron.

A clean uniform piece of the substance was dried, as much as could be, by bibulous paper. A fragment of it being heated red, lost 62·26 per cent. of water.

The remainder of the original substance, weighing 188 grains, was dissolved in muriatic acid, and evaporated at a high heat on the sand-bath, acid was again added, and the evaporation repeated. Water was poured on it, and the silica separated: when well washed, dried, and heated red, it weighed 7·5 grains.

The filtered solution was precipitated by carbonate of potash, and the precipitate boiled in solution of pure potash. The solution was separated from the solid matter, neutralized by sulphuric acid, and precipitated by carbonate of ammonia. The precipitate, when well washed and dried, weighed 0·95 of a grain. It was soluble in sulphuric acid, and possessed the properties of alumina.

Diluted sulphuric acid was added to the solid matter not acted upon by the potash; the whole boiled for some time, and then filtered. The sulphate of lime obtained weighed, after being heated red, 136 grains, which, estimating the lime at 43 per cent., is equivalent to 58·48 grains of lime.

The sulphuric solution was precipitated by ammonia, and two grains of oxide of iron were obtained.

Supposing the quantity of water in every part of the piece first taken to be uniform, it would follow that the 188 grains contained 117·05 of water; so that 70·95 was the quantity of dry matter acted upon. The results were—

Silica 7·50
Alumina 0·95
Lime 58·48
Oxide of iron 2·00

The loss is therefore rather more than two grains, which may, perhaps, actually have taken place, and the difference may have been derived from the unequal diffusion of water throughout the piece.

Supposing 100 parts of the specimen to have been taken, the analysis will stand thus:—

Lime 82·424
Silex 10·570
Iron 2·820
Alumina 1·340
Loss 2·846
It is perhaps worthy of observation, that during the solution of the substance in muriatic acid, a part only of the silica separated; the greater part remained in solution until heat was applied, when it gelatinized, as in the case where it is separated by an acid and heat from its combination with alkali.

Observations on the preceding Paper.
by Sir H. DAVY, V.P.R.I., F.R.S.

The Duchess of Montrose was so good as to send me the caustic lime which is the subject of the preceding analysis; her Grace received it immediately from Tuscany. It was in a bottle, carefully sealed and full of water. Some of the exterior portions had become combined with carbonic acid before they were collected, and from the colour, it appeared that there were different portions of protoxide of iron in different parts of the substance.

On examining the water, it was found to be a saturated solution of lime, and it contained fixed alkali, but in quantities so minute, that after the lime was separated, it could be made evident only by coloured tests.

It appears from Mr. Faraday's analysis, that the menstruum which deposits the solid substance must be a solution of silica in lime-water, and heat is evidently the agent by which the large quantity of lime deposited is made soluble and is enabled to act on silica; and the fact offers a new point of analogy between the alkalies and the alkaline earths.

Vestiges of extinct volcanoes exist in all the low countries on the western side of the Apennines; and the number of warm springs in the Tuscan, Roman, and Neapolitan States, prove that a source of subterraneous heat is still in activity beneath a great part of the surface in these districts.

Carbonic acid is disengaged in considerable quantities in several of the springs at the foot of the Apennines; and some of the waters that deposit calcareous matter are saturated solutions of this substance. Calcareous tufas of recent formation are to be found in every part of Italy. The well-known Travertine marble, Marmor Tiburtinum, is a production of this kind; and the Lago di Solfaterra near Tivoli, of which I shall give a particular account on a future occasion, annually deposits masses of this stone of several inches in thickness.

It is scarcely possible to avoid the conclusion, that the carbonic acid, which by its geological agency has so modified the surface of Italy, is disengaged in consequence of the action of volcanic fires on the limestone, of which the Apennines are principally composed, and liberated at their feet, where the pressure is comparatively small; but the Tuscan laguna offers the only instance in which the action of these fires extends, or has extended, to the surface at which the water collected in the mountains finds its way to the sea, so as to enable it to dissolve caustic calcareous matter.

  1. Quarterly Journal of Science, i. 260.

    I reprint this paper at full length. lt was the beginning of my communications to the public, and in its results very important to me. Sir Humphry Davy gave me the analysis to make as a first attempt in chemistry at a time when my fear was greater than my confidence, and both far greater than my knowledge; at a time also when I had no thought of ever writing an original paper on science. The addition of his own comments and the publication of the paper encouraged me to go on making, from time to time, other slight communications, some of which appear in this volume. Their transference from the ‘Quarterly’ into other Journals increased my boldness; and now that forty years have elapsed and I can look back on what the successive communications have led to, I still hope, much as their character has changed, that I have not, either now or forty years ago, been too bold.—M. F.