Jacques Louis de Bournon

“ This reproduction of the carbonate of lime, occasioning it to occupy a more considerable space, compresses the particles of the plaster, which it has been shown had a very loose texture; and this joined to the hardness of the carbonate of lime itself, must impart to the plaster that additional solidity, which, in such cases, it obtains by age.
When, on the contrary, the gypsum includes either quartz, sand, or clay, as these substances undergo no change by calcination, they produce an additional want of contiguity between the molecules regenerated from the plaster ”

“ In these two natural operations, so interesting and at the same time so remarkable, the change which occasions the decomposition and transformation of the substance begins at the surface, without in the least affecting its figure or dimensions. It then proceeds gradually, till the decomposition and regeneration are complete. Specimens of pyrites are found, in which only the surface to a very slight depth is in the state of hepatic iron, as well as prisms of phosphate of lead, which are precisely in the same circumstances. ”

“ Action if Heat. A moderate degree of heat renders such of its varieties as are indeterminate of a dull white colour; but has no such effect on those that are crystallized; yet when the crystalline variety of the salt-works of Bex, enclosed in a mass of compact bardiglione mixed with gypsum and sea-salt, is exposed to heat, it gives both to its crystal and to their fragments whitish tint, frequently accompanied with a pearly lustre. ”

“ This difference in opinion between the Abbé Haüy and myself with regard to the true primitive form of the laumonite may perhaps again give rise to the unjust reproaches which a similar diversity under circumstances nearly alike has already occasioned. By rendering the science responsible for the errors which those who cultivate it may commit, an attempt has been made to insinuate its uncertainty and even its inutility. ”

“ I have a specimen of it that exhibits an interesting fact. Its surface shews the union of an immense number of small rectangular prisms, all, or nearly all, having their planes in a parallel direction; but a very great number of them are very distinct in all the extent of their square surface. This texture appears to have been occasioned by a very slight alteration. The matrix of this bardiglione is an indeterminate massive quartz, to which there adheres a fragment of foliated micaceous rock, mingled with galena and some iron pyrites. ”

“ But in the transition of bardiglione, now in question, there is no decomposition indicated, no loss of any principle, no replacing of one principle by another; it is simply the fortuitous introduction of a new principle, which, as far as it appears, has but an extremely weak affinity for the molecule of the substance, which however it must divide into its constituent molecules, since it has to form with them integrand molecules of a different figure belonging to a new compound. ”

“ Bardiglione of a determinate Figure.
The crystalline state is that in which this substance most commonly occurs; but its forms are seldom determinate. The finest crystals, and the greatest variety, have been found in the mine of rock-salt at Hall in the Tyrol; which is also the place where it has been most commonly observed of a determinate crystalline figure. The crystals occur there in groups, either colourless, or of a pale red slightly inclining to violet. They are commonly pretty large, frequently flattened, and often of very little thickness. ”

“ Hence we em readily explain the cause why plaster, when in contact with water, absorbs this liquid, and acquires solidity; while bardiglione, in similar circumstances, undergoes no change. From what has been said of the effect of calcination on the integrand molecules of gypsum, it appears that the moment when plaster is brought into contact with water, the molecules of the fluid are attracted into the vacuities of similar shape with themselves by the compound attraction of the bare surfaces of the constituent molecules of lime and sulphuric acid, and are fixed there anew. ”

“ A certain quantity of the water is absorbed, and after the evaporation of the superabundant part of this liquid, which is promoted in some degree by the caloric evolved in the operation, the mixture acquires solidity; still however retaining a slighter cohesion between its particles than existed between those of the gypsum previous to its calcination, and without receiving any degree of transparency. In the transition of Plaster to this state of solidity, the gypsum has hitherto been said to resume its water of crystallization ”

“ It is of a deep grey at the salt-works of Bex, and of a bluish tint at Vulpino, which has occasioned some mineralogists to confound it with the bluish variety of sulphate of strontian.
Electricity. The transparent violet-coloured bardiglione of Hall is pretty strongly electrical by friction. That of the salt-works of Bex is likewise electrical, but in a less degree. I could not find this property in any of the other varieties of this substance: even in that of Hall the intensity of the electricity varies considerably; for in some pieces it is very weak, and is excited with difficulty. ”

“ From the moment that the laumonite is exposed to the atmosphere, it begins to alter, and this alteration, as well as the progress of it, is proportional to the warmth of the air; it advances with such rapidity when the temperature is high, that having inadvertently undertaken during a very hot day the examination of its crystals, many of these, in which the alteration, although begun, was not yet very considerable, so quickly disintegrated that it became impossible, even in the short space of time necessary for their examination, to touch them without reducing them to powder. ”

“ Let us allow it to proceed towards this point, without obstructing its course. Difference of opinion when maintained with candour and decorum is perhaps not without advantage to the security and promptitude of its progress. ”

“ In this mass different cavities are perceivable, in which the gypsum, likewise coloured red, is in small crystals; and some of these also include sea-salt, perfectly pure, and of a red colour. The crystals of bardiglione which are scattered through this mass, while they remain adhering to it, appear themselves reddish, on account of their transparency; but, when they are detached, their colour is a deep grey. This variety comes from the salt-works of Bex, and is that which I have already mentioned as having its colour destroyed by heat. ”

“ As the integrand molecules of gypsum, in their regeneration from plaster, can only crystallize, as has been said, in a very irregular and confused manner, the space occupied by the whole mass, after this reproduction, must be in proportion to this irregularity. Accordingly it is found that plaster acquires a very evident increase of bulk, a circumstance that is often very detrimental in the purposes for which it is employed ”

“ I have a variety from Somma in very fine short hexahedral prisms, which, if viewed perpendicularly to their axis, are of a green, more or less deep in proportion to their thickness ; but when viewed parallel to that axis, through their sides, are of a very deep reddish orange yellow. A specimen in my collection, likewise from Somma, and perhaps unique for the beauty of the very bright, slightly greenish-yellow topazes it includes, contains small crystals of mica, in incomplete acute hexahedral pyramids, very transparent, and having the lustre of the hardest stones ”

“ But is all the water, that may be included in these mineral substances, to be so considered? Certainly not. In many of them the water is foreign to their substance, and has entered merely in consequence of the attraction (to which I have given the name of attraction by approximation) more or less powerful, exerted upon it by their integrand molecules; in this case it is only imbibed and interposed between them, but does not act, in any degree, the essence of the substance containing it ”

“ It is obvious, that the one does not necessarily exclude the other, for they are totally unconnected with each other. It is obvious too, that we cannot consider the water, which a substance may contain, as making one of its component principles, unless there he solid reasons for such a conclusion. For instance, in the case in question, the combination of lime with sulphuric acid gives rise to rectangular tetrahedral prismatic molecules with square or rectangular bases ”

“ I conceive it to be the product of the retrogradation by five rows in breadth and four laminæ in height on the edges of the prism with square bases. The angle it forms with the primitive planes would consequently be 128° 40′. I place this crystal here anew, under the aspect proper to it, because it appeared to me better adapted than any other, to set in its true light the opinion of the Abbé Haüy, as well as my own; and may more easily enable the reader to decide between the two. Whichever may be adopted, the crystals represented will remain the same ”

“ This will always be the case, while authors endeavour to give mineral substances a significant name; it being sometimes taken from a quality supposed to be perceived for the first time, but soon after shewing itself to be too general to designate one substance. in particular; and being at other times founded on a quality merely peculiar to the individual, or accidental and of which a great number of the individuals that compose the species are afterwards found to be destitute. ”

“ If, in order to find the measure of the angles of these rhombs, different angles be placed on them till their sides apparently coincide, an angle of 100°, or nearly so, appears to agree very well with the obtuse angles, so that the rhombs have apparently 100° and 80° for the measures of their angles. ”

“ This fresh example, if it were necessary, might serve to shew the difference which exists between the retrogradations on primitive crystals, where the geometrical form is one in which all the pants are perfectly symmetrical, and those on primitive crystals, in which the parts are not so circumstanced. A symmetry is established between the new planes produced on the first, while the planes which occur in the second, are more or less distant from it. ”

“ Very small dodecahedral crystals with pentagonal faces of iron pyrites, are disseminated through its substance. Small globules of compact bardiglione, very easily separable from the substance in closing them, are likewise included in it. These little globules are destitute of the brilliant laminæ observable in the surrounding mass, are of a brown colour, and appeared to me larger and more numerous in the grey compact variety, than in the reddish. I observed similar globules in one of the varieties of bardiglione, consisting of small blue laminæ, from Vulpino. ”

“ I am indebted to M. Gillet de Laumont for another very fine specimen of this mineral which I have received since that mentioned in the first part of this paper, in which the crystals of laumonite are very large and regular and beautifully grouped with crystals of carbonate of lime. The base of this specimen bears very evident marks of its having been detached from the schist which I have just described. ↑ In all the shove specimens, however, of this substance the rapidity of its spontaneous efflorescence is less remarkable than in those from Huel-goet. ”

“ The circumstances that probably led the Abbé Haüy into an error, are the different varieties of crystals I have mentioned as forming the aggregations to which the fasciculated masses of the laumonite belong, but which, when this substance has undergone any alteration, are detached with much facility, producing so many isolated crystals, a great number of which present varieties differing one from the other. ”

“ This crystallisation generally appears in the state of crystalline masses, often of considerable size, and which at first sight present only a deeply striated or fasciculated surface; but these same masses, which apparently have no determinate crystalline forms, on being broken become immediately a rich field of observation by the great quantity of extremely perfect and diversified crystals, which the portions into which they divide, afford. ”