them (19) is a support with a hinge, to receive at its upper extremity the crystal to be measured (21); this support slides in a groove (22) in order to bring it nearer to, or further from, the measuring radius, or to withdraw it altogether. The other (20) is a sight-vane, by means of which we can ascertain, after the crystal is fixed on its support, if the edge which separates the two planes, whose incidence is to be measured, be in a perfectly horizontal position. To use the goniometer, we place the crystal on the support, in the position just mentioned, then slide the support along the groove, till the measuring radius lies exactly above it, and turn the graduated moveable semicircle, having previously removed the nonius (12) that its motion may be unobstructed. The rest of the operation consists in placing the arm of the measuring radius very accurately on the plane of the crystal, which is opposed to it. This is done by moving the rule (7) to which the semicircle is fixed; and to do this more easily, and avoid jerks, it is necessary to use both hands, one placed at each extremity of the rule.* If the radius be not placed exactly on the face of the crystal, it is to be gently withdrawn, in order to elevate or lower the moveable semicircle, which is to be done in like manner, by using both hands, seizing with one the extremity of the measuring radius, and with the other, the opposite extremity of the moveable circle, till the measuring radius lies so evenly on the face of the crystal, that no day-light can be seen between them when examined with a lens. That done, the moveable nonius (12) is lowered till it meets the stop (23) placed at zero of the moveable semicircle, and fixed by the screw (17). The support is then to be withdrawn, in order to pass the measuring radius on the other side of the crystal, when it is replaced, and the business finished, by repeating on that face of the crystal the same operation as was performed on the other. The value of the angle of incidence of the two planes, in degrees and minutes, is then read off on the graduated semicircle and its nonius, 7 The instrument is represented in the figure as having finished its operation on one of the faces of the crystal, and with the nonius fixed. The use of this goniometer is very easy; it has the advantage of being fixed, and of not depending for the accuracy of the observations on the manual dexterity of the observer; nor does it require that perfect brilliancy of surface, in the crystal to be measured, which it is often so difficult to find. As the measuring radius, when once placed on the face of the crystal, remains * A regulating screw, with its carriage, was at first used to adjust the motion of the 'rule, as shown in the figure, at the place where it was fixed; but it was found not to be necessary, as the adjustment can be made quite as accurately, and much more quickly by hand, provided the rule itself, and the rollers on which it slides, be made perfectly true. i fixed in that position till the obverser changes it, if his eyes be fatigued by the operation, he can leave it as long as he pleases, and examine the accuracy of his work at a future occasion, or have it verified by an assistant. The great condition necessary, is that the faces of the crystals be perfectly plane, which unfortunately is not always so easily fulfilled as might be wished. 630 Remarks upon Mr. Daniell's Work on Hygrometry. (To the Editors of the Annals of Philosophy.) b 3 (11 As none of the Scientific Journals in their notices of Mr. Daniell's work on Hygrometry make any remarks on a part of that work which, if I mistake not, is erroneous, I beg leave to direct the attention of your readers to the subject. ; A principal object with Mr. Daniell, is the application of his improvements to the correction of the barometer when that instrument is used for ascertaining the heights of mountains but it is not a little surprising that he should have erred in his manner of computing what is commonly the greatest and most important correction required in using the instrument. That to which I allude, is the compensation for difference of fl temperature at the two stations, which Mr. D. considers as a case of apparent dilatation of the mercury, and gives, for the purpose of correction, a table calculated by Mr. Rice, from the results of Dulong and Petit's experiments: now, besides that the last mentioned gentlemen are egregiously wrong in their way of deducing the apparent expansion, it is not only inapplicable to the present case, but is as a standing number quite useless for every other purpose, varying as every one knows. It has always been understood that, other circumstances being alike, mercury in the barometer will have its altitude affected by the existing temperature in no other way than as that tempera"ture alters its specific gravity; so that whether the tube expand or contract, or were it possible, do neither, whatever the material of which it is made, whatever its sectional form, equality or inequality of calibre, still the absolute dilatation and not the apparent must regulate the correction for difference of temperature. After a detailed account of the many operations gone through by Mr. Daniell and his fellow-labourer Mr. Newman, while making a barometer for the Royal Society, during which every thing is done to attain accuracy, we are told that "a scale is engraved on the front, of the correction to be applied for the sd dilatation of the mercury and the mean dilatation of glass, by which the observation may be at once reduced to the standard temperature of 32°;" that is to say, the barometer for the use of the Royal Society so carefully made, is to have its observed height corrected for temperature, by using the constant denominator 11,664 (64.8 × 180) for each degree of Fahrenheit, or by Mr. Rice's table. This number, however, has no connexion with the computation in question, for seeing that Mr. D. is partial to Dulong and Petit, their number 9990 (55.5 × 180) ought to have been Mr. Daniell's choice; and with regard to the mean dilatation of glass, repeated so mal à propos, D. and P. take no mean whatever, asserting its uniformity, and besides do not deduce the number which Mr. D. ought to have used from any such consideration, but on the contrary and conversely, use that number and the apparent dilatation falsely taken, to find that of glass, where they again err. At pages 358-9 examples are given for the various corrections, but all those for temperature are on the same erroneous principle as that engraved on the instrument; and previously, page 183, there occurs one where seems to be taken as the fraction of dilatation. This example I profess not to understand, unless it be intended to show that we may take any number at random for this correction. In taking the liberty of making these remarks on Mr. Daniell's meritorious work, it may be admitted that, though he and Mr. Rice have inadvertently assumed as the fundamental number on a false principle, still it may not be so far from the truth as it might have been, since the absolute dilatation of mercury is given so variously, that in adopting the right principle, Mr. Daniell might have a number still more erroneous. Important as a knowledge of the real amount of the dilatation of mercury would be, in many scientific determinations, in none perhaps is it of such consequence as in this correction for the mountain barometer, respecting which, nevertheless, there is too much cause for regretting that, even in this age of refined experimental knowledge, we are so embarrassed with conflicting authorities, as to be forced to entertain more than a suspicion that the real quantity is still unknown. Mr. Dalton calls it; Dulong and Petit; and General Roy; while philosophers, of equal and undoubted reputation, vouch for the authenticity of almost every intervening denominator, and some for even a greater. If such be the actual state of our knowledge with regard to the quantity of the absolute dilatation of mercury, it is difficult to conceive how Mr. Daniell can so confidently assert, that "this effect has been most minutely appreciated, and its correction applied with the utmost ease and precision." M. Biot fixes this number at, which is generally received in France, and by many here; yet even this celebrated philosopher is wrong, by having made a false conclusion from his own pre mises, Traité, tome i, page 52. From what he there states, a different number ought to have been obtained; and the error affects many of his subsequent formulæ relating to the barometer, expansion of fluids, of gases, &c. In common with many others, Mr. D. seems to think that boiling the mercury in the tube of a barometer is of great consequence; this is at least doubtful, but certain it is that no human art can render a mass of this fluid such mere mercury that it shall not contain something, which in all its mechanical effects may not be called air. Sir H. Davy is in the right when he says so; and it is not a little to his credit to have perhaps by induction inferred that such is the fact, when there is no reason to think him aware of certain proofs to which it were needless here to appeal. Mr. Daniell speaks also of filtering the mercury; though performed a thousand times no good effect can follow this practice. A knowledge of this metal gained from a peculiar application of it, warrants the assertion, that the mercury of commerce is not improvable by either distillation or filteration, in so far as its application is purely mechanical, and that its fitness for barometers can be completely known by bare inspection. There is a probable source of error in the barometer hitherto little attended to, and of which Mr. D. takes no notice; in making the correction for temperature, it has ever been taken for granted, that the expansion proceeds pari passu, or that the fraction of dilatation is, for example, the same from 20 to 30, as it is from 90 to 100. This is, however, quite a gratuitous assumption; and although there are reasons for here suspecting something different from the utmost degree of precision, it is possible that the experimental means which we at present possess are inadequate to ascertain any appreciable discrepancies. Much learning has been brought to bear on the other corrections requisite in using the mountain barometer; and Mr. Daniell could not have conferred a more substantial benefit than by having set the question at rest, as to the absolute dilatation of mercury, without a certain knowledge of which, all other minute attentions are little better than mere drivelling. Mr. Daniell's account of the manufacture of barometers and thermometers is most certainly not overcharged. Throughout the continent, and even in England, the business is in the hands of itinerant Piedmontese; and these artists supply not only the general public with their glittering baubles, but furnish greater part of the most reputable instrument-makers with their whole stock of meteorological wares. Such of these as choose to graduate their own scales, must confide entirely as to the quality of their tubes and the excellence of the filling, in one who has but indirect interest in the matter, or equivocal reputation to lose; responsibility is thus shuffled from both, and rests on neither. Such, however, are the people who by unaccountable prescription supply the city of London, and the philosophers of England, with the instruments which Mr. Daniell so well describes. If common notoriety did not bear Mr. Daniell out in his assertions, the shameful disagreement of the thermometers used by Captain Parry in his last voyage, would fully do so. On one occasion this amounted to no less than 13 degrees; Capt. Parry could do nothing else than give a mean, though in such 48° had as good a chance of being the truth as a case 359. ARTICLE XII. Instructions for the Assay of Chloride of Lime. X.. THE uncertainty which has hitherto existed in the modes of ascertaining the quality, and consequently the commercial value of chloride of lime, and in no small degree retarded its coming into general use, has determined me to publish the following instructions on the subject. I shall divide the work into two parts; in the first I shall expose the principles on which the assay of the chloride of lime is founded, and in the second I shall describe the instrument which I call a Chlorometer, and the manipulations necessary for making the assay with sufficient accuracy for the purposes of those arts in which chlorine is employed. PART I. Principles on which the Assay of Chloride of Lime by means of Indigo is founded. ing & It is known that chlorine destroys vegetable colours, by forming new compounds with their component principles. It is in consequence of this property which it possesses, whether in the state of gas, in solution in water, or in combination with an alcali, that it is employed in the arts of bleaching, calico printThe same quantity of chlorine, in either of those three states, destroys the same quantity of colouring matter; and since by combination with an alcali, it becomes fixed, has scarcely any smell, keeps better, is more portable, and more capable of concentration, the advantages of preparing it in that form are obvious. Caustic potash, soda and lime, and even their carbonates, combine very readily with chlorine. Its combination with the From the Annales de Chimie. |