then moved until its tangent point is ascertained. In order to accomplish this exactly, a good magnifying glass is requisite; also a perpendicular direction from the eye to the scale. Although in such an operation it is supposed that the globules are always orbicular, and of the same specific gravity, this is not the case, because, during the fusion, the globules are compressed by virtue of their gravity, and conserving their flat form after refrigeration; also the degree of density is dependent upon the time allowed for cooling. However, in the limits between which the scale may be employed, the influence of the temperature, as well as the form of the globule, are of no avail, as HARKORT has ascertained from numerous carefully conducted experiments. PLATTNER'S Scale-its use in quantitative Silver Assayings. The examinations for silver, described in the preceding pages, being effected from one decigramme, a standard representing the hundredweight in small operations,-PLATTNER constructed a scale, in order to use it for metallic globules obtained from an assay of this weight. He triturated, therefore, intimately, a quantity of ruby silver, with an equal quantity of a poor calcareous silver spar, and cupelled the mixture; this operation was repeated several times, and constantly gave 122.5 loths, or 5.48 per cent. silver. The assay was then submitted to the Blowpipe, and the same result obtained. The annexed wood engraving-Fig. 48— W is a representation of this scale, divided into 50 parts, the first of which answers to the globule obtained from one decigramme of an ore containing 5.48 per cent. The constant factor L3 is therefore equal to 0.00098 for the division in loths; and 0.00004384, if the content be expressed decimally. 5.48 503 = It is evident that a scale so constructed might be greatly extended, in order to serve for measuring metallic globules of a far greater volume than what is indicated in the foregoing remarks. This, however, is inconvenient; as the weight of spheres varies directly as the cubes of their diameters, the differences of weight for each division of the scale increase considerably,therefore, a button, having a greater gravity than can be ascertained upon any scale, may readily be weighed upon the balance. The divergence of the two lines a b and a c requires a remark. The smaller the divergence is, in general, the less will be the variation in weight for each descending line, and the more accurately the diameters of globules can be compared ; but this is again limited in practice, for if the divergence be so minute that the difference between the diameters of two globules cannot be ascertained, the scale becomes useless. The most convenient divergence has been found to be two-sevenths of a line, in a length of fifty English lines.-The English line is equal to 3.17494 millimetres. The value of the scale is considerably increased by marking at the side of each division the per centage indicated in a metallic globule corresponding to it. For the smallest of these three decimals are sufficient; if, however, the next decimal number exceed 5, the preceding may be increased by 1. A silver globule, when placed upon the scale, is often found not to coincide with any of the perpendicular lines: in this case, the distance of the touching point from the upper line is ascertained; then the difference of weight corresponding to the divisions enclosing the globule is multiplied into the known fraction, and the product added to the weight indicated by the under line. Thus if a silver globule, placed at two-thirds of the distance between 43 and 44, corresponds to a richness of silver comprised between 83.48 and 77-916 loths, or, in decimals, 2:37160 and 2.21545 per cent., the difference of the centesimal value being 0.15615,— this, multiplied by two-thirds, gives 0.10410, which, added to 2-21545, shows the product to be 2:31955 per cent. The exactness of these calculations depends chiefly on the skill exercised in placing the smallest globules upon the scale, to ascertain their diameters. The operator may, however, control his results, by weighing a number of globules obtained from an ore of a mean value, in one lot, upon the balance, after measuring them singly upon the scale, and dividing the whole weight by the number of globules taken. 2. Application of the Scale for determining the weights of Gold Globules, obtained in Quantitative Assayings. It is very easy to comprehend, that upon such a scale as the one just described, small gold globules obtained in cupellation may also have their diameters and weights determined. If gold globules were as much compressed in refining as silver, their contents might be ascertained from the relative specific gravity of the two metals, but the cohesion of molten gold, being much greater than that of silver, prevents it from assuming the flat form that generally accompanies the latter metal. It was therefore necessary to calculate a new factor for W. This, PLATTNER effected, by dissolving 946 milligrammes of fine gold in nitrohydrochloric acid,-aqua regia,-and precipitating the metal with protosulphate of iron, then filtering, edulcorating, desiccating, and igniting the residue. The protosulphate in this instance, is converted into sesquisulphate, and sesquichloride of iron. The metallic gold was next mixed with 14.544 grammes of calcareous spar, and portions of this submitted to cupellation, and also to the Blowpipe assaying. The mixture, 15.5 grammes, contained 946 milligrammes of gold, or 6·103 per cent. The metallic buttons obtained in the cupellation agreed very closely with the above, each weighing 609,-and the globules resulting from the analysis with the Blowpipe weighed, altogether, 30-5, indicating a mean gravity of 6·08 for each, corresponding to 214-5 loths in a cwt. The experiments agreeing, PLATTNER placed one of the globules between the lines a b and a c upon the silver scale, and found its tangent points to be in the middle, between the numbers 46 and 47. Consequently, the fact that a gold globule placed upon this part weighed 6.07 milligrammes, 214-5 loths per cent., establishes the subjoined general equation: w= 13 = |