Then, Sp. Gr. — w — W' Instead of absolute weighings, relative weights may be determined on a scale by the stretching of a spring as in the Jolly balance or by the distance the apparatus sinks in water, as in the hydrometer; the results are ap proximate.
Liquids of high specific gravity such as con centrated solutions of mercuric and potassic iodide or of silver thallium nitrate. or organic liquids •like bromoform or methylene iodide, are often used for quick distinctions between similar appearing substances, one higher in specific gravity than the liquid, the other lower. They may also be conveniently used for cer tain exact determinations, being equally accu rate for minute fragments and coarser mate rial. A liquid is chosen which will float the material; the proper diluent is then stirred in drop by drop until a stage is reached at which the substance, if pushed down, will neither sink nor rise but stay where pushed. The specific gravity of the liquid may then be determined either roughly by dropping in fragments of material of known specific gravity until one is found which just sinks and another which floats, the liquid being of a specific gravity be tween these; or for more accurate determina tion a special balance, such as the Westphal, may be used.
Numerous other non-directional characters, some of which, such as fusibility and elasticity, are susceptible of exact determination, are ap proximately expressed by convenient terms. Fusibility, for instance, is determined in terms of a scale of seven minerals by comparing the effect of the blowpipe flame on small fragments of similar size. Elastic substances are distin guished as elastic and flexible. Tenacity is ex pressed as brittle, sectile, malleable, ductile or tough. The fracture surface is said to be con chotdal, even, uneven, splintery; and terms are used describing taste, odor and the sense of touch.
Certain characters are limited to a few minerals rather than exhibited by all. Such a character is luminescence or the property of emitting light at ordinary temperatures after being subjected to some exciting influence, such as light, friction, X-rays, ultraviolet light or radium.
3. Chemical Minerals are either elements or are formed by the uniting of atoms of different elements in definite propor tions in accordance with the laws of chemistry and for either identification or classification their chemical composition is their most im portant characteristic.
The methods of analyses and the calculation of formula are in general the same as in the analyses of other definite chemical substances. Much attention has to be paid to securing homogeneous material and in general the prob lem is complicated by the fact that most min erals are isomorphous mixtures (or mixed crystals) rather than simple salts.
True molecular formula are not generally determinable. The empirical formula is calcu lated from the analysis. Thus, for instance, beryl : Proportionate Percentage Atomic Number Composition Weights of Groups BeO 14.01 + 25 = .56 AhOs 19.26 + 102 = .189 SRN 66.37 + 60 = 1.106 or closely in the ratio 3BeO.A1,0s.6SiO, or, summing up, Be.A1,(SiOs)e. If the material is an isomorphous mixture, the sum of the ratios of the replaceable elements or groups is considered, for instance, in the sphalerite analysis which follows, the sum of the propor lions in which Zn Fe Cd and Pb are found is 1.040 and the S 1.039.
Pro- Number Per- Atomic por ofcentage Weight tion Groups S 33.25 + 32 = 1.039 Zn + 65.4 == .756 Fe 15.44 + 56 = .276 1.040 I Cd .30 + 112 = .003 Pb 1.01 + 207 = .005 Such a composition could be expressed either by RS or (Zn.Fe.Cd.Pb)S, the letter R being used to represent a varying group of isomorphic or equivalent elements, and the parentheses with periods between the elements to show that the zinc, iron, etc., takes together accompany one atom of sulphur.
The question whether the water given off during heating is due to the destruction of an acid or basic salt or a hydroxide or is more loosely held as so-called water of crystallization, or is present in solid solution or is adsorbed or adhering atmospheric water, is often difficult to answer. Carefully worked-out water curves showing the loss at frequent intervals of tem perature and the rate of loss at each tempera ture will often assist the judgment.