Of late petrographers have begun to demand, with considerable reason, an arrangement "which shall bring the essential chemical features both the percentage figures and the molecular ratios-prominently and compactly before the eye, so that the general chemical character and the relations of the various constituents may be seen at a glance."

In accordance with this demand it is now our practice to follow pretty closely the arrangement proposed by Pirsson and very recently strongly advocated by Washington (loc. cit.), namely:

SiO2, Al,O,, Fe,О, FeO, MgO, CaO, Na,O, K,O, H2O (above 105110° C.), H2O (below 105-110° C.), CO2, TiO„, ZÃO, PO, SO,. CI, Fl, S (FES), Cr,O,, V2O, NiO, CoO, CuO, MnO, SrO, BaO, Li̟2O, C. NH3.

By this arrangement the nine constituents which in the great majority of cases determine the character of the rock are placed at the head of the list, thus greatly facilitating the comparison of different analyses similarly arranged, especially when, as Washington recommends, the molecular ratios are calculated for these leading constituents and placed immediately after the corresponding oxides. The order of the remaining members is determined somewhat by the following considerations: CO, is placed next after H2O, since these two are generally a measure of the alteration the rock may have undergone. TiO, and ZrO, naturally follow CO, on chemical grounds, and SO, and Cl, being common constituents of the sodalite group, are conveniently placed together.

IV. TIME Needed FOR MAKING AN ANALYSIS.

The question has often been put, "How long does it take to complete an analysis of this kind?" This will depend, of course, on the mineral complexity of the sample and on the personal factor of the individual worker. If there is a competent assistant to do the grinding, and specific-gravity determinations are not required, it is quite possible after long experience for a quick worker to learn to so economize every moment of time in a working day of seven hours, with an abundance of platinum utensils and continuous use of air and water baths through the night, as to finish every three days, after the completion of the first analysis, barring accidents and delays, one of a series of rocks of generally similar character, each containing from eighteen to twenty quantitatively determinable constituents, excluding, for instance, fluorine, carbon as such, nitrogen, metals of the hydrogen sulphide group, and cobalt. On one occasion a series of fourteen rocks, of comparatively simple composition, was completed in one month, with the help of an assistant who made the phosphorus

H. S. Washington, The statement of rock analyses. Am. Jour. Sci. 4th series, Vol X, p 61, 1900

and ferrous iron determinations. But such an output of work is more than exceptional and implies an unusual freedom from those occasional setbacks to which every chemist is exposed.

It should here be remarked that the Survey laboratory is most exceptionally well supplied with all kinds of platinum vessels and utensils, so that it is rare indeed for delay to arise through lack of dishes of even the largest sizes.

V. TWO USEFUL AIDS IN CHEMICAL MANIPU-
LATION.

In connection with the foregoing remarks it is in place to mention two aids to the chemist which are in constant use in this laboratory and have come to be well-nigh indispensable. Neither is novel in principle and both are in use elsewhere, but they are not so commonly known as they deserve to be, hence this allusion to them.

ago.

Fig. 1 represents a form of platinum-tipped crucible tongs devised by Dr. A. A. Blair many years With them a crucible can be securely grasped and brought into any desired position while still hot. To the contents, if in fusion over the blast flame, can be imparted the rotatory motion so often desirable. Above all, the cover need not be in the slightest degree displaced, as when using the common form of platinum-tipped tongs.

Fig. 2 represents a very useful adjunct to the worktable and especially to the draught cupboard, whereby the liquid contents of crucibles can be speedily evaporated at almost any desired temperature and the dehydration of many solids effected much more safely than on an iron plate or sand bath. I do not recall who originated this form of air bath, but it has been in use here for over fifteen years and is identical in principle with the Nickelbecher of Jannasch. Nickel undoubtedly has a certain advantage in not rusting as does iron, but the form depicted in R of fig. 2 can easily be made anywhere of sheet iron riveted at the joint, the bottom (not shown in the figure) being securely held by a flange at the extremity of the truncated cone. A crucible placed on the platinum triangle becomes uniformly heated by hot air, and large quantities of liquid, even sulphuric acid, can be thus volatilized in a short time without ebullition or spattering.

FIG. 1.-Platinum-tipped crucible tongs. The parts A B, also of heavy platinum, are hollow

to serve as sockets for

the cheaper metal of the handles.

VI. LIMITS OF ALLOWABLE ERROR IN SUMMATION OF ANALYTICAL RESULTS.

As is well known, a complete silicate rock analysis which foots up less than 100 per cent is generally less satisfactory than one which shows a summation somewhat in excess of 100.

R

FIG. 2.-Radiator for rapid and safe evaporation. R is of sheet

This is due to several causes. Nearly all reagents, however carefully purified, still contain or extract from the vessels used. traces of impurities, which are eventually weighed in part with the constituents of the rock. The dust entering an analysis from first to last is very considerable, washings of precipitates may be incomplete, and if large filters are used for small precipitates the former may easily be insufficiently washed. Given the purest obtainable reagents, an ample supply of platinum, facilities for working, and a reasonably clean laboratory, there is no excuse for failure on the part of a competent chemist to reach a summation within the limits 99.75 and 100.50. Failure to attain 100 per cent in several of a series

iron, also nickel (Jannasch). Various sizes. A convenient height of analyses of similar nature should be the

is 7cm, width at top 7em and at bottom em

strongest evidence that something has been overlooked. Excess above 100.5 per cent should be good ground for repeating portions of the analysis in order to ascertain where the error lies, for it is not proper to assume that the excess is distributed over all determined constit

uents.

It is quite as likely, in fact more than likely, to affect a single determination and one which may be of importance in a critical study of the rock from the petrographic side.

VII. QUALITY OF REAGENTS.

All analyses performed in the Survey laboratories have been made with the purest reagents obtainable, either by purchase in the open market or by special preparation on the part of manufacturers or in the laboratory. The best acids made in this country are of a high grade and need no redistillation except for special experiments. Ammonia has always been redistilled at short intervals; and no sodium carbonate which exceeds 2 milligrams of total impurity (see p. 50) in 20 grams (0.012 per cent) is used for the main portions, in which silica, alumina, etc., are to be estimated. For other portions, as phosphoric acid, fluorine, sulphur, a poorer grade is entirely allowable, provided it is free from the element to be determined, and from any other which might interfere with its estimation.

Hydrofluoric acid was always freshly distilled with potassium permanganate until the introduction of ceresine bottles afforded an article sufficiently pure for all but the most exacting work. Care must be exercised even yet, however, that no particles of paraffin or ceresine are floating on the acid, and that the latter is free from traces of chlorine whenever it is to be used for attacking silicates with a view to estimating chlorine (p. 103).

Potassium bisulphate has usually been prepared in the laboratory from sulphuric acid and potassium sulphate, since it is not always to be bought of satisfactory quality. Even then the normal sulphate had first to be examined, for it has been found to contain, on different occasions, notable amounts of lead, calcium, and silica.

The phosphorus salt used for precipitating magnesium has been found to contain iron and silica, and calcium is almost always a constituent of ammonium oxalate. The latter has therefore to be purified or specially prepared, as also oxalic acid, ammonium chloride (in which latter manganese has been observed), and occasionally other reagents. Some hydrogen peroxide contains fluorine, which renders it unfit for use as a chemical reagent.

A "C. P." label is no guaranty whatever of the purity of a reagent; hence no chemicals should be taken on trust because of bearing such a label. Every new purchase should be examined, if it is one in which purity is a desideratum. In general all so-called "C. P." chemicals should at least stand the tests laid down by Krauch.1

Of late years the appearance upon the market of so-called guaranteed reagents promised to meet a long-felt want. But experience has

Die Prufung der chemischen Reagentien, 3d ed., Berlin, Julius Springer, 1896.

shown that with the pioneer in this line at least the guaranty amounts to nothing, the reagents being sometimes worse than the "C. P.” articles emanating from sources which make no claim to special purity for their goods, and redress being unobtainable. The "guaranteed reagent" needs checking as much as any other.

VIII. PRELIMINARY QUALITATIVE ANALYSIS.

A complete qualitative analysis of a rock, preceding the quantitative examination, is in most cases a sheer waste of time. A few constituents may now and then be specially looked for, but in general time is saved by assuming the presence of most of them and proceeding on that assumption in the quantitative analysis.