with small handles, turned towards the periphery of the disk, and resting in appropriate grooves. Behind every cavity is stamped on the plate a number, from 1 to 6; the handles of the pans bear corresponding numbers, so that every pan has its own proper cavity. The distance

Fig. 36.

from the centre of the plate to the centre of the pans is 6.5 centimetres; the rims of the pans are level with the surface of the plate. Five of the pans are intended to receive the samples (ores, parts of plants, &c.); the sixth receives the thermometer, to which end a brass ring is fitted into it, projecting 3 centimetres above the surface. The pan, which is thereby heightened, is filled with brass or copper filings, and the bulb of the thermometer immersed in these down to the bottom. The heat is made to act upon the centre of the plate.

f. Substances which suffer no alteration at a red heat, such as sulphate of baryta, potash, &c., are very readily freed from moisture. They need simply be heated in a platinum or porcelain crucible over a gas- or spirit-lamp until the desired end is attained. The crucible, having first been allowed to cool a little, is put, still hot, under a desiccator, and finally weighed when cold.

III. GENERAL PROCEDURE IN QUANTITA-
TIVE ANALYSES.

$ 32.

It is important, in the first place, to observe that we embrace in the following general analytical method only the analysis and quantitative estimation of the metals and their combinations with the metalloids, and the inorganic acids and salts. With respect to the quantitative analysis of other compounds, it is not easy to lay down a universally applicable method, except that their respective constituents usually require to be converted first into acids or bases, before their separation and quantitative estimation can be attempted; this is the case, for instance, with sulphide of phosphorus, chloride of sulphur, chloride of iodine, sulphide of nitrogen, &c.

As has already been observed before, the quantitative analysis of a substance presupposes an accurate knowledge of the nature and properties of the same, and of its several constituents; as this will enable the operator at once to decide whether the direct quantitative estimation of each individual constituent be necessary or not; and, consequently, whether he need operate only on one sample of the substance, or whether he has to determine in several different portions respectively, the relative amount of each individual constituent. Let us suppose, for instance, we have a mixture of chloride of sodium and anhydrous

sulphate of soda, and wish to ascertain the exact proportion in which each of these two substances is contained in the mixture. Here it would be superfluous to determine separately the proportional amount of every individual constituent, since the determination either of the quantity of the chlorine, or of that of the sulphuric acid, is quite sufficient to answer the purpose; still the double estimation of both the chlorine and the sulphuric acid will afford us an infallible control for the correctness of our analysis; since the united weights of these two substances, added to the known proportion of their respective equivalents of sodium and soda, must be equal to the weight of the analysed portion of the mixture.

This double estimation may be made, in this instance, either in one and the same sample of the mixture, by first precipitating the sulphuric acid with nitrate of baryta, and subsequently the hydrochloric acid from the filtrate with solution of nitrate of silver; or a separate portion of the mixture may be appropriated to each of these two operations. The latter method, whenever it is at all applicable, is preferable to the former, in cases where we have to deal with perfectly homogeneous substances; and where we have a sufficient quantity of substance to act upon. It is more convenient than the former method, and leads to more accurate results; since, when operating upon one and the same sample of a substance, the unavoidable washing of the first precipitate swells the amount of liquid so considerably, that the analysis is thereby delayed, and, moreover, loss of substance less easily guarded against.

The student will always do well, at least in operations on a larger scale or of a more complex nature, to draw up a fixed, written plan, and to accurately note, during the entire process, every successive phase, with its results. To trust to memory, is most unwise; and to work without a maturely considered, fixed plan, is equally so. A mere unthinking and purposeless stringing together of a series of filtrations, evaporations, ignitions, and weighings is not chemistry, however well these several operations may be performed.

We will now proceed to describe the various operations constituting the process of quantitative analysis.

§ 33.

1. WEIGHING THE SUBSTANCE.

The amount of matter required for the quantitative analysis of a substance depends upon the nature of its constituents; it is, therefore, impossible to lay down rules for guidance on this point. Half a gramme of chloride of sodium, and even less, is sufficient to effect the estimation of the chlorine. For the quantitative analysis of a mixture of common salt and sulphate of soda, 1 gramme will suffice; whereas, in the case of ashes of plants, complex minerals, &c., 3 or 4 grammes, and even more, are required. The average quantity of substance required for the purposes of quantitative analysis may, accordingly, be said to range, in most cases, between 1 and 4 grammes. the estimation of constituents present in very minute proportions only, as, for instance, alkalies in limestones, phosphorus or sulphur in castiron, &c., much greater quantities are often required-10, 20, 50, grammes, and more.

For

The greater the amount of substance operated upon, the more accurate will be the results of the analysis; the smaller the quantity, the less time will be required for the performance of the operation. We would advise the student to endeavor to combine accuracy with economy of time. The less substance he takes to operate upon, the more carefully he ought to weigh; the larger the amount of substance, the less harm can result from slight inaccuracies in weighing. For somewhat large quantities of substance, the accuracy of weighing may be safely limited to within about 1 milligramme; for minute quantities, to about To of a milligramme.

If several different quantities of a substance are to be operated upon, the best way is to weigh off the several portions successively; which may be accomplished most readily by weighing in a glass tube, or other appropriate vessel of known weight, the whole amount of substance intended to be devoted to the analytical process. Out of this tube the portions required for the several operations are taken, and the weight is ascertained each time from the diminution in the weight of the tube.

The work may often also be materially lightened, by weighing off a larger portion of the substance, dissolving this in sufficient liquid to give exactly, or a litre of solution, and taking out for the several estimations aliquot parts of the fluid, by means of a pipette of 50 c. c. or 100 c. c. capacity. The first and most essential condition of this proceeding, of course, is that the pipettes must accurately correspond with the measuring flasks. The method of ascertaining this fact has been described in § 18 and § 20. It may, however, be determined, also, in a more simple way, by emptying the pipettes 5 times, 10 times, &c., into the measuring flasks, and observing whether the volume thus poured in corresponds exactly with the line-mark of the flask.

$ 34.

2. ESTIMATION OF THE AMOUNT OF WATER CONTAINED IN A

SUBSTANCE.

If the substance to be examined contains water, it is usual, in the great majority of cases, to begin by determining the amount of this water. This operation is generally simple; in some instances, however, it has its difficulties. This depends upon various circumstances, viz., whether the compounds intended for analysis yield their water readily or not; whether they can bear a red heat without suffering decomposition; or whether, on the contrary, they give off other volatile substances, besides water, even at a lower temperature.

The correct knowledge of the constitution of a substance depends frequently upon the accurate estimation of the quantity of water contained in it; in many cases-for instance, in the analysis of the salts of known acids-the estimation of the amount of water contained in the analysed compound suffices to enable us to deduce the formula. The estimation of the amount of water contained in a substance, is, therefore, one of the most important, as well as most frequently oceurring operations of quantitative analysis. The proportion of water contained in a substance may be determined in two ways, viz., 1, from the diminution of weight consequent upon the expulsion of the water; 2, by weighing the amount of water expelled.

$ 35.

a. ESTIMATION OF THE WATER FROM THE LOSS OF WEIGHT CONSEQUENT UPON ITS EXPULSION.

This method, on account of its simplicity, is almost invariably resorted to in quantitative analyses, except in cases where it is inapplicable from the nature of the substance examined. The modus operandi depends upon the nature of the substance under examination.

a. The Substance bears a Red Heat, without losing other Constituents besides Water, and without absorbing Oxygen.

The substance is weighed in a platinum or porcelain crucible, which is then heated over the flame of a gas- or spirit-lamp, very gently at first, but increasing gradually to the desired point. When the crucible has been maintained some time at a red heat, it is removed from the flame, let cool a little, put still warm under the desiccator, and finally weighed when cold. The same operation is then repeated, and the weight again ascertained. If no further diminution of weight has taken place, the process is at an end, the desired object being fully attained. But if the weight is less than after the first heating, the operation must be repeated until the weight remains constant.

In the case of silicates, the heat must be raised to a very high degree, since many of them (e.g. talc, steatite, nephrite), begin only at a red heat to give off water, and require a yellow heat for the complete expulsion of that constituent. (Th. Scheerer, in Liebig and Kopp's "Annual Report" for 1851, 610.)

In the case of substances that have a tendency to puff up, or to spirt, a small glass flask may sometimes be advantageously substituted for the crucible. Care must be taken to remove the least traces of aqueous vapor from the vessel, by suction through a glass tube.

Decrepitating salts (chloride of sodium, for instance) are put-finely pulverized if possible-in a small covered platinum crucible, which is then placed in a large covered crucible, and a gentle heat applied, which is gradually increased to the requisite degree.

B. The Substance does not yield other Constituents besides Water, upon the application of a Red Heat; but it has a Tendency to absorb Oxygen (as many salts of Protoxide of Iron, for instance).

The substance is put into the bulb of a bulb-tube, made of difficultly fusible glass, and gradually heated to redness, whilst a slow current of carbonic acid gas, dried previously by transmission through sulphuric acid, is kept passing through the tube; the substance is maintained at a red heat, until the complete expulsion of the water is accomplished; it is then allowed to cool in the bulb, the transmission of the carbonic acid gas being continued all the while; when cold, the substance is weighed. When employing this method, it is always advisable to collect the expelled water in a chloride of calcium tube, connected with the bulb-tube by means of a perforated cork, and to weigh this before and after, by way of control. (Compare $ 36.)

Volds has lately recommended* to estimate the water in substances of this kind by mixing them in a tared platinum or porcelain crucible,

* "Annal. der Chem. u. Pharm." 94, 216.

with a weighed quantity, in excess, of pure bichromate of potassa, thoroughly freed from water by fusion at a gentle heat, adding some water, evaporating cautiously, drying at 392° F., and calculating the amount of water by the diminution of weight. This method, certainly, will give, in many cases, satisfactory results; but it does not appear to me to excel the other method either in accuracy or simplicity. It is quite inapplicable, of course, in presence of organic substances, or of ammoniacal salts. Neutral chromate of potassa must be substituted for the acid salt, in cases where the application of the former would be attended with evolution of oxygen or chlorine, as, for instance, in the estimation of the water in salts of protoxide of iron containing chloric acid or sulphuric acid.

The Substance loses other Constituents besides Water, upon the Application of a Red Heat (Carbonic Acid, Sulphuric Acid, Fluoride of Silicon, &c.). Here the analyst has to consider, in the first place, whether the water may not be expelled at a lower degree of heat, which does not involve the loss of other constituents. If this may be done, the substance is heated either on the water-bath, or where a higher temperature than 212° F. is required, in the air-bath or oil-bath, the temperature being regulated by the thermometer. The expulsion of the water may be promoted, if necessary or desirable, by the co-operation of a current of air (compare § 29 and § 30); or by the addition of pure dry sand to the substance, to keep it porons ("Ann. der Chemie und Pharm." 53, 233). The process must be continued under these circumstances also, until the weight remains constant.

In cases where, for some reason or other, a degree of heat below redness is insufficient to effect the purpose in view, the analyst has to con, sider whether the desired end may not be attained at a red heat, by adding some substance that will retain the volatile constituent whose loss is apprehended. Thus, for instance, the crystallized sulphate of alumina loses at a red heat, besides water, also sulphuric acid; now, the loss of the latter constituent may be guarded against, by adding to the sulphate an excess (about six times the quantity) of finely pulverized, recently ignited, pure oxide of lead. But the addition of this substance will not prevent the escape of fluoride of silicon from silicates when exposed to a red heat (List, "Ann. d. Chemie und Pharm." 81, 189).

The amount of water in iodine of commerce may be determined by triturating the iodine together with eight times the quantity of mercury, and drying the mixture at 212° F. (Bolley, Dingler's "Polytechnic Journal," 126, 39.)

d. The Substance contains several differently combined Equivalents of Water which requires different Degrees of Temperature for Expulsion.

Substances of this nature are heated first in the water-bath, until their weight remains stationary; they are then exposed in the oil- or air-bath to temperatures of 302°, 392°, or 482° F., &c., successively; and finally heated to redness over the flame of a gas- or spirit-lamp.

In this manner differently combined equivalents of water may be distinguished, and their respective quantities correctly estimated. Thus, for instance, crystallized sulphate of copper contains 28.87 per cent. of water, which escapes at a temperature below 282° F., and 7-22 per cent., which escapes only at a temperature between 428° and 500° F. It is