With respect to the amount of a substance necessary for its quantitative analysis, this depends entirely upon the nature of its constituents, and it is, therefore, impossible to lay down any general rule. One half gramme, and even less, of common salt, is sufficient to enable us to determine the proportion of chlorine which it contains. For the analysis of a mixture of common salt and sulphate of soda, one gramme would be sufficient; but if we have ashes of plants, complex minerals, etc., to deal with, it is necessary to take from three to four grammes or more. From one to three grammes may, however, in the great majority of cases, be considered sufficient for analysis.

The greater the amount of substances operated upon, the more accurate are the results of the analysis; the less their amount, the less time is required for the completion of the operation. We would advise the student of quantitative analysis, to endeavour to combine accuracy with economy of time. The less substance we take to operate upon, the more accurately we ought to weigh; the greater the amount of substance, the less harm can result from slight inaccuracies in weighing. In analyses of tolerably large proportions of substances, it is customary to limit the accuracy of weighing to about one milligramme; but in cases where we have very minute proportions to deal with, we must weigh accurately within the one-tenth of a milligramme.

If several different quantities of a substance are to be operated upon, it is most advisable to weigh the different portions successively; and this may be accomplished best by weighing the whole quantity intended for the analytical operations in a glass tube, or other appropriate vessel, the weight of which has, likewise, been previously accurately ascertained. From this tube, the portions required for the several operations are projected into the appropriate vessels, and the weight determined every time by the diminution in the weight of the tube. (Compare § 10, 4.)

§ 17.

2. DETERMINATION OF THE AMOUNT OF WATER CONTAINED IN

SUBSTANCES.

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 up their water readily or not; whether they can stand a red heat without undergoing decomposition, or whether, on the contrary, they yield up other volatile substances, besides their water, even upon the application of a less degree of heat.

The correct apprehension of the constitution of a substance depends frequently upon the accurate determination of the quantity of water contained in it; in many cases, such as, for instance, in the analysis of the salts of known acids, the determination of the amount of water contained in the analysed compound suffices to enable us to infer, and establish its formula. The determination of the amount of water contained in a substance, is, therefore, one of the most important, as well as most frequently occurring operations of quantitative analysis. The proportion of water contained in a substance may be determined in two ways, viz. 1. by the diminution of weight which the substance undergoes by the expulsion of the water; 2. by weighing the amount of water expelled.

A. DETERMINATION OF THE PROPORTIONAL AMOUNT OF WATER CONTAINED IN A SUBSTANCE, BY THE DIMINUTION OF WEIGHT WHICH THE LATTER UNDERGOES UPON THE EXPULSION OF THE WATER BY HEAT.

This method is almost invariably adopted in the analysis of inorganic compounds, and is inadmissible only in very rare instances. The modus operandi and the apparatus, are exactly the same with those of exsiccation described in § 14.

If the substance intended for analysis will bear a red heat without losing any other constituent besides water, the operation may be conducted simply in a covered platinum or porcelain crucible, or in a small test tube, over the flame of a spirit-lamp, taking care to commence with a gentle heat, and to increase it gradually to the desired point. If the substance under examination does not admit of the application of a red heat without losing some other constituent besides water, (ammonia, carbonic acid, &c.) it is exposed to the temperature of a water-bath; or, should it be desirable to apply a higher degree of heat than 212° to the heat of an air-bath, or an oil-bath, provided with a thermometer, as we have seen in § 14.

Salts containing several distinct proportions of water, which separate at different temperatures, are first heated in the waterbath until they cease to lose weight, upon continuing the operation they are subsequently exposed to temperatures of 302, 392, 482, &c. in the air,-or oil-bath; and finally heated over a free fire.

In this manner variously combined proportions of water may be distinguished, and their respective amounts correctly determined. Thus, for instance, the common perphosphate of iron, dried over sulphuric acid, contains 13 equivalents of water; 6 of which volatilize at 212°, 4 at between 392° and 572°, and 3, finally, at a red heat.

B.-DETERMINATION OF THE PROPORTIONAL AMOUNT OF water CONTAINED IN A SUBSTANCE, BY DIRECT WEIGHING OF THE WATER EXPELLED FROM THE LATTER BY HEAT.

The method which we have just described cannot be pursued in the analysis of substances which, upon the application of heat in the common way, lose other constituents besides water. Substances of this description must be freed from their water by the application of heat in such a manner as to admit of the conden

sation of the aqueous vapours, and of their transfer upon some hygroscopic substance, the exact weight of which has previously been accurately ascertained.

This operation may be conducted in various ways; the following, however, is one of the most appropriate :

B represents a gasometer, filled with air; b a flask, half filled with concentrated sulphuric acid; c and e are chloride of calcium tubes; d is a glass tube, expanded into a bulb in its centre.

The substance intended for examination is accurately weighed in the perfectly dry tube d; the weight of which is also to be previously ascertained; d is then connected, by means of sound and well-dried perforated corks, with the chloride of calcium tubes c and e; the latter of which must previously be accurately weighed.

The operation is then commenced by opening the stopcork g a little, so as to allow the air to pass through b and c, (where its moisture will be completely retained,) into d; the latter is then heated at f, beyond the boiling point of water, by means of a spiritlamp, (always taking care not to burn or singe the corks;) and, finally, the bulb of the tube d, which contains the substance, is exposed to a slight red heat, whilst the above indicated temperature is maintained at f. After all the water is expelled, the air is still allowed, for a few moments, to pass through the apparatus; the latter is then taken asunder, and the chloride of calcium tube (after cooling) accurately weighed. The increase in its weight gained

E

during the operation, indicates the amount of water originally contained in the substance examined.

The expulsion of the aqueous vapour from the tube containing the substance under examination, into the cloride of calcium. tube, may also be effected by other means than a stream of air furnished by a gasometer; viz., the substance, the amount of water of which we intend to determine, may be heated to redness in a perfectly dry tube, together with carbonate of lead, since the carbonic acid of the latter escaping at a red heat, serves here the same purpose as a stream of air. This method is principally applied in cases where it is desirable to retain an acid which, in the former method, would volatilize together with the water; thus it is applied, for instance, for the quantitative determination of the water existing in the acid sulphate of potass, etc.

PLATE XIII.

a

Plate XIII., represents the apparatus used for the application of this method.

a b is a common combustion furnace; cf a tube filled from c to d with carbonate of lead, (this must have been heated to incipient decomposition, and subsequently cooled in a close tube;) from d to e is placed the substance to be analysed, intimately mixed with carbonate of lead; and from e to f pure carbonate of lead. The chloride of calcium tube g, after being accurately weighed, is connexed with the tube cf by means of a well-dried perforated cork ƒ.

The operation is commenced by surrounding the tube cf with red-hot charcoal, advancing from f' towards c; the inferior part of the tube which protrudes from the furnace, should, during the whole operation, be maintained at such a degree of heat as hardly to permit the operator to lay hold of it with his fingers. All further particulars of this operation will be found below in the chapter on organic elementary analysis. The mixing of the car