These two engravings represent washing or rinsing bottles adapted for the washing of gelatinous precipitates requiring a very protracted application of the process; such as hydrated alumina, for instance.

It will be seen from the engravings, that the principle is the same in both. There is only this difference between them, that in XXIV. the two tubes are fixed together into one piece, whilst in XXV. they are separate. The construction of XXIV. is somewhat more difficult of illustration than that of XXV. A brief exposition of the principle of the apparatus will show this very clearly. Plate XXVI. represents the cork with the tubes apart from the bottle.

The arrangement of these bottles is so contrived, that no water can flow out of them by a simple inversion, but it begins to flow immediately upon bringing the finger, or a small piece of paper, or some other substance, in contact with the water in the point c; the water now flows incessantly, whilst air rushes into

PLATE XXVI.

a

the bottle through the tube a b; the efflux ceases upon removing the fingers or the piece of paper.

I deem it of great importance, for the proper construction and use of this rinsing apparatus, that the student should be en

abled to form a clear and correct notion of the principle upon which its properties depend. I will, therefore, briefly explain this principle. In the first place, the nonefflux of water from c is owing to the circumstance that the pressure of the column of water, occupying the space between the lines e ƒ and g h is not altogether but nearly sufficiently powerful to overcome the capillary attraction which the tube a b exercises upon the fluid; in the second place, the efflux of the water from c, upon being brought into contact with a substance to which it may adhere, is, owing to the draught of the water column fh, assisted now by the power of adhesion

of the humiated substance overcoming the capillary attraction in a b. If the point c is placed completely under water, the efflux will likewise cease, since this shortens the column fh. If the tube dc be lowered, so as to place c below gh, the water will flow incessantly, the draught of the increased column of water overcoming by itself the capillary attraction in a b. But if, on the contrary, c d be moved upwards, the draught of the column fh, together with the power of adhesion of the substance in contact with it, will no longer be sufficiently powerful to overcome the capillary attraction in a b, and consequently no water will flow out. It will be readily conceived from the preceding remarks, that the construction, as well as illustration, of the joint-tube in XXIV. offers some difficulties, since it is not by any means easy to fill the extremity c exactly at the required distance from the point where the tube a b is united to c d. In XXV., the tube cd requires simply to be turned upward and downward until the desired point is attained. In the construction of the latter, care should be taken that the aperture a of a b be situated somewhat higher than b; if this precaution be neglected, water will be forced out every time the air rushes through this tube; moreover, d must abut somewhat higher than 6, or else, as soon as the surface of the water ceases to close b, the rest will flow out in a continued stream.

*

The process of washing by means of either of these rinsing bottles is easily performed. The bottle is placed inverted into the aperture contrived for its reception in the second arm of the filter-stand, (vide plate XX. b.,) and kept suspended over the funnel in such a manner that c just dips under the surface of the fluid. If the apparatus is well arranged, the water will now flow out of c in the same measure as it runs off through the funnel. To substitute for these rinsing bottles, narrow-necked flasks, inverted directly into the funnel, is quite inadmissible in cases when we intend to determine the exact amount of the precipitated substance, since the air-bubbles which such a contrivance must unavoidably create, would invariably carry minute particles of the precipitate up into the flask.

Care should be taken whilst washing the precipitate (no matter whether with or without a rinsing apparatus) to prevent the formation of channels in it, through which the rinsing water might flow off at once, without previously pervading the whole mass of the substance. Should such channels have formed, however, the precipitate must be carefully stirred with a glass rod or platinum spatule.

The operation of rinsing may be considered complete when all soluble matter has been completely washed away; whether this end has been attained is generally ascertained by evaporating a drop of the last rinsing water upon a clean platinum plate, observing whether this leaves any residue behind or not. In some cases, however, when the precipitate is not absolutely insoluble in the rinsing water, (precipitated sulphate of strontian, for instance,) we have recourse to some more special tests which we shall subsequently have occasion to point out.

The operation of washing precipitates claims the greatest care and attention in the hands of the analytical chemist, since it is self-evident that the imperfect rinsing of a precipitate must materially interfere with the accuracy of his results. This operation should never be deemed concluded on the mere impression and judgment of the operator, the application of the appropriate test or tests alone can safely be relied on.

§ 28.

a. FURTHER TREATMENT OF PRECIPITATES PREPARATORY TO

DETERMINING THEIR WEIGHT.

Before we proceed to weigh a precipitate, it is indispensable to convert it first into a state in which its composition is perfectly and accurately known to us. This is done either by drying or heating to redness. The former process is more protracted and tedious in its application than the latter, and is, moreover, liable to yield less accurate results. The application

is therefore generally confined to precipitates which cannot bear exposure to a red heat without undergoing total or partial volatilization, and the residue of which remaining, in the latter case, are not of a kind to admit of any retrospective inference regarding the exact original composition of the precipitate; thus, for instance, drying is applied to sulphuret of mercury, sulphuret of lead, and other metallic sulphurets, and likewise to cyanide of silver, chloride of platinum, and potassium, &c., &c.

But whenever the nature of the precipitated substance leaves the operator at liberty to choose between drying and heating to redness, the latter is almost invariably preferred to the former, (thus precipitates of sulphate of barytes, sulphate of lead, and a great many more compounds, are heated to redness.)

$ 29.

ry. Drying of the precipitate.

When a precipitate has been collected, washed, and dried on a filter, minute particles of it adhere so firmly to the paper that it is found impossible to remove them. The weighing of dried precipitates involves therefore the drying and weighing of the filter. In former times, chemists used to collect the precipitate upon two filters of equal size, the one placed within the other; after the precipitate was dried, the outer filter was taken off and placed on the balance as a counterpoise to the inner filter which contained the precipitate. It was at the time assumed that filters of equal size were likewise of equal weight. This assumption, however, is inadmissible in minute and accurate analysis, since every experiment shows that even small filters, although of equal size, yet differ in weight to the extent of twenty, thirty, and even more millegrammes. To obtain accurate results, it is necessary to dry and to weigh the filter previous to using it; the temperature at which the filter is to be dried must be the same to which it is intended subsequently to expose the precipitate. The filter

ing paper must not contain any substance liable to be dissolved by the fluid filtering through it.

The process of drying filters is conducted either in the waterbath, air-bath, or oil-bath, according to the degree of heat required. The dried filter must be weighed in vessels with a cover, platinum crucibles are generally used for this purpose. The filter, as soon as it appears dry, is introduced into the heated crucible, allowed to cool, (this may be done most advantageously by placing the crucible with the filter over a vessel of sulphuric acid under a bell-jar,) and weighed; the crucible with the filter is then again exposed for some time to the required degree of heat, and after cooling, weighed again. If the weight does not differ from that found the first time, the filter may be considered dry, and we have simply to note the collective weight of the crucible and filter.

After the rinsing of the precipitate has been concluded, and the water allowed to run off as much as possible, the filter with the precipitate is taken off the funnel, folded up, and placed upon blotting-paper, and is kept for some time in a moderately warm place, protected from dust; this, in some measure, dries it, and thus facilitates the further process. It is finally introduced into the same platinum crucible in which it has been weighed previously to the filtration, and the crucible is then exposed to the appropriate degree of heat, either in water, air, or oil-bath, according to the nature of the precipitate. The crucible with its contents is then repeatedly weighed, and the drying continued until the weight remains constant. From the collective weights. of the precipitate crucible and filter thus ascertained, we have simply to subtract that of the crucible and filter which, as stated above, we had accurately determined previous to the commencement of the process; the remainder expresses the exact weight of the precipitate.

It happens sometimes that the precipitate nearly fills the filter, or retains a considerable amount of the rinsing water, or sometimes the paper is so thin that its removal from the funnel cannot