measuring the length of a b with the greatest possible accuracy, and subtracting it from the actual state of the barometer. Suppose, for instance, the barometer stands at 758 mm.,

and the length of a b is = 100 mm., the actual pressure upon the gas will be 758-100 = 658 mm. mercury.

Fig. 4.

If we have water, or some other fluid (solution of potassa, for instance), over the mercury, we proceed generally as if this were not the case; i. e., we either place the mercury on a level inside and outside the cylinder, or measure the difference between the surface of mercury in the cylinder and that in the trough. The pressure of the column of water, &c. &c., floating over the mercury, is mostly so trifling that it may safely be disregarded altogether. The more correct way, of course, would be to measure the height of the column of water, &c., to reduce this upon mercury, and to subtract the result from the actual state of the barometer. But this correction may usually be omitted, since, as has already been stated, absolutely correct results cannot be arrived at under such circumstances.

$ 16.

Ad. 4. INFLUENCE OF MOISTURE.

In measuring gases saturated with aqueous vapor, it must be taken into account that the vapor, by virtue of its tension, exerts a pressure upon the confining fluid; the result must, therefore, be corrected accordingly. This is an easy task, however, since we know the respective tension of aqueous vapor for the various degrees of temperature. To this end it is, of course, necessary that the gas should be actually saturated with the vapor. It is, therefore, indispensable in measuring gases to take care either to have the gas absolutely dry, or thoroughly saturated with aqueous vapor.

The

The drying of gases confined over mercury is effected by means of a ball of fused chloride of calcium, stuck on a platinum wire; this is prepared by inserting the wire, bent at the lower end in form of a hook, into a pistol-bullet mould of about 6 mm. inner diameter, and then filling the hollow with chloride of calcium heated just to the fusing point; the chloride of calcium used must be free from caustic lime. neck sticking to the ball is, after cooling, removed with a knife. When it is intended to dry a gas, this ball is, with the aid of the wire, pushed through the mercury, and inserted into the orifice of the cylinder containing the gas; after having been left there for the space of an hour or so, the ball is withdrawn, the gas being now perfectly dry. Whilst the ball remains within the cylinder, the end of the wire must be kept below the surface of the mercury in the trough, since otherwise we should inevitably have diffusion of the confined gas and the outer air, at that part of the wire which is not covered by the mercury.

Where it can be done, it will always be found more convenient to saturate gases with moisture before measuring them. Bunsen effects this object by introducing an iron wire, with a droplet of water the size of a pin's head adhering to it, into the empty measuring cylinder and

casting the water off in the top, without wetting any other portion of the tube. The quantity of water thus introduced in the cylinder is more than sufficient, at the common temperature, to saturate with aqueous vapor the gas subsequently passed into the cylinder.

It is quite obvious from the preceding remarks, that volumes of different gases can be compared only if measured at the same temperature, under the same pressure, and in the same hygroscopic state. The temperature is generally reduced to 32° F., the hygroscopic state of the gases to 0, and the pressure to 0.76 met. barometer. How this is effected, as well as the manner in which we deduce the weight of gases from their bulk, will be found in the chapter on the calculation of analyses.

$ 17.

b. THE MEASURING OF FLUIDS.

In consequence of the vast development which volumetrical analysis has of late acquired, the measuring of fluids has become an operation of very frequent occurrence in analytical researches. According to the different objects in view, various kinds of measuring vessels are employed. The number of vessels proposed or suggested for the measuring of fluids in volumetrical processes has indeed now increased to such an extent, that I must forbear discussing all the forms and dispositions recommended, and simply confine myself to the description of such measuring apparatus as I have found the most practical and convenient in my own laboratory.

Before entering into details, I have to observe that the operator must, in the case of every measuring vessel, carefully distinguish whether the vessel is graduated for holding or for delivering the exact number of cubic centimetres marked on it. If you have made use of a vessel of the former description in measuring off 100 cub. cent. of a fluid, and wish to transfer the latter completely to another vessel, you must, after emptying your measuring vessel, rinse it, and add the rinsing to the fluid transferred; whereas, if you have made use of a measuring vessel of the latter description, there must be no rinsing.

Fig. 5.

a. MEASURING VESSELS GRADUATED FOR HOLDING THE

EXACT MEASURE OF FLUID MARKED ON THEM.

aa. Measuring vessels which serve to measure out definite quantities of fluid.

We use for this purpose

$18.

1. Measuring Flasks.

Fig. 5 represents a measuring flask of the most practical and convenient form.

Measuring flasks of various sizes are sold in the shops, holding respectively 200, 250, 500, 1000, 2000, &c., cub. cent. As a general rule, they have no ground-glass stoppers; it is, however, very desirable, in certain cases, to have measuring flasks with ground stoppers. The flasks must be made of well-annealed glass of uniform thickness, so that fluids may be

heated in them. The line-mark should be placed within the lower third or, at least, within the lower half-division of the neck.

Measuring flasks, before they can properly be employed in analytical operations, must first be carefully tested to determine their exact capacity. The best and simplest way of effecting this is to proceed thus:Put the flask, perfectly dry inside and outside, on the one scale of a sufficiently delicate balance, together with a weight of 1000 grammes in the case of a litre flask, 500 grammes in the case of a half-litre flask, &c., restore the equilibrium by placing the requisite quantity of shot and tinfoil on the other scale, then remove the flask and the weight from the scale on which they are placed (leaving, of course, the shot and tinfoil on the other scale), put the flask on a perfectly level table or shelf, and in that position pour in distilled water of 60.8° F.,* until the lower border of the dark zone formed by the surface stratum of the water around the inner walls is on an exact level with the line-mark. After having thoroughly dried the neck of the flask above the mark, replace the flask upon the scale: if this restores the perfect equilibrium of the balance, the water in the flask weighs, in the case of a litre measure, exactly 1000 grammes, in the case of a half-litre measure, exactly 500 grammes, &c. If the scale bearing the flask sinks, the water in it weighs as much above 1000 grammes, &c., as the additional weights amount to which you have to put in the other scale to restore the equilibrium; if it rises, on the other hand, the water weighs as much less as the weights amount to which you have to put in the scale with the flask to effect the same end.

If the water in the litre-measure weighs 999 grammes,† in the halflitre measure, 499.5 grammes, &c., the measuring flasks are correct. Differences up to 0.100 grm. in the litre-measure, up to 0.070 grm. in the half-litre measure, and up to 0.050 grm. in the quarter-litre measure, are not taken into account, as one and the same measuring flask will in repeated consecutive weighings, though filled each time exactly up to the mark, with water of the same temperature, be found to differ within these limits.

Though a flask should, upon examination, turn out not to hold the exact quantity of water which it is stated to contain, it may yet possibly agree with the other measuring vessels, and may accordingly still be perfectly fit for use for most purposes. Measuring vessels agree among them if the number of cubic centimetres marked upon them severally bears in all of them respectively the same relative proportion to the

To use water in the state of its highest density, viz., of 39.2 F., 1 cub. cent. of which weighs exactly 1 grm., and, accordingly, 1 litre, exactly 1000 grammes, is less practical, as the operations must in that case be conducted in a room of the same temperature; since, in a warmer room, the outside of the flask would immediately become covered with moisture, in consequence of the air cooling below dew-point. Nor can I recommend F. Mohr's suggestion to make litre flasks, and measuring vessels in general, upon a plan to make the litre flask, for instance, hold, not 1000 grammes of 39-2 F., but 1000 grammes of 63.5 F., since in an arrangement of the kind proper regard is not paid to the actual purport and meaning attached to the term "litre" in the scientific world; and measuring-vessels of the same recognised capacity, made by different instrument-makers, are thus liable to differ to a greater or less extent. One litre-flask, according to Mohr, holds 1001-2 standard cub. cent. I consider it impractical to givo to the cub. cent. another signification in vessels intended for measuring fluids than in vessels used for the measuring of gases, which latter demand strict adhesion to the standard cub. cent., as, in the analysis of gaseous bodies, it isoften required to deduce the weight of a gas by calculating from the volume.

+ With absolute accuracy, 998 981 grammes.

ascertained weight of the water; thus, for instance, supposing your litremeasure to hold 998 grammes of water of 60.8° F., and your pipette of 50 cub. cent. nominal capacity, to give 49.9 grammes of water of the same temperature, the two measures agree, since

1000: 50:998:49.9.

To prepare or correct a measuring flask for use, counterpoise the drylitre- half-litre- or quarter-litre-flask most accurately, and then weigh into it, by substitution (§ 9), 999 grammes, or, as the case may be, the one-half or one-fourth of that quantity of distilled water of 60·8° F. Put the flask on a perfectly level table or shelf, place your eye on an exact level with the surface of the water, and mark the lower border of the dark zone by two little dots made on the glass with a point dipped into thick asphaltum varnish, or some other substance of the kind. Now pour out the water, place the flask in a convenient position, and cut with a diamond a fine distinct line into the glass from one dot to the other.

Measuring flasks are occasionally also graduated for delivering the exact number of cubic centimetres of their nominal capacity; these, however, can properly be used only in operations where minute accuracy is not absolutely indispensable, since the water-drops which remain adhering to the glass inside the flask vary not inconsiderably in number and bulk, which may give rise to perceptible differences in the results of several measurements with one and the same flask. If you wish to graduate a flask for delivering, or to test one so graduated, pour water into it, empty it again, let it drain, and then weigh into it the exact weight of distilled water of 60.8° F. corresponding to the number of cub. cent. which the flask is stated to hold.

bb. Measuring vessels which serve to measure out indefinite quantities of fluid.

$ 19.

2. The Graduated Cylinder.

This instrument, represented in Fig. 6, should be about 3 centimetres wide, of a capacity of 300 cub. cent., and graduated into cub. cent. It must be ground level at the top, that it may be covered quite close with a ground glass plate. The measuring with cylinders is not quite so accurate as with measuring flasks, as in the latter the volume is read off in a narrower part. The accuracy of measuring cylinders may be tested in the same way as in the case of measuring flasks, viz., by weighing into them water of 60.8° F.; or, also equally well, by letting certain definite quantities of fluid flow into the cylinder from a correct pipette or burette graduated for delivering, and observing whether or not the quantity is correctly indicated by the scale of the cylinder.

aa. Measuring vessels which serve to measure out definite quantities of

fluid.

$ 20.

3. The Graduated Pipette.

This instrument serves to transfer a definite volume of a fluid from one vessel to another; it must accordingly be of a suitable shape and construction to admit its being freely inserted into the neck of the flask.

We use pipettes of 1, 5, 10, 20, 50, 100, 200 cub. cent. capacity. The proper shape for pipettes up to 20 cub. cent. capacity is represented in Fig. 7; Fig. 8 shows the most practical form for larger-sized pipettes. To fill a pipette with the fluid which it is intended to transfer from one vessel to another, the lower point of the instrument is dipped into the fluid, and suction applied to the upper aperture, either direct with the lips or through a caoutchouc-tube, until the fluid in the pipette stands a little above the required mark; the upper, somewhat narrowed, ground orifice is then closed with the point of the index of the right hand, which to that end had always better be moistened a little, and, holding the pipette in a perfectly vertical direction, the excess of fluid over the quantity required is made to drop out, by lifting the finger a little. When the fluid in the pipette has fallen to the required level, the drops which may happen to adhere to the outside of the pipette are carefully wiped off, and the contents of the tube are then finally transferred to the other vessel. In this process it is found that the fluid does not run out completely, but that a small portion of it remains adhering to the glass in the point of the pipette; after a time, as this becomes increased by other minute particles of fluid trickling down from the upper part of the tube, a drop gathers at the lower orifice, which may be allowed to fall off from its own weight, or may be made to drop off by a slight shake. If, after this, the point of the pipette be laid against a moist portion of the inner side of the vessel, another minute portion of fluid will trickle out, and, lastly, another trifling droplet or so may be got out by blowing into the pipette through the upper orifice. Now, supposing the operator follows no fixed rule in this respect, letting the fluid, for instance, in one operation simply run out, whilst in another operation he lets it drain afterwards, and in a third blows off the last particles of it from the pipette, it is evident that the respective quantities of fluid delivered in the several operations cannot be quite equal. I prefer in all cases the second method, viz., to lay the point of the pipette, whilst draining, finally against a moist portion of the inner sides of the vessel, which I have always found to give the most accurately corresponding measurements.

The correctness of a pipette is tested by filling it up to the mark with distilled water of 60-8° F., letting the water run out, in the manner just stated, into a tared vessel, and weighing; the pipette may be pronounced correct if 100 c. c. of water of 60-8° F. weigh 99.9 grammes.

Testing in like manner the accuracy of the measurements made with a pipette, we find that one and the same pipette will in repeated consecutive weighings of the contents, though filled and emptied each time with the minutest care, show differences up to 0010 grm. for 10 cub. cent. capacity, up to 0040 grm. for 50 cub. cent. capacity.

The accuracy of the measurements made with a pipette may be heightened, by giving the instrument the form and construction shown in Fig. 9.

It will be seen from the drawings that these pipettes are emptied only