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Bain Marie or Balneum

balance, weighing, placed, water, fig, piece, substance, weight and pipette

BAIN MARIE or BALNEUM waterbath. The substance to be heated being placed in a proper vessel, the latter is placed into another vessel containing water and capable of stand ing the application of fire by which heat is gradually communi cated. An arrangement of this kind, as shown in Fig. 55, is used in developing carbon prints and for similar purposes. One of its advantages is that the water contained in the inner vessel can never reach boiling point, and a small amount of solution can be kept at an even temperature.

BALANCE.—An instrument for determining the relative weights or masses of bodies. The ordinary balance consists of a beam with its fulcrum in the centre and its arms precisely equal. From the extremities of these arms are suspended two scales, the one to receive the object or substance to be weighed, and the other the counterpoise.

The possession of a good balance is of great importance to photographers and experimenters, as in many delicate processes a trifling error in the weighing of the necessary chemicals will make an important difference in the results.

A good balance should possess both stability and sensibility, or, in other words, it should not oscillate for any considerable time after being disturbed, and it should sensibly move upon the addition of the minutest particle of matter to one of the scales. For weights, see under Weights and Measures. For quantitative analysis, a very deli cate balance, such as that shown in Fig. 59, will be requisite.

Fig. 57 shows the construction of a handy form of balance. It combines two forms. The small pan and light beam are suitable for weighing small quantities up to Go grains, and the large pan and beam weighing from one quarter to 8 ounces. In Fig. 58, we have a balance specially de signed for weighing liquids instead of measuring them. They are pro vided with a tare beam for balancing the bottle before commenc ing to weigh the solution.

The following contrivance for weighing very minute quan tities can easily be constructed by those possessing skill and perseverance. It is called a torsion balance. Fig. 6o will serve to show its construction. A thin platinum wire is stretched horizontally through the stables from the wooden supports A B, firmly fixed on a stout deal board. An upright piece of wood, G F, is also firmly at tached to the board, strengthened, if necessary, by a side piece. A very thin delicate lever, D, cut in wood, is fixed to the centre of the platinum wire by means of a clip, H, securing it firmly in such a manner that it is raised per ceptibly out of the horizontal line. If a centigramme be placed on the lever at D, it is lowered to a certain point, twisting the platinum wire, and its po sition marked on a piece of paper attached to the upright piece G F. The space between the two points marked by the lever with and without the weight is then divided and marked into ten equal divisions, which will rep resent the distance that will be trav ersed by the lever under the weight of a milligramme. If we wish to weigh,

say, five milligrammes, a sufficient quantity of the substance is placed on the lever until it falls five divisions.

Another simple contrivance, which is within the power of anyone to con struct, is a Nicholson aerometer, Fig. 63, which serves as an excellent bal ance. A glass balloon, B, filled with air and hermetically sealed with a cork, is made to receive a piece of wood sur mounted by a wooden disc, D. At the lower end of the balloon is attached a small tray to hold pieces of lead It is that,rill.trori trinec M1 with water and the upper portion, and the wooden stem, held in position by means of a ring attached to four pieces of wire which firmly clip the sides of the vessel. Sufficient lead is then placed on the tray C until the stem rises almost entirely above the level of the water. This stem should be divided in such a manner that each divisicn represents the volume of a cubic cen timetre. We have now a perfect balance ; the sub ject to he weighed is placed on the disc D, and the number of divisions the stem sinks corresponds to the number of cubic centimetres of water displaced, or the weight of the substance in grammes. Thus, if the stem sinks six divisions, this represents the dis placement of six cubic centimetres of water, and the weight of the substance is therefore six grammes.

The difficulty of weighing out liquids which readily volatilize, of fuming acids, etc., has been experienced by every chemist and photographic experi menter. To simplify the process, Messrs. H. Schweitzer and E. Lundwitz have devised a pipette, shown in the accompanying 3 illustrations (Figs. 6i, 62). The arrangement is thus described *: The exhaustion tube terminates in a capillary, which is bent within the bulb of the pipette toward the upper portions of its walls ; the opposite side of the bulb is flattened in order that the pipette can be placed on the pan of the balance ; the mouthpiece may be drawn out to a capillary, and the same applies to the discharge tube. A sufficient quantity of the liquid to be weighed is sucked up into the bulb so that the end of the capillary within the bulb projects beyond the surface of the liquid when the instrument is laid down on the flattened portion. After wiping the end of the tube, the pipette and its contents are weighed ; the necessary quantity of liquid is then run out, and the weight redetermined. For weighing solid fats the instrument may be warmed ; the error introduced by weighing a slightly warm liquid in a small pipette being for practical purposes quite negligible.