3850. Ignition

3850.    Ignition. The heating of a substance to redness. It is especially resorted to for the calcination of a substance at a high degree of heat. (See No. 3849 (Calcination).)

3851. To Bend Glass Tubes

3851.    To Bend Glass Tubes. Small glass tubes may be bent over the flame of a spirit lamp; for larger tubes, the heat of a blow-pipe flame is necessary. The tube should be heated to a dull red about an inch either way beyond the point of curvature, by revolving it in the flame; as soon as the glass begins to yield, bend the tube very gradually until curved as desired. Stopping one end of the tube, and blowing into the other while bending it, will prevent wrinkling or collapsing at the point of curvature. It requires some tact to bend a tube with an even curve and without collapsing its sides; and it is recommended by an experienced chemist to use a Bunsen burner, having the extremity flattened out so as to give a short and thin, but broad flame, something like the flame of an ordinary gas burner. The tube is placed in this flame and turned around until a good heat is given to the tube; it is then withdrawn from the flame and bent, when it does so with a perfect curve and no collapse on the sides of the tube. Of course this is only intended for the smaller tubes, but a tube of one-third of an inch and more can be thus bent very readily.

3852. To Find the Dry Weight of a Pulp or Moist Precipitate

3852.    To Find the Dry Weight of a Pulp or Moist Precipitate. Pulps or precipitates, such as the metallic colors, chrome

| yellow, white lead, etc., are of different consistence at the top from what they are at or near the bottom of the vessel in which they are contained; and the actual weight of the precipitate in the dry state can therefore not be arrived at by merely taking a sample from top or bottom, but, in most cases, only guessed at. When, however, the specific gravity of such a precipitate in its dry state is known, as well as that of the surrounding liquid, the operation of obtaining the accurate dry weight of the same while in pulp can be reduced to the simple manipulation of weighing it in a vessel. Find the weight of a vessel full of the pulp; then weigh the same vessel full of the same liquid that the pulp is moistened with, and note down the difference between the weights. Next divide this difference of weight by the difference between the specific gravities of the pulp and the liquid ; lastly add this quotient to the difference of weight already noted down, and the sum will bo the dry weight of the pulp.