APPENDIX. 



fixed <vcr the wide end \vith a piece of fine cord, and the dialyser, 

 of iii-in"- floated on the water, is suspended so that the parch- 

 ment is ////. below the surface. This is effected by strings which pass 

 from its' neek to u irlass rod laid over the month of a cylindrical glass 

 j ;II - containing the water (fig. 338). Diffusion is prompted by using a 

 irface of parchment, or by frequently gently shaking the dialyser. 

 Tin- proems may be further accelerated by heat and by evaporation, for 

 which purple ill'- basin containing the dialyser may be advantageously 

 placed in the warm chamber or bath at a temperature of 37 C. 



213. Drying. Glass vessels, in which substances are to be weighed, 

 are dried by heat. In the case of flasks and tubes, this may be done by 

 warming them over thetlame of a spirit-lamp, then blowing air through 

 them with the bellows. For most purposes the hot-air bath is used a 

 copper vessel either cubical or cvlindrical in shape, and provided with 

 a door or movable cover ifiir. 889), It is heated by a lamp or burner, 

 and must be furnished witli a thermometer, so fixed as to indicate the 

 temperature of tin- air of the clnmber. For all purposes which re- 

 quire a temperature not exceeding 100 C., the hot-air bath must con- 



tWO Casings, the space between which is filled with water. 



Di'1/in'f and Cnoli/iy <>ctr Sulphuric Acid. When substances, espe- 

 eiallv hv'-roscopic powders, are dried in the air-bath and then allowed 

 to cool, 'they take up moisture and gain weight. To prevent this, they 

 must be allowed to cool under a bell jar, under which is a dish con- 

 taining sulphuric acid (fig. 340). The acid absorbs moisture with 

 avidity, and keeps the air under the jar dry. The acid may be placed 

 in a shallow dish, and the substances to he dried supported over it on a 

 sheet of perforated /inc, which rests on the edges of the dish or on a 

 small tripod. Another method is to put the acid in a beaker, covered 

 with a ground-glass plate greased at the edges, and to support the cru- 

 cible on a leaden support ; the support is made of a bit of strong leaden 

 wire by bending one end of it into a circle which lies at the bottom of 

 the beaker, and the other end into a smaller circle which rises above 

 the -url'aee of the acid and holds the crucible. To prevent dried hydro- 

 substances from taking up moisture during weighing, they 

 should not be placed in an open vessel, but inclosed between two watch- 

 held together by a spring. 



Wild, it is desired to' dry substances without the aid of heat, they are 

 to be placed under the receiver of an air-pump and over sulphuric acid, 

 tsl mentioned. 



pitates may be rapidly dried by supporting the funnel containing 

 them over a very small flame by means of a beaker with the bottom 

 out. a triangle of iron wire and a piece of wire gau/,e, arranged as in 

 lig. :;il. 



214. Ignition. Substances are exposed to a red heat in order to dry 

 them th. iron-Id Y. to drive away volatile matters, or to burn off organic 

 constituents, and allow the fixed inorganic solids to be determined. A 

 small quantity of a substance may be iirnited on a piece of platinum 

 foil or in a platinum spoon, larger quantities in porcelain or platinum' 

 crucibles. Platinum vessels should not be used if the substance to be 



, contains iodine. bromine, phosphorus, or easily reducible 



. such as cupper, lead, silver, u'old, or tin. When precipitates 



ted in a filter arc ignited, they must be first carefully dried. The 



crucible is then to be placed on a piece of -la/ed paper, the precipitate 



d from the filter b v nibbing the sides together, and then shaken 



gently into the crucible. The filter is then either folded and placed in 



the Crucible, r it is set lire to and held over it bv a pair of forceps, so 



that the ashes may fall into it. Anv ashes or precipitate that has fallen 



on the paper havimr been collected' ami added to the rest, the crucible 



