APPENDIX. 



.'), consists of a wide glass tube, a, divided into two 

 in upper :iiid a lower, by a hori/onial septum. In the middle of 

 : nun is an opening, from which a tube runs down nearly to the 

 . of the lower division. The tube is closed by a perforated cork 

 or India-rubber stopper. Through this passes a tube. H, with a hori- 

 zontal limb. K. Inside B is a smaller and shorter tube, r, which lias a 

 .iall opening opposite D. The sides of i; and c are luted together 

 with cement at K. In using this regulator, a quantity of mercury is 

 poured into A. and of course runs down into the lower division, partly 

 tilling it. and partly compressing the air it contains. 



The month of A. la then closed by the cork, and the tube c connected 

 by India-rubber tubing with a gas-pipe, and the tube K with a small 

 MUT. Tin s down the tube r through its lower opi n 



end, up again between it and B, and out at K. and thence to the burner. 

 The regulator and a thermometer are then immersed in the water-bath, 

 .ted. and the bath warmed till the thermometer indicates 

 MV other desired temperature. The tubes 13 and c are then 

 , down till the mercury touches the lower end of c and closes it. 

 thai pre\ented from passing onwards to the burner, and the 

 \\onld go out entirely were it not that the small hole in c, oppo- 

 site D. allo\\s sullicient gas to pass through it to preserve Ihe flame from 

 ompletely extinguished. As soon as the flame is thus diminished, 

 i'er-bath and the regulator immersed in it begin to cool, and the 

 mercury, and still more the air in the regulator, consequently contracts. 

 The mercury, therefore, sinks, and lea\ s the mouth of c open, so that 

 ely through it. the flame increases, and the tem- 

 ilie hath B The mercury and air again expand ; 



he temperature is reached to which the regulator was 

 adjusted. Hi' s the mouth of c. and cuts oil' the gas 



till the temperat i.dls. In this way the temperature mav be 



i- months at -U) without varying much more than half a'degree. 

 mercury is \ery clean, however, it will adhere slightly to 

 the lower end of c. and the \ariations will thus be greater. The water 

 bath must also lie kept at a constant level, as otherwise the part 

 of the regulator heated by it is sometimes greater and sometimes less. 

 The mercury consequently does not always expand in the same pro- 

 portion to the rise in the temperature of'the water in which it is par- 

 tially immersed, and variations of -everai degrees may thus be produced. 



209. Precipitation.- In precipitating a substance by the addition 

 of another, the reagent is generally added a little at a time, and mixed 



iiis of a -tirring rod. till a further addition of the reagent produces 

 no perceptible increase in the amount of the precipitate. In order to 



iin that the precipitation is complete, a little of the liquid is tested 

 by throum- it on a tiller, and the reagent added to tin; clear filtrate. 

 It no further precipitate occurs, the precipitation is complete ; but if 

 one is formed, the liltrate is again mixed with the rest of the fluid and 



the process repeated. 



210. Washing of Precipitates on Filters. Precipitates are gene- 

 rally collected on a filter and \\ashed by directing u stream of water or 

 alcohol on them by means of a wash-bottle. Tn' filter should never be 

 tilled up to the top, as the upper part of the precipitate cannot then be, 



i-hed. It is always advisable to let the precipitate settle in 



. and to allow the' clear liquid to passs through the filter 



throwing the precipitate itself upon it ; and the whole of the 



fluid from which the precipitate has subsided must be allowed to pass 



through the filler before the washing is begun. A stream of water is 



then directed on the part of the precipitate nearest the edge of the filter, 



by which it is gradually washed towards the centre. The stream 



