10 



PRINCIPLES OF CHEMISTRY 



oxide and carbonic acid gas is equal to the weight of the copper car- 

 bonate 17 originally taken, and thus by carefully following out the 

 various stages of all chemical reactions we arrive at a continuation of 

 the law of the indestructibility of matter. 



3. Red mercury oxide (which is formed as mercury scoria by heat- 

 ing mercury in air) is decomposed like copper carbonate (only by 

 heating more slowly and at a somewhat higher temperature), with the 

 formation of the peculiar gas, oxygen. For this purpose the mercury 

 oxide is placed in a glass tube or retort, 18 to which, by means of a cork, 

 a gas delivery tube is attached. This tube is bent downwards, as shown 



FIG. 1. Apparatus for the decomposition of red mercury oxide. 



in the drawing (Fig. 1). The open end of the gas delivery tube is im- 

 mersed in a vessel filled with water, called a pneumatic trough. 19 When 



17 The copper carbonate should be dried before weighing, as otherwise besides copper 

 oxide, and carbonic anhydride water will be obtained in the decomposition. Water 

 forms a part of the composition of malachite, and has therefore to be taken into considera- 

 tion. The water produced in the decomposition may be all collected by absorbing it in 

 sulphuric acid or calcium chloride, as will be described further on. In order to dry a 

 salt it must be heated at about 100 until its weight remains constant, or be placed under 

 an air pump over sulphuric acid, as will also be presently described. A* water is met 

 with almost everywhere, and as it is absorbed by many substances, the possibility of its 

 presence should never be lost sight of. 



18 As the decomposition of red oxide of mercury requires so high a temperature, near 

 redness, as to soften ordinary glass, it is necessary for the experiment to take a retort 

 (or test tube) made of infusible (German) glass, which is able to stand high temperatures 

 without softening. For the same reason, the lamp used must give a strong heat and a 

 large flame, capable of embracing the whole bottom of the retort, which should be as 

 small as possible for the convenience of the experiment. 



19 The pneumatic trough may naturally be made of any material (china, earthenware ) 

 or metal, &c.), but usually a glass one, as shown in the drawing, is used, as it allows the 

 progress of experiment being better observed. For this reason, as well as the ease with 

 which they are kept clean, and from the fact also that glass is not acted on by many sub- 



