INTRODUCTION 



11 



the gas begins to be evolved in the retort it is obliged, having no other 

 outlet, to escape through the gas delivery tube into the water in the 

 pneumatic trough, and therefore its evolution will be rendered 

 \ i.sible by the bubbles coming from this tube. In heating the retort 

 containing the mercury oxide, the air contained in the apparatus is 

 first partly expelled, owing to its expansion by heat, and then the 

 peculiar gas called 'oxygen' is evolved, and may be easily collected as it 

 comes off. For this purpose a vessel (an ordinary cylinder, as in the 

 drawing) is filled quite full with water and its mouth closed ; it is then 

 inverted and placed in this position under the water in the trough ; 

 the mouth is then opened. The cylinder will remain full of water- 

 that is, the water will remain at a higher level in it than in the sur- 



stances which affect other materials (for instance, metals), glass vessels of all kinds 

 such a> retorts, test tubes, cylinders, beakers, flasks, globes, &c. are preferred to any 

 other for chemical experiments. Glass vessels may be heated without any danger if the 

 following precautions be observed : 1st, they should be made of thin glass, as otherwise 

 they are liable to crack from the bad heat-conducting power of glass ; 2nd, they should be 

 surrounded by a liquid or with sand (Fig. 2), or sand bath as it is called ; or else should 



Fiu. 2.- --Apparatus for distillinsr under a diminished pressure liquids which decoiui>ose at their 

 boiling poim ; under the ordinary pressure. The apparatus in. which the liquid is distilled is con- 

 oeoted with a large jilobe from which the air is pumped out; the liquid is heated, and the receiver 



cr.nl,., I 



stand in a current of hot gases without touching the fuel from which they proceed, or in 

 the flame of a smokeless lamp. A common candle or lamp forms a deposit of soot on a 

 cold object placed in their flames. The soot interferes with the transmission of heat, and 

 so a glass vessel when covered with soot often cracks. And for this reason spirit lamps, 

 which burn with a smokeless flame, or gas burners of a peculiar construction, are used. 

 In the Bunsen burner the gas is mixed with air, and burns with a non-luminous and 

 smokeless flame. On the other hand, if an ordinary lamp (petroleum or benzine) does 

 not smoke it may be used for heating a glass vessel without danger, provided the glass is 

 placed well above the flame in the current of hot gases. In all cases, the heating should 

 be begun very carefully by raising the temperature by degrees, and not all at once, or the 

 glass will break. 



