12 PKINCIPLES OF CHE3I1STKY 



rounding vessel, owing to the atmospheric pressure. The atmosphere 

 presses on the surface of the water in the trough, and prevents the 

 water from flowing out of the cylinder. The mouth of the cylinder is 

 placed over the end of the gas delivery tube, 20 and the bubbles 

 issuing from it will rise into the cylinder and displace the water con- 

 tained in it. Gases are generally collected in this manner. When a 

 sufficient quantity of gas has accumulated in the cylinder it can be 

 clearly shown that it is not air, but another gas which is distinguished 

 by its capacity for vigorously supporting combustion. In order to show 

 this, the cylinder is closed, under water, and removed from the bath ; 

 its mouth is then turned upwards, and a smouldering taper plunged 

 into it. As is well known, a smouldering taper will be extinguished in 

 air, but in the gas which is given off from red mercury oxide it burns 

 clearly and vigorously, showing the capacity this gas has for vigorously 

 supporting combustion, and thus enabling it to be distinguished from 

 air. It may be observed in this experiment that, besides the forma- 

 tion of oxygen, metallic mercury is formed, and, being volatilised at the 

 high temperature required for the reaction, condenses on the cooler parts 

 of the retort as a mirror or in globules. Thus two substances, mer- 

 cury and oxygen, are obtained by heating red mercury oxide. In this 

 reaction, from one substance two are produced that is, decomposition 

 ensues. The means of collecting and investigating gases were already 

 known before Lavoisier's time, but he first sho.wed the real part they 

 played in the processes of many chemical changes which before his era 

 were either wrongly understood (as will be afterwards explained) or were 

 not explained at all, but only observed in their superficial aspects. This 

 experiment on red mercury oxide has a special significance in the 

 history of chemistry contemporary with Lavoisier, because the oxygen 

 gas which is here evolved is contained in the atmosphere, and plays a 

 most important part in nature, especially in the respiration of animals, 

 in combustion in air, and in the formation of rusts or scorise (earths, as 

 they were then called) from metals that is, of earthy substances, like the 

 ores from which metals are extracted. The law of the indestructibility 

 of matter could not be discovered or confirmed by the balance until the 

 part played by the atmosphere as regards the participation of its oxygen 

 in the numerous chemical phenomena, known either from the everyday 

 experiences of life (combustion, respiration) or from the researches of 



20 In order to avoid the necessity of holding the cylinder, its open end is widened (and 

 also ground so that it may be closely covered with a ground-glass plate when needful), and 

 placed on a stand below the level of the water in the bath. This stand is called ' the bridge.' 

 It has several circular openings cut through it, and the gas delivery tube is placed under 

 one of these, and the cylinder for collecting the gas over it. 



