18 PRINCIPLES OF CHKMISTKY 



poured four ounces of pure mercury into a glass retort (fig. 3), whose 

 neck was bent as shown in the drawing and dipped into the vessel R s, 

 also full of mercury. The projecting end of the neck was covered 

 with a glass bell jar P. The weight of all the mercury taken, and the 

 volume of air remaining in the apparatus, namely, that in the upper 

 portion of the retort, and under the bell-jar, were determined before 

 beginning the experiment. In this experiment it was most important 

 to know the volume of air in order to learn what part it played in the 

 oxidation of the mercury, because, according to Stahl, phlogiston is 

 emitted into the air, whilst, according to Lavoisier, the mercury in 



FIG. 3. Lavoisier's apparatus for determining the composition of air and the 

 reason of metals increasing in weight when they are calcined in air. 



oxidising absorbs a portion of the air ; and consequently it w r as abso- 

 lutely necessary to determine whether the amount of air increased or 

 decreased in the oxidation of the metal. It was, therefore, most import- 

 ant to measure the volume of the air in the apparatus both before and 

 after the experiment. For this purpose it was necessary to know the 

 total capacity of the retort, the volume of the mercury poured into it, 

 the volume of the bell-jar above the level of the mercury, and also 

 the temperature and pressure of the air at the time of its measure- 

 ment. The volume of air held in the apparatus and isolated from the 

 surrounding atmosphere could be determined from these data. Having 

 arranged his apparatus in this manner, Lavoisier heated the retort 

 holding the mercury for a period of twelve days at a temperature near 

 the boiling point of mercury. The mercury became covered with a 

 quantity of small red scales ; that is, it was oxidised or converted into 

 an earth. This substance is the same mercury oxide which has already 

 been mentioned (example 3). After the lapse of twelve days the 

 apparatus was cooled, and it was then seen that the volume of the air 

 in the apparatus had diminished during the time of the experiment. 

 This result was in exact contradiction to Stahl's hypothesis. Out 

 of 50 cubic inches of air originally taken, there only remained 42. 



