26 CHEMISTRY FOR AGRICULTURAL STUDENTS 



amount of carbon combining with 8 parts of oxygen {i.e. 

 the amount which was found to combine with loo of mercury). 

 Numerous experiments have shown that carbonic acid gas 

 invariably has a uniform composition ; like mercury, carbon 

 combines with oxygen in definite proportions by weight. 



VI. AQUEOUS VAPOUR, WATER, AND ICE 



Besides rusting and burning and the conversion of lime into 

 chalk, another change that is often noticed taking place in the 

 air is the deliquescence of certain substances, such as common 

 salt and chloride of calcium. Place some of the latter in a 

 covered dish and weigh, then expose to the air till partially 

 liquid and weigh again. Should the substance now weigh 

 more, it may be assumed that the liquefaction is due to the 

 absorption of atmospheric moisture in which the chloride of 

 calcium has dissolved. To confirm, heat the partially liquid 

 substance in a test-tube, and notice whether moisture is given off 

 and deposited on the sides of the tube. This change may be 

 observed even in the driest weather, showing 

 that water vapour is always a constituent of the 

 atmosphere. 



Besides existing in the gaseous state, as water 

 vapour or sFeam, and in the liquid state, water 

 exists in the solid state as ice. The tempera- 

 tures at which these changes of state take place 

 are called the boiling and freezing points. 

 Determine the boiling point of water by finding 

 the temperature of the steam escaping from 

 rapidly boiling water contained in a flask fitted 

 with a cork, tube, and centigrade thermometer 



Fio. II. 



