PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 431 



The four metals of the alkaline earths, barium, strontium, cal- 

 cium, and magnesium, by a star having four points. 



The consideration of metals and metallic compounds would 

 open too wide a field for the present discussion. 



Having spoken of true chemical combinations, and symbolized 

 water and carbonic acid, we are prepared to speak of the mixture 

 of gases by virtue of the law of ditFusion. Figure 9 will convey 

 some idea of the proportion of gases found in common air, nearly 

 four-fifths being nitrogen, and one fifth oxygen. There is a trace 

 of ammonia, carburetted hydrogen, and nitric acid in air which 

 is not represented. 10,000 parts of common air common air con- 

 tain from three to six parts of carbonic acid. The proportion of 

 water varies with the temperature of the air. Every 15° C. of 

 additional heat doubles the capacity of a given quantity of air for 

 holding vapor. 



The symbols now presented are intended to impress upon the 

 mind the leading characteristics of the non-metallic elements, and 

 by proper grouping to show their relation. This ground-work 

 thoroughly mastered will enable the student to erect in his own 

 mind a permanent superstructure. Thus prepared, he will quickly 

 catch the significance of every chemical structure, and readily 

 apply the fundamental principles of the science to the experi- 

 mental work of the laboratory. 



An interesting discussion followed this explanation of new sym- 

 bols by Professor Tillman, in the course of which the chemists 

 present expressed their hearty commendation of the scheme. 



Improved Stoves. 

 Mr. J. C. Cochrane, exhibited a model of his improved stove 

 which he claimed to possess all the requirements for perfect com- 

 bustion. In ordinary stoves, much of the fuel goes up the chimney 

 unconsumed. To remedy this, air is admitted just behind the fire, 

 but in this process the temperature of the draft is lowered ; now 

 it is claimed that if air is applied on the flame, the temperature is 

 increased, and if an obstruction is placed against the flame the 

 temperature is raised. This stove is constructed on the principal 

 of the Davy Safety Lamp. The flame is arrested by a wire guaze 

 through which the air will pass, though the flame will not, 

 whereby the temperature is elevated and not depressed ; and all 

 the combustible gases are consumed, the carbonic oxide gas which 

 is so prolific in causing smoke alone escaping. This gas passes 



