of the Heat of Combustion. 109 



bustion, it will be necessary to deduce our calculations from 

 the electric intensity which is required in order to reduce the 

 product of combustion to the state in which its elements were 

 prior to combustion. The following is a list of these intensi- 

 ties, reckoning the decomposition of water into its gaseous ele- 

 ments as unity. 



47. Intensity necessary to decompose oxide of zinc into gase- 

 ous oxygen and metal, from (4'2.) and (44.), is 3'7 pairs of 

 Smee's batter}', or \-32h. 



48. Intensity necessary to decompose protoxide of iron into 

 gaseous oxygen and metal. — From (43.), 3"3 of Smee's pairs 

 = ?■; and from (41.), 2-8 pairs = h ; whence 2'8 i — 3*3 //, 

 or i— 1-18. 



49. Intensity necessary to decompose potassa into potassium 

 and gaseous oxygen. — From (44.) and (37.) we have 1*93 +1*9 

 : 4-06 + 1-9 :: 1-32 /i : 2-05//, the intensity required, which 

 may be otherwise expressed by 5'74 of Smee's pairs. 



Heat evolved by Comhustioii, 'when it terminates in the forma- 

 tion of an Electrolyte. 



50. Finding that our information on the quantity of heat 

 evolved by the combustion of metals was not very satisfactory, 

 I have, without wishing to depreciate the labours of Dulong, 

 Despretz and others, thought it right to bring forward such of 

 my own experiments as aie necessary in order to make my in- 

 vestigation complete. 



51. I provided two glass jars. The smaller had an intei- 

 nal capacity of 90 cubic inches; and when placed within the 

 other jar, as represented by fig, 7, the space left between the 

 two was sufficient to contain three pounds of water. By 

 means of a scale, s, suspended by wire from a thick fold of 

 moistened paper, 1 was able to introduce a combustible within 

 an atmosphere of oxygen, and by means of a heavy weight I 

 could keep the paper sufficiently close to the top of the jar to 

 prevent the escape of any considerable quantity of heated air, 

 while at the same time it was not so tight as to prevent the 

 admission of air as the oxygen was consumed. The increase 

 of the temperature of the water was measured by a thermo- 

 meter of great sensibility. 



52. The heat evolved by the combustion of zinc was ascer- 

 tained in the following manner. The smaller jar was filled 

 with oxygen, placed in the other jar, and surrounded by three 

 pounds of water, the heat of which was contrived to be as 

 much below the ten)perature of the surrounding air as it was 

 expected to exceed it at the close of the experiment. A piece 



