no Mr. J. P. Joule on the Electric Origvi 



of phosphorus, weighing 0*4 grain, was then put into the 

 scale, and over it I placed a heap of fine zinc turnings, weigh- 

 ing 50 grains. I now ignited the phosphorus, and plunged 

 the scale into the inner jar. After the combustion had termi- 

 nated, and the heat thereby evolved had been evenly distri- 

 buted throughout the water by stirring, the increase ol tempe- 

 rature was noted. The contents of the scale were then thrown 

 into dilute sulphuric acid, and the volume of hydrogen thereby 

 evolved indicated the quantity (generally about 15 grains) 

 which had not been burnt. Two-tenths of a degree of heat 

 were deducted from the observed heat, on account of the phos- 

 phorus, and an allowance having been made, on account of 

 the capacity of glass for heat, the results were reduced to the 

 standard of one pound of water. 



53. The mean of several experiments conducted in the above 

 manner, showed that the heat evolved by the combustion of 

 32*3 grains of zinc is able to increase the temperature of a 

 pound of water by 10°'8. 



54. The heat evolved by the combustion of iron was ascer- 

 tained in a similar way. The iron was in the state of fine 

 wire, and that portion of it that was not burnt was carefully 

 collected, weighed, and deducted from the original quantity. 

 The mean of several trials indicated that 28 grains could in- 

 crease the temperature of a pound of water by 9°'48. 



55. Heat evolved by the combustion ofpotassium. — This metal, 

 in pretty large lumps, was introduced into an atmosphere com- 

 posed of equal bulks of oxygen and air. I then introduced 

 a stout iron wire, sharpened at the end, into the jar, and with 

 it I cut the potassium into small pieces. Under this treat- 

 ment it soon became so soft, that every time the rod was lifted 

 it would draw out a string of metal. In this state it often ig- 

 nited, and the experiment was spoiled on account of the par- 

 tial formation of peroxide. However, by carel'ul manage- 

 ment, I succeeded in making some good experiments, in which 

 nearly all the potassium was converted mio j}otassa \ and the 

 exact quantity of unoxidized metal was ascertained by obser- 

 ving the volume of hydrogen evolved when the contents of the 

 scale were exposed to the action of water. The mean of these 

 showed that the heat evolved by the conversion of 40 grains of 

 potassium into potassa is able to increase the temperature of a 

 pound of water by 1 7°*6. 



5G. Heat evolved by the combustion of hydrogen. — The gas 

 was burned in an atmosphere of oxygen, diluted with common 

 air, by means of a jet furnished with a very narrow bore. A 

 grain of hydrogen evolved as much heat as is able to increase 

 the temperature of a pound of water by 8°-S6. 



