Mr. Joule on the Mechanical Value of Heat. 435 



of a pearly lustre. When sublimed, its crystals resemble 

 those of naphthaline, and when formed by spontaneous evapo- 

 ration from aether they are deposited in large, very regular 

 rhombohedrons. I formerly stated that alkalies do not evolve 

 ammonia from nitro-theine; I find that in this I was mistaken, 

 for when boiled with solution of potash it gives off abundance 

 of ammonia. 



When subjected to analysis, — 



I. 0*2628 gr. of the substance dried at 212° gave 0*398 

 carbonic acid and 0*1005 of the water. 



II. 0*2529 gr. gave 0*3855 carbonic acid and 0*0975 water. 

 When burnt with oxide of copper, ten tubes gave carbonic 



acid and nitrogen in the proportion of five to one. 



I. II. 



Carbon 41*87 42*15 



Hydrogen 4*24 4*28 



Nitrogen 19*39 19*56 



Oxygen 34*50 34*01 



100*00 100*00 



Nitro-theine appears to be a neutral body, and as I have 

 not been able to determine its atomic weight, I have not 

 thought it worth while to attempt to deduce any formula from 

 these analyses. Theine does not yield more of this substance 

 than from 5 to 6 per cent. 



LI I. On the Calorific Effects of Magneto-Electricity r , and on 

 the Mechanical Value of Heat. By J. P. Joule, Esq. 

 [Continued from p. 355 and concluded.] 

 Part II. — On the Mechanical Value of Heat. 

 TTAVING proved that heat is generated by the magneto- 

 *■* electrical machine, and that by means of the inductive 

 power of magnetism we can destroy or increase at pleasure the 

 heat due to chemical changes, it became an object of great in- 

 terest to inquire whether a constant ratio existed between it and 

 the mechanical power gained or lost. For this purpose it was 

 only necessary to repeat some of the previous experiments, 

 and to ascertain, at the same time, the mechanical force ne- 

 cessary in order to turn the apparatus. 



To accomplish the latter purpose, I resorted to a very simple 

 device, yet one peculiarly free from error. The axle b, fig. 1, 

 (p. 264) was wound with a double strand of fine twine, and the 

 strings (as represented in fig. 8) were carried over very easily- 

 working pullies, placed on opposite sides of the axle, at a di- 

 stance from each other of about 30 yards. By means of weights 

 placed in the scales attached to the ends of the strings, 1 could 



2 F2 



