Dr. Woods on the Thermic Equivalent of Magnesium* 49 



and hence for dry air - =1-4144. Considerable uncertainty, 



however, attaches to this correction. 



Hence, then, we can calculate the mechanical equivalent of 

 heat, a point to which I intend to return in a subsequent com- 

 munication. 



Ziitphen, November 19, 1864. 



VI. The Thermic Equivalent of Magnesium. 

 By Thomas Woods, M.D.* 



THE great amount of light produced by the combustion of 

 magnesium would make it desirable, if there was no other 

 reason, to know if a corresponding quantity of heat is evolved 

 during the same process. I however felt interested in this mat- 

 ter, chiefly on account of the researches I made a few years ago 

 " On the amount of Heat produced by the combination of several 

 Metals with Oxygen," and the occurrence of these amounts in 

 multiple proportions (see Phil. Mag. November 1852, and July 

 1856). 



As a matter of curiosity in the history of magnesium, I must 

 mention that at the time I speak of (in 1856) I wrote to a firm 

 in London to know the cost of some of the rare metals I intended 

 to experiment with, and the price fixed for magnesium was six- 

 teen shillings for three grains ! 



The method I adopted to find the quantities of heat of oxida- 

 tion of the metals, mentioned in the paper in this Magazine for 

 November 1852, I used in the present instance also to ascertain 

 that evolved by magnesium ; that is, I dissolved it in ddute sul- 

 phuric acid, marked the rise of temperature of the fluid, and cor- 

 rected the result by making the proper allowances for the heat 

 absorbed by the decomposition of the water, and that evolved by 

 the combination of the magnesia with the acid. 



No description of the apparatus, or the several steps of the 

 process, need be detailed, as examples are given in the papers 

 referred to ; and since then the same experiments (without ac- 

 knowledgment) have been worked out by Eavre and Silberman, 

 and published in the Annates de Chimie for 1853. 



It will suffice to state that the result of many experiments is 

 that the quantity of heat developed by the combustion or oxida- 

 tion of an equivalent of magnesium is exactly twice as much as 

 that produced by the oxidation of an equivalent of zinc. 



Zinc will raise the temperature of 1000 grains of water 9 0, 6 E. 

 by the combustion of 4 grains, or 1 equivalent, oxygen =1. 



* Communicated by the Author. 



Phil. Mag. S. 4. Vol. 30. No. 200. July 1865. E 



