Prof. Regnault on the Specific Heat of some Simple Bodies. 107 



If for special experiments it is desired to obtain temperatures 

 lower than —40°, some liquid ammonia, which is prepared 

 easily and in abundance by an apparatus which I have described 

 (Memoires de V Academic des Sciences, vol. xxvi.), should be placed 

 on the annular space. The air-current passing through the liquid 

 ammonia with sufficient velocity, lowers its temperature to about 

 — 80°*. By regulating the current, the temperature maybe 

 kept stationary at any point between —40° and —80°; the great 

 latent heat of evaporation of ammonia renders it very easy to do 

 this. 



Magnesium. 



The specific heat of magnesium has not hitherto been deter- 

 mined. I used for this experiment a beautiful specimen of 

 magnesium, lent by M. Rousseau. The metal was prepared 

 by decomposing chloride of magnesium by sodium at a high 

 temperature ; a single regulus was formed with a considerable 

 depression in the centre. The regulus was wrapped in several 

 pieces of lead-foil to preserve it from contact with the air. 

 Magnesium can be kept for a long time in dry air without under- 

 going any perceptible alteration. 



M . . 



. . 91§ r *870 



92^*22 



Lead 



. . 49^*410 



29s r *69 



T . 



. . 97°*63 



98°*28 



& . 



. . 23°-81 



22°-99 



A0' . . 



. . 3°*8289 



3°-9203 



A . 



. . . 466^*69 



466s r *69 



C . 



. . . 0? r -2466 



0^-2533 



Mean of C .... 0s r -2499. 



This specific heat, multiplied by 150, the number now usually 

 taken as the atomic weight of magnesium, gives the product 

 37*49. Magnesium is consequently included in the law of spe- 

 cific heats of simple bodies, and its specific heat confirms the 

 accuracy of the formula, MgO, given to magnesia. 



Lithium. 

 In the memoir which I published [Ann. de Chim. et de Phys. 

 3rd series, vol. xlvi. p. 276), I gave the specific heat of chloride of 

 lithium, and, reasoning on this datum, I endeavoured to show 

 that the atomic weight which has been adopted for lithium, 

 80*37, ought to be halved, that is, reduced to 40*18. The for- 

 mula of lithia would then be Li 2 0, like those of potash and soda, 

 which, according to my experiments, should be written K 2 and 

 Na" 2 0. But for a convincing proof in the case of lithia, it was 



* MM. Loir and Drion have recently announced that by the evaporation 

 of ammonia in vacuo a cold of — ( .)(j° is obtained (Phil. Mag^ vol. xxi. p. 4 ( J5). 



