Atomic Weight of Mercury. 445 



" But we might also consider the atom of mercury as only 

 12.5 or the half of 25. In that case, the red oxide would be a 

 compound of one atom of mercury and one atom of oxygen, and 

 the black oxide of two atoms of mercury, and one atom of oxy- 

 gen. There is nothing in these compounds that can determine 

 which of these views is the right one. Both oxides are capable 

 of combining with acids, and of forming salts. The red oxide 

 is the most permanent and intimate combination, but the black 

 is always first formed when we attempt to combine mercury 

 with oxygen. In such cases as this we are left to conjecture or 

 analogy to assist us in deciding what number should be taken 

 to denote the true atomic weight of the body. We see that 

 the atom of mercury weighs either 25, or the half of 25, but 

 which of the two, it might, in the present state of our knowledge, 

 be impossible to determine. In such a case, we may be allowed 

 to refer to analogy, to enable us to decide the point. It was 

 first observed by Dulong and Petit, that when the atomic weight 

 of a body is multiplied into its specific heat, the product is a 

 constant quantity. And I have shown, in my treatise on Heat, 

 that this product is always 0.376. Therefore, if we divide .376 

 by the number denoting the specific heat of mercury, the quo- 

 tient should be the atomic weight of that body. But the spe- 

 cific heat of mercury is .03 and .376 -4- .03 = 12.52. This 

 circumstance furnishes a reason for considering the true atomic 

 weight of mercury to be 12.5." — Vol. \, p. 9. 



" The specific gravity of the vapour of mercury, as deter- 

 mined by Dumas, is 6.976. From this determination, which 

 must be very near the truth, it follows as a consequence, that 

 the atomic weight of mercury is 12.5. For the atomic weight 

 of a gaseous body multiplied by .5555, is equal to the specific 

 gravity in the gaseous state. Now 12.5 x .5555 = 6.9747 

 a number which almost coincides with that found by Dumas." — 

 Vol. I, p. 612. 



" I have shown (Ann. of Phil. 2d series, ii. 126,) by experi- 

 ments which I consider as decisive, that the real atomic weight 

 of mercury is 12.5." — Vol. l,p. 615. 



If the arguments* advanced by Mr. Allinson, are added to 



Which are briefly these : the protoxide of mercury is decomposed more readily 

 than the deutoxide ; the protochloride ihan the deutochloride, &c : these facts contravene 

 the universal law, that compounds (of elements) consisting of one atom of each con- 

 stituent are less easily decomposed than those consisting of one and two. Hence, there 

 is a strong probability that what is called the deutoxide is, in reality, the protoxide, &c 



