Skey. — On the Diniorphisim of Magnesia. 389 



When this periodide is exposed to the au- it gradually acquii-es a blue 

 colour, but if allowed contact with sodic-bicarbonate in quantity this colour 

 is produced instantaneously and voluminously with a beautiful and striking 

 effect. In shade it is not to be distinguished from that of the compound of 

 iodine with starch. 



The new salt thus produced is almost, if not entirely, insoluble in water, 

 hot or cold, also in potassic-iodide or alcohol, nor is its appearance changed 

 by these liquids. 



Treated with any ordinary acid, even with the acetic, it is instantly 

 decomposed, carbonic acid and iodine being liberated. Any free alkali 

 also decomposes it ; carbonic acid, even, in conjunction with water, decom- 

 poses it, but very slowly, iodine being liberated in a free state. By 

 spontaneous evaporation, it can be had in acicular crystals. 



It appears to be composed of iodine and carbonic acid with lead and 

 oxygen. 



The effect of a long-continued contact of this salt with carbonic acid and 

 water is to produce another compound of quite a different kind. This is 

 insoluble in acetic acid and quite colourless. 



There are two points in connection with this subject which I would 

 particularly desire to bring to your notice ; they are — first, that this reaction 

 of per-iodide of lead with carbonic acid is a very delicate and beautiful test 

 for this acid ; thus when shaken up in a finely granular form (as precipi- 

 tated) with ordinary spring or rain water, mixed with a little borax if need 

 be, or if breathed upon, the presence of this acid is revealed by a greenish 

 colour being instantly communicated to the water, which colour soon passes 

 to a blue if the acid is present in more than a very minute quantity. This 

 is a striking demonstration for the lecture-room, and the only chromatic 

 test as yet known for the presence of the acid cited. 



The next point I wish to note is the striking resemblance of the colour 

 of this new compound to that of iodine with starch. This is strongly 

 suggestive of the idea that in both compounds the iodine is in the same 

 molecular state, and consequently that the compounds themselves possibly 

 are in some fundamental points analagous. 



Art. LIV. — The Vimorphisim of Magnesia. 



By William Skey, Analyst to the Geological Survey Department. 



[Bead before the Wellington Philosophical Society, ith December, 1880.] 



While trying to electro-deposit gold from sea-water, so as to verify Sonstrad's 



observation regarding gold in sea-water, and if possible to estimate it, I ap- 



