472 BEPOiiT — 1893. 



heat ; a little hfematite was visible, and that qaite close to the aphthitalite 

 near the cooling surface. 



The aphthitalite occurred in thin plate-like crystals, very much resem- 

 bling those of specular iron. They were simple hexagonal plates or com- 

 pound featbers ; only in the smaller crystals were the rhombohedral faces 

 developed along the edges of the plate. Some were white with a faint 

 opalescent tinge, but that variety was rare. Others were more opalescent 

 and some quite milky. These latter shaded into examples of the most 

 beautiful cerulean blue, and thence through varying tints of bird's-egg 

 green to light chrome green and greenish yellow. The blue was due to 

 copper impurities, and the green to iron and copper sulphates combined. 

 The crystallisation was best developed in the whiter or more translucent 

 specimens. Crystals of different tints were often blotched with a reddish 

 coppery lustre, due to the contemporaneous deposition of fine transparent 

 laminte of htematite. 



In some fissures of the hot lava were moss-like deposits of the 

 mineral euchlorine (called by Scacchi encloruia), of bright emerald green 

 colour. The deposit of this, however, was very limited. Several rifts 

 in the new lavas have been most beautifully coated with very delicate 

 feathery deposits of mixed sodium and potassium chlorides. Some had 

 grown to such dimensions and solidity that they could be removed. This 

 form of sublimate, although one of the commonest, is extremely rare in 

 collections, for it is so fine and light that the slightest current of air 

 I'educes it to a fine powder. 



At other spots thick saline crusts were deposited, but these proved to 

 be very composite in nature, consisting of mixed sulphates and chlorides 

 of the alkalies and alkaline earths, with much iron and a little copper. 



If we carefully examine the history of eruptions at Vesuvius in which 

 a record of the time and place of sublimates is made, and if we make 

 constant observations of the vapour components, one fact becomes evident. 

 The vapour that first escapes from boiling lava when it reaches the 

 surface of the earth consists in great part of sulphurous acid and 

 probably alkaline sulphites. Later, and more slowly, the hydrochloric 

 acid and chlorides volatilise. Thus, when much new lava is issuing 

 sulphurous acid is very obvious at the central crater, and around this 

 incrustations of sulphur and sulphites prevail over chlorides, which are 

 only deposited or produced by the escape of hydrochloric acid gas at 

 more distant fumaroles. At the point of exit of the lava sulphites are 

 deposited, but after the lava has flowed some distance chlorides are more 

 abundant. 



In May of this year when visiting the Atrio at the point of exit of the 

 lava a phenomenon, of which I have never seen or heard of the like, 

 could be studied. One of the curious conical spiracles, similar to those I 

 described in the report of this committee for 1891, was puffing away with 

 violent intermittent but not i-hythmic blasts, and with the occasional 

 escape of small fragments of lava. Watching this action, the cause of 

 which has always appeared to me obscure, I noticed a large ball of 

 incandescent pasty lava appear at the mouth of the spiracle at intervals, 

 and as soon as the high pressure vapour found exit by its side it again 

 fell back, and for a moment or two almost completely stopped any vapour 

 escaping. It seemed to have nearly blocked the lower opening from 

 which at times it was blown up by the vapour when the pressure in- 

 creased. In fact it was somewhat like a ball valve. After watching this 



