102 



THE AMEKICAN MONTHLY 



[June, 



and irregularly fractured as is the case 

 in volcanic ashes, lose their distinct- 

 ive characters. Their microscopic 

 size does not permit their optical 

 properties to be learned ; their irreg- 

 ular and fragmentary form interferes 

 w^ith the determination of their char- 

 acteristics. The phenomena of pleo- 

 chroism, and the particnlar tint of the 

 mineral, lose so much of their bright- 

 ness that they do not serve for a siu'e 

 identification. It results from oiu* ob- 

 servations that a mineral having the 

 characters of those composing volca- 

 nic ashes, cannot be certainly deter- 

 mined when its dimensions descend 

 below 0.05 mm. ; while the vitreous 

 fragments are susceptible of determi- 

 nation even when their dimensions 

 are less than 0.005 mm. The ab- 

 sence or rarity of crystals or frag- 

 ments of volcanic crystals should not 

 be considered a proof that pulverulent 

 matter collected from the air is not of 

 eruptive origin, owing to the sorting 

 to which the particles are subjected 

 after the eruption. 



The most certain diagnosis of the 

 volcanic nature is always found in the 

 structure of the vitreous particles pro- 

 jected in the form of cinders. This 

 special sti"ucture is seen in the frac- 

 ture, and its imprint is found even on 

 the smallest fragments, when the mi- 

 croscope cannot reveal other charac- 

 teristic properties. To prove that 

 these characters of vitreous volcanic 

 materials remain constant even to 

 the ultimate limits of pulverization 

 we have ground in a mortar diverse 

 varieties of pumice. The powder 

 thus obtained was extremely fine, 

 neveilheless its infinitesimal particles 

 still showed the distinctive features 

 of which the ashes of Krakatau gave 

 such a perfect demonstration. The 

 diagnostic character to which we al- 

 lude is not the extraordinary prepon- 

 derance of the vitreous substance ; but 

 the great number of gaseous bubbles 

 imprisoned in the pumice and in the 

 vitreous portion of the ashes. These 

 bubbles of gas are due to the expan- 

 sion of gas dissolved in the magma, 



and which causes the eruption. Ad- 

 mitting that these incoherent volcanic 

 products are derived from the pulver- 

 ization of a fluid magma, one under- 

 stands that these particles are rapidly 

 cooled, remaining in a vitreous state, 

 and that the dissolved gas by expan- 

 sion forms numerous pores, which, 

 owing to the method of projection, 

 assume a drawn out or elongated 

 arrangement. The existence of these 

 bubbles or of this filamentous struct- 

 ure, therefoi-e, afibrds the means to 

 discern the volcanic nature of these 

 particles despite their condition of ex- 

 treme division. It is also their porous 

 nature that permits them to be trans- 

 ported such great distances. 



Vitreous fragments of a brown color 

 are quite rare in the ashes from Kra- 

 katau ; those of a deeper tint enclose 

 magnetite. In a general way it may 

 be said all the crystals except those 

 enclosed in vitreous matter are broken. 

 The minerals of the ashes of Krakatau 

 that are capable of positive determina- 

 tion are plagiocalse, angite, rhombic, 

 pyroxene, and magnetite. In pro- 

 portion as the ashes ai^e distant from 

 the volcano, the ashes become less 

 rich in minerals. 



Lighton's Immersion Ilhiminator. 



The following extremely simple 

 plan for an immersion illuminator 

 was first brought to the notice of mi- 

 croscopists a few years ago, and, in 

 the absence of the inventor, was kind- 

 ly described by Prof. Albert McCalla 

 at the meeting of the American So- 

 ciety of Microscopists at Columbus, 

 O. It consists of a small disc of sil- 

 vered plate glass c, fig. 20, about one- 



C R 



Fig. 20. — Lighton's Illuminator. 



eighth of an inch thick, which is 

 cemented by glycerin or some ho- 

 mogeneous immersion medium to the 



