ON SOME BLOWPIPE-REACTIONS. ' 251' 



invariably, indeed, if the blast be long continued, tbe bead becomes 

 more or less milky or opalescent on cooling. This latter reaction was 

 apparently regarded by Plattner as essentially due to the presence of 

 alkaline or earthy bases, such as exhibit the reaction per se. He states 

 ( Prohirkunst : Dritie Aujlage, Jt.68) — "Da man nun von mehreren. 

 Silikaten ein Glas bekommt, welches, so lange es heiss ist, zwar klar 

 erscheint, aber unter der Abkiihlung mehr oder weniger opalisirt, so 

 muss man sich von der ausgeschiedenen Kieselsaure iiberzeugen, so 

 lange das Glas noch heiss ist, und dabei die Loupe zu Hiilfe nehmen. 

 Die so eben erwahnte Erscheinung tritt gewohnlich bei solchen Sili- 

 katen ein, deren Basen Kalkerde, Talkerde, Beryllerde oder Yttererde 

 sind, die ftir sich mit Phosphorsalz, bei gewisser Sattigung des Glases^ 

 unter der Abkiihlung oder durch Flattem milchweiss oder opalartig 

 werden." Dr. Theodor E-ichter, the editor of the 4th edition of 

 Plattner's work, leaves out the "gewohnlich" of the above quotations 

 and so makes the implication still stronger. In this vierte Aufiage, 

 the statement runs — " Bei solchen Silikaten deren Basen fiir sich mit 

 Phosphorsalz, bei gewisser Sattigung des Glases, unter der Abkiih- 

 lung oder durch Flattern milchweiss oder opalartig werden (Kalkerde,. 

 Talkerde, Beryllerde, oder Yttererde) wird die Perle unter der 

 Abkiihlung mehr oder weniger triibe." It is true enough that sili- 

 cates in which these bases are present, exhibit the reaction; but as 

 other silicates, practically all, indeed, exhibit the reaction also-, the 

 inference implied in the above statement is quite erroneous. The 

 opalescence of the glass arises entirely from precipitated silica. If the 

 blast be sufficiently kept up, a certain amount of silica is almost 

 always dissolved, but this becomes precipitated as the glass cools. A 

 simple experiment will shew that this is the true cause of the opales- 

 cence. If some pure silica (or a silicate of any kind) in a powdered 

 condition, be dissolved before the blowpipe-flame in borax until the 

 glass be nearly saturated, and some phosphor-salt be then added, and 

 the blowing be continued for an instant, a precipitation of silica will 

 immediately take place, the bead becoming milky — or, in the case of 

 many silicates, opaque- white — on cooling. This test may be resorted 

 to for the detection of silica in the case of silicates which dissolve 

 with difficulty in phosphor-salt alone, or which do not give a well- 

 pronounced "skeleton" with that reagent.* 



* By whom was tlie formation of a " silica skeleton " first made known ? There is no reference 

 to it in the early treatise of Von Engestkom attached to his translation of Cronstedt's " Miner- 



