Intelligence and Miscellaneous Articles. 289 



talline and glassy constituents to one another. Holocrystalline 

 aggregates, of basic composition, are found passing, as the quantity 

 of acid glass increases, into various " ophitic," " intersertal," and 

 " pilotaxitic " types, finally assuming the " vitrophyric " form of 

 " pitch stone-porphyries " — rocks which have a distinctly acid com- 

 position. The rocks of the Tertiary dykes in Southern Scotland and 

 the North of England, which have been described by Dr. A. Geikie, 

 Mr. Teall, and other authors, were shown to agree with these later 

 Tertiary andesites, both in their mineralogical constitution and in 

 the peculiar phases which they present to us. The latest Tertiary 

 ejections were shown in 1874 to bear the same relations to the five 

 grand volcanoes of the Western Isles which the chains of " puys " in 

 Auvergne do to the great central volcanoes of that district ; and this 

 conclusion is strikingly confirmed by petrographical studies of which 

 the results were given in the present memoir. 



XXXI. Intelligence and Miscellaneous Articles. 



NOTE ON THE GRADUAL ALTEKATION IN GLASS PRODUCED BY 

 ALTERING ITS TEMPERATURE A FEW DEGREES. BY SPENCER 

 U. PICKERING, M.A.* 



nPHE alteration experienced by glass when heated to tempera- 

 ^- tures above 50° C. is well known, but I do not think that it 

 has previously been noticed that a rise or fall of even a few degrees 

 at ordinary temperatures will produce an appreciable alteration of 

 a similar nature. 



That such is the case, however, the following experiments will 

 show. During the course of a series of density-determinations, in 

 which the apparatus used consisted of an ordinary 25 cub. cent. 

 density-bottle with perforated stopper — but of which the stopper 

 was so accurately ground that it made the instrument thoroughly 

 reliable — the water-contents of the bottle were frequently deter- 

 mined. Between November 1887 and May 1888, during which 

 time the temperature of the laboratory had been kept at about 18°, 

 no appreciable alteration in the capacity occurred, the values all 

 being within -0002 grm. of 25*0070 grm., and this in spite of the 

 fact that the bottle had been temporarily heated to temperatures as 

 high as 38°. But during November and December of the latter 

 year the temperature of the laboratory had been maintained 10° 

 lower than during the previous twelve months, and the effect of 

 this lowering on the bottle was such that its water-contents, mea- 

 sured at the same temperature, were *001 gram less. Although the 

 amount of this contraction is small, the following values will, I 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 29. No. 178. March 1890. Z 



