80 Intelligence and Miscellaneous Articles. 



place in France seventeen centuries later : had they passed a roller 

 over this frame, they would have obtained glass of uniform thickness, 

 and nothing more would have been required except to polish the 

 surfaces, an operation to which they were no strangers ; for Pliny 

 says that they made use of obsidian to make mirrors which they at- 

 tached to the walls ; and this could only be done when the obsidian 

 was polished. 



The Pompeian window-glass is of a bluish-green tint, like our 

 common glass to within the last fifty years. Its analysis by M. F. 

 Claudet gave the following result : — 



Silica 69-43 



Lime 7*24 



Soda 17*31 



Alumina 3*55 



Oxide of iron 1*15 



Oxide of manganese . . 0*39 



Oxide of copper traces 



99-07 



This composition agrees almost exactly with that of the window- 

 glass made now-a-days. The analysis of this given by Dumas shows- 

 Silica 68-65 



Lime 9*65 



oocia . ., ....... ...... •••• i. i / u 



Alumina 4*00 



In this, traces of iron and manganese may have been neglected, and 

 the glass was of inferior quality, the average of composition of our 

 window-glass being as follows : — 



Silica 72-50 



Lime 13'10 



Soda 13-00 



Alumina 1 -00 



Oxides of iron and manganese 0*40 

 — Comptes Rendus, May 5, 1862, p. 980. 



ON A QUESTION IN THE THEORY OF PROBABILITIES. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 The law of form noticed by Mr. Cayley in the June Number of the 

 Philosophical Magazine, p. 471, is true when the data are the pro- 

 babilities of events unconnected by any conditions expressed or im- 

 plied ; but it is then true, not as a first principle, but as a conse- 

 quence of acknowledged principles in the theory of probabilities. As 

 it has not been proved to be true beyond this case (indeed it is cer- 

 tainly not generally true), no argument ought to be built upon the 

 assumption that it is true. 



I am, Gentlemen, 



Your obedient S ervant, 



George Boole. 

 June 11, 1862. 



QoLS.ifc- 



