1878. J lUd [Frazer. 



These represent in the rock — 



p. c. of (Si0 4 ) combined in ortho-silicates 22.67 



p. c. of (Si0 3 ) combined in mono-meta-silicates 28.12 



Total 50.79 



It is interesting to note in conclusion that the rational formula calcu- 

 lated for labradorite according to the modern chemical system and which 

 regards this mineral as one of the para-silicates is 



(Ca H Al, Ti ) Tiii (Si 3 iv O 10 u ) viii . 

 This might be viewed as a mixed ortho- and mono-meta-silicate* in which 

 there are two molecules of the radical (Si0 4 ) and one of (SiO : .). In the 

 latter one of the atoms of O is employed in saturating alone, and the pro- 

 portion which this bears to the total amount of oxygen in both radicals is 

 evidently 1 : 11. 



In Pyroxene all the silica is present as mono-meta silicic acid. 



In a mixture containing exactly one molecule each of Labradorite and 



Pyroxene, there would then be : 



Orthosilicic Mono-meta- 



Labradorite. 

 Pyroxene. . 



Acid. 



silicic Acid . 



That is, the number of molecules of Ortho- and Mono-meta-silicic acid 

 would be equal, or if the p. c. by weight of the latter were as above sup- 

 posed, 22.67 in the rock, that of the latter would be in such a mixture, 

 27.17 p. c. which is very nearly that actually given. 



On the Total Solar Eclipse of July 2Wi, 1878. 

 By George F. Barker. 



{Read before the American Philosophical Society, Nov. 15th, 1878.) 



The purpose of the present paper is to put on record in the Proceedings 

 of the Society some account of the observations made by certain of its 

 members upon the total solar eclipse of the 29th of July, 1878. 



The expedition was organized in June, by Professor Henry Draper of 

 New York, out of compliment to whom, his associates named it the 

 Draper Eclipse Expedition. The party consisted of Dr. Draper as Direc- 

 tor, with Mrs. Draper as assistant, who were in charge of the photographic 

 and photospectroscopic work, as also of the observations with the slitless 

 spectroscope ; of President Morton, of Hoboken, to whom was confided 

 the general observations, as well as those with the polariscope and pocket 

 spectroscope ; of Dr. Thomas A. Edison, of Menlo Park, who was to 

 use his newly invented tasimeter. in order to determine whether it was 



*See " Tables for the determination of minerals," Frazer. 1874. 



