xxvm 



it was a veritable triumph for his method of work that he was able 

 to establish that there was a kind of potash-felspar distinct from 

 orthoclase ; it was anorthic instead of mono-symmetric in its sym- 

 metry, although approximating to the latter in the development of 

 its forms, and its optical characters, instead of being unstable, are 

 stable at all temperatures ; to this he applied the term microcline. 

 Indeed, it is to Des Cloizeaux that we owe our first precise know- 

 ledge of the optical characters of all the plagioclastic felspars, and 

 the determination presented constant difficulty to him by reason of 

 the lowness of the symmetry and the smallness and rarity of well- 

 developed crystals. 



Much of his work has been incorporated in the treatise to which 

 he gave the modest title ' Manuel de Mineralogie,' and which is now 

 the standard book of reference for all that relates to the optics of 

 minerals. In its compactness and freedom from unnecessary words, 

 and indeed in its general characters, it bears a close resemblance to 

 (Brooke and) Miller's edition of ' Phillips' s Mineralogy '; and it is 

 interesting to know that at first he had intended merely to translate 

 that book, but was eventually compelled, by the extension of his 

 optical researches, to prepare an independent treatise. He adopted 

 the same general plan in giving a stereographic projection of the 

 observed faces for all the more important minerals, and elaborate 

 lists of measured and calculated angles useful in the recognition of 

 the substance. The preparation of this manual was a work of great 

 labour, and involved a vast amount of physical observation and 

 numerical calculation ; he made it a rule never to cite an angle with- 

 out verification by observation or recalculation. The first volume of 

 the manual was published in 1862 ; the first part of the second 

 volume in 1874, and the second part in 1893. The third and last 

 volume had not been issued at the time of his death, but is now 

 being prepared for publication by his successor at the Museum and 

 former pupil, Professor Lacroix. 



Professor Des Cloizeaux was elected Metnbre de 1'Institut in 

 1869, and President of the Academic des Sciences in 1889. The 

 Royal Society awarded him the Bumford Medal in 1870, and 

 elected him a Foreign Member in 1875. He was a founder and the 

 first President (1878) of the Mineralogical Society of France, and 

 again served as its President eleven years later. Other Societies in 

 many parts of the world recognised the value of his scientific work 

 by enrolling him on the list of Honorary Members. An aged widow, 

 a widowed daughter (the Vicointesse d'Herouville), and three young- 

 grandchildren more especially mourn his loss ; but the memory oi 

 his kindly character and encouragement will long be treasured up 

 by those who were in any way associated with him. 



L. F. 



