J 30 Prof, N." St Maskelyne on the Crystallography 



clined at small normal angles on each other, it is often difficult 

 to discriminate the faces lying in a particular zone, as the 

 greater number of the determinations have to be made with 

 faces which offer no images or such as are very confused, and 

 which can be measured only by the method of maximum illu- 

 mination. 



The figures 1 and 2 represent the same forms as those 

 already given in the Journal of the Chemical Society. Fig. 3 

 exhibits the forms which exist on the crystals obtained from the 

 French and the juniper oils, together with some other forms, 

 of which the existence on some of the crystals is probable, 

 though no reliable measurements could be obtained from them. 

 The crystals rapidly lose by exposure such lustre as they have 

 when fresh — apparently in consequence of evaporation taking 

 place at ordinary temperatures, as evinced by a faint odour 

 perceptible even while a little crystal is exposed on the go- 

 niometer. 



The measurements obtained from crystals of the different 

 sorts are arranged in columns, — the first column consisting of the 

 calculated angles as given in the earlier communication to the 

 Chemical Society ; column II. containing the angles obtained 

 by measurement of the crystals thus described, which were 

 obtained from American and ordinary oil of turpentine. Co- 

 lumns III. and IV. give the averages of the angles (omitting the 

 extremes) as obtained by measurements from the crystals made 

 from French and from juniper turpentines respectively ; while 

 column V. gives the angles calculated on average data, obtained 

 from what seemed to be the better measurements, yielded by the 

 two latter varieties of the crystal. The forms on these " French" 

 and "juniper" nitrosoterpenes were by no means uniformly 

 the same. Some of the crystals were very complicated, ex- 

 hibiting numerous forms of which even the zone-positions 

 could be only approximately ascertained ; others were much 

 simpler, the faces of the forms {11 1}, {201}, {110}, in one 

 case 31 apparently, with traces of 1, forming the combi- 

 nation. On others the form u or {312} seemed the most im- 

 portant face. The faces of the form {110} are in the latter 

 varieties always corrugated by a series of planes inclined at 

 from 1° to 3° on each other, approximately in the zone 

 [1 1 0, 1]. The following letters represent the faces and 

 forms of the crystals : — 



a, {100}; m, (110); b, {010}; m',(l 1 0); m',(l 1 0); 



£,{101}; S,{201}; c,{001}; A, {011}; n,{031\; 



JP, {HI}; Ull2};y,{3 3 2} 5 ^{312}; p,{512}; 



X, {3 51}. 



