212 



SCIENCE 



[N. S. Vol. XXIX. No. 736 



favor the growth of many small crystals. 

 This is well illustrated by the laboratory 

 experience of Day with the crj'stallization 

 of various lime-soda-feldspars. Thiis it 

 was found that 100 grams of liquid anor- 

 thite crystallized completely in ten minutes 

 to fair-sized crystals, and it required quick 

 chilling to prevent its crystallization and 

 to produce glass. A mixture of equal 

 parts of anorthite and albite (AbiAni) 

 required a gradual cooling extending over 

 several days to effect complete crystalliza- 

 tion, whereas liquid albite could not be 

 induced to crystallize through days of 

 cooling in an open crucible. Comparing 

 the size of the crystals of anorthite pro- 

 duced in 10 minutes with those of oligo- 

 clase-andesine (AbgAni) which were pro- 

 duced by gradual cooling through two 

 days, the former were from 3 to 5 mm. 

 thick, the latter about 0.005 mm. thick. 

 That is, the more liquid anorthite produced 

 crystals one thousand times as thick in 

 about one three-hundredth the time, a 

 ratio of 300,000 : 1. 



The rate of separation of solid from 

 liquid also depends on the solubility and 

 the amount of any substance in solution. 

 The greater each of these factors the more 

 rapid the rate of crystallization and the 

 larger the crystals, other things being con- 

 stant in compared cases. This law has 

 been expressed definitely by von Piekardt 

 as follows: "The velocity of crystalliza- 

 tion (separation in solid phase) is dimin- 

 ished by the addition of foreign substances 

 to the liquid phase of a substance, the 

 diminution of the velocity being the same 

 for equimolecular quantities of all sub- 

 stances. ' ' 



The order of succession in the separa- 

 tion of different kinds of minerals from 

 molten magma is a subject upon which 

 there has been some difference of opinion 

 among petrologists. It has been clearly 

 demonstrated that the order is not an in- 



variable one. The laws relating to the 

 order of separation of solids from mixed 

 solutions have been definitely determined 

 for solutions of various compounds in one 

 another and the general principles are ap- 

 plicable to the study of igneous rocks. 

 The attention of petrographers has been 

 called to these laws by Vogt. 



The order of separation of several com- 

 pounds in solution in one another depends 

 on the degree of saturation of each, that 

 with the highest degree of saturation, or 

 that one whose saturation point is reached 

 first upon the cooling of the solution sepa- 

 rates first. The relation between satura- 

 tion, molecular concentration and the melt- 

 ing point of each compound has been 

 established in general terms for different 

 sets of cases by Meyerhoffer, and further 

 elaborated by Roozeboom for cases of 

 crystals of isomorphous compounds. 



In all cases where the mixed compounds 

 do not unite chemically to form new com- 

 pounds, or physically as mixed crystals, 

 there is one minimum point of tempera- 

 ture for a mixture of two compounds, and 

 more than one in more complex mixtures, 

 at which a certain proportioned mixture 

 remains liquid. At this temperature the 

 two components of a binary mixture will 

 crystallize simultaneously. This minimum 

 temperature and particular mixture are 

 called eutectic. 



Miers has shown that when supersatura- 

 tion sets in and the labile condition is 

 taken into account, the minimum tempera- 

 ture of separation and corresponding 

 proportions of the mixture do not coincide 

 with those already described as eutectic. 

 These he has called hyperteetic. 



A study of these principles shows that 

 there can be no invariable order of separa- 

 tion, or crystallization, of the constituent 

 compounds in a series of mixed solutions 

 composed of like compounds. And that 

 simultaneous crystallization of pairs, or of 



