146 a. l. day mineral relations from laboratory viewpoint 



Individuality of different Minerals in Melting 



The laboratory student, proceeding from the physical standpoint, recog- 

 nizes melting as a "change of state'^ involving, according to accepted 

 molecular theory, a complete change in the molecular structure of the 

 substance. Such a change of state will carry with it various visible evi- 

 dences of its occurrence, as, for example, the disappearance of crystalline 

 structure; a change of density; a change in the electrical conductivity; 

 a more or less sudden appearance of fluidity causing it to take the shape 

 of the containing vessel; a change in the specific heat — in a word, there 

 appears a more or less conspicuous discontinuity in all its physical prop- 

 erties. In respect of these outward evidences, different substances will 

 obviously vary. 



In pure diopside, for example, the crystalline structure disappears 

 with considerable promptness at the moment of melting, while silica and 

 the more acid feldspars are extremely slow in transition from the one 

 state to the other. The density change accompanying the change of 

 state in anorthite is small, while for albite it is relatively much larger. 

 If we assume that the relative density of the cooled products offers an 

 approximate measure of its magnitude, the density change from anorthite 

 to its glass is but 2 per cent, while for albite it is five times as great. 

 In a substance like ordinary borax, which will no doubt prove to have 

 several analogues among the minerals, the density change proceeds nearly 

 as far in the opposite direction, while among organic compounds many 

 are found to undergo no measurable change in either direction. Pure sil- 

 limanite flows so freely at the moment of melting that in an oxyhydrogen 

 flame the liquid mineral is freely blown about by the flame, while pure 

 silica and the feldspars again offer no outward evidence of flow during 

 the melting process. Some of the phenomena attending the change of 

 state are accessible to established methods of observation, while others 

 can not readily be detected by any method suggested by existing labora- 

 tory experience. There is therefore a certain amount of individuality 

 in substances which is altogether characteristic, but which may render a 

 particular method which has proved fully competent with one substance 

 to be very inefficient when applied to another. Considerations like these 

 not only increase the scope of the laboratory problem, but detract some- 

 what from its assumed simplicity. If we would determine the melting 

 temperature, or change of state, of a great body of substances, it is not 

 merely necessary to he able to measure temperatures accurately and con- 

 veniently, hut also to ohtain sufficient knowledge of the individuality of 

 the substances under investigation to enable us to he quite sure that the 

 method employed for detecting the change of state when it occurs is an 

 appropriate one for each particular substance. By way of illustration. 



