488 Johnston, Mericitt, and Williamson — 



elongation, and maximum and minimum refractive indices* of 

 1*62 and 147. The material, when dried at 120°, lost only an 

 insignificant amount of water, and retained its original optical 

 properties; its density proved to be 2 , 51, considerably lower 

 therefore than aragonite (2*9) and calcite (2*7) and somewhat 

 lower than /u.-CaC0 3 (2 - 54). These several properties thus 

 differ so much from those of the other forms that it would 

 seem that this material is a distinct form. 



But there are other lines of evidence which are opposed to 

 this conclusion. A complete analysis of the material, dried at 

 120°, showed that it was CaCO„ with about 4 per cent of 

 water and about 1 per cent KOH.f Now such an amount of 

 water (which would be about 10 per cent by volume of the 

 material) if held as such in submicroscopic interstices among 

 the radiating fibres of calcite would reduce the density and 

 refractive index of the latter, yielding apparent values cor- 

 responding very closely to those observed for " vaterite." 

 Again these aggregates preserve their structure intact, even 

 after being heated to 700°, a temperature sufficient to invert 

 both aragonite and /u-CaC0 3 to coarse-grained calcite ; they 

 then consist undoubtedly of calcite. These two lines of 

 evidence point unmistakably to the inference that "vaterite" 

 is spherulitic calcite containing water enmeshed among the 

 fibres. This conclusion is corroborated by another line of 

 evidence, as follows : Along with the typical spherical 

 aggregates, there appeared in our experiments small numbers 

 of aggregates of other shapes, all too small to be broken into 

 fragments upon which optical measurements could be made, 

 but the fibres of some of them are so nearly parallel that they 

 extinguish simultaneously, and thus allow one to measure the 

 maximum and minimum refractive index. These observed 

 refractive indices range from those characteristic of calcite to 

 those of " vaterite," though the aggregates themselves would 

 have been called " vaterite." 



We conceive then that these spherulites consist essentially 

 of radiating prisms of calcite too fine to be resolved by the 

 highest powers of the microscope ; the individual calcite fibres 

 are not all in actual contact — for, when crushed, the aggregates 

 fall apart along radial lines — but are separated by very thin 



*In one case the refractive indices of material precipitated froni a much, 

 more alkaline solution were 1"585 and 1"455; practically all the spherulites 

 yielded indications that the material about the center differed slightly from 

 that in the outer layers. 



f One would expect from the mode of preparation and state of the sub- 

 stance that it would contain potash which could not be removed by washing. 

 Incidentally it may be observed that the fact of the retention by this material 

 of as much as 4 per cent of free water is an additional example of the impos- 

 sibility of distinguishing between combined and free water in fine-grained 

 material by the loss of water on heating at any definite temperature. 



