DEVELOPMENT OF THE ENAMEL 139 



during the calcification of the shell. The first experiments 

 were made with a solution of carbonate of potash in gum 

 (which contains salts of lime). The bodies so produced were 

 globular in form, showing concentric lamination and radial 

 striae. Professor Harting of Utrecht (12) independently 

 carried out a similar research, and succeeded in producing 

 a great variety of these spherical bodies in various sub- 

 stances, his most instructive experiment being one in which 

 small fragments of calcium chloride and of sodium carbonate 

 were placed on the opposite sides of a flat dish containing 

 egg albumin. The dish was left perfectly still and undis- 

 turbed for two or three weeks ; the salts diffusing through 

 the albumin from the opposite sides formed a crust upon 

 the surface. This crust was found to consist of a deposit of 

 carbonate of lime in the globular form, showing a great 

 variety of spheres and disks of varying size, and very similar 

 to those produced in gum in Rainey's experiments. He 

 found that they retained their form after they had been 

 subjected to the action of an acid, some portion of the lime 

 having become so intimately blended with the albumin 

 that it remained in combination with it. To this substance, 

 left after decalcification, Harting gave the name of ' calco- 

 globulin ', distinguishing the calcified globular bodies by 

 the name of ' calcospherites '. If a microscope slide be 

 prepared with gum containing carbonate of potash or soda, 

 and examined under the microscope, minute granules appear 

 after a short time, which increase in size by coalescence 

 until small globular bodies are formed the process of 

 molecular coalescence described by Rainey. 



In this experiment the formation of the particles is con- 

 sidered to be due to surface tension ; by this agency ' the 

 first ultramicroscopic particles are brought together ' , and 

 these further coalesce into visible particles, and finally into 

 the large calcospherites. 



It is surface tension that maintains the form of a drop of 

 fluid, and in considering the chemistry of colloids Ostwald 

 has shown that when a substance begins to separate out 

 from a solution it always makes its appearance first as 

 a liquid. In colloidal suspensions the material appears first 

 in a condition of solution, and as it passes from this to the 



