CALCIUM PHOSPHATE AND CALCIUM CARBONATE. 293 



There was also an alkaline series made similarly to the above. 

 In each series there were ten grades as follows : 



Acid series treated with — » — , ■ — • • • - — - HC1. 



16 32 64 8192 



Alkaline series treated with — » — , — • • • NaOH. 



16 32 64 8192 



The pH of the acid series of washed gelatins in one per cent, 

 solution ranged from 3.2 to 6.0. Untreated gelatin, both in sheets 

 and in powder, which had been used for all the previous experi- 

 ments was found to possess a pH of 6.0. 



The weakest alkaline member of the series registered 6.2 and 

 the fifth member of the series a pH of 9.0, above which no record 

 was kept. 



The amount of precipitate visible to the naked eye was less in 

 the acid series than the alkaline one. It was least of all in those 

 specimens where the pH was in the neighborhood of the isoelectric 

 point of gelatin, which occurs in the acid series at 4.7, and which 

 marks the point where the gelatin is most insoluble and most inert. 

 In these slides there was practically no diffusion and spreading of 

 the precipitate, only a narrow band of deposit occurring along the 

 line of contact of the reacting solutions. Such a condition would 

 lead one to infer that the salts in solution are intimately bound to 

 the gelatin. 



It was also noted that crystals (Figs. 17, 18, 19) were abundant, 

 and very often large, in the various acid gelatins. Crystals were 

 also present in the weaker alkaline specimens, but in gelatin treated 

 with M/2048 NaOH or stronger alkaline solutions (Fig. 23) not 

 a single crystal has yet appeared in a period of six months since 

 the preparation of these slides. Here the precipitate is still in the 

 form of very small spherules, often so densely packed as to prevent 

 Brownian movement, but elsewhere still actively in motion, which 

 has gone on continuously for six months without a change, and • 

 gives every indication of continuing indefinitely. The larger of 

 these spherules, approaching one micron in diameter, have a slow 

 movement and so can be well observed, and have given definite 

 proof of what was already suspected from a study of other slides, 



