THE BIOMETER: HOW TO USE IT 121 



off carbon dioxide, will not be coated with the precipitate, 

 but will have on its surface some crystals of barium car- 

 bonate, which becomes more heavily precipitated as the 

 respiration goes on. By repeating the same experi- 

 ments after interchanging the chambers, using the left 

 for the tissue and the right for a blank, it will be possible 

 to eliminate any possible error which might come from 

 some technical fallacy characteristic of one particular 

 chamber. For a casual observer the initial granule 

 will not be distinct from a granular spot on the glass. 

 The granules, however, will soon increase over the surface 

 of the drop and will gradually collect downward at the 

 edge of junction of the drop with the glass tubing. 

 The thick band of white precipitate around the bottom of 

 the drop will gradually extend toward the top of the 

 drop, so that after the band reaches more than half of the 

 hemispherical drop of barium hydroxide one can see 

 with the naked eye, not only from the side, but also 

 from above, the whole drop, now resembling a contract- 

 ing iris. When the very top of the drop is filled with the 

 precipitate, the whole drop of barium hydroxide will 

 look very opaque, covered with a thin layer of the car- 

 bonate. If one take a small piece of sciatic nerve of a 

 frog, say about 20 mg., he can see these different stages 

 of precipitation very distinctly, but when the amount 

 of tissue taken is very large it is very difficult to observe 

 these phenomena on account of the too rapid formation 

 of the precipitate all over the surface of the drop. It 

 is therefore best to take a very small piece of the nerve 

 for the purpose of following these different stages of the 

 precipitation of carbon dioxide as carbonate, for the 

 practice of distinguishing these different stages is very 



