CELL DIFFERENTIATION 



515 



way of finding out which beaker is which, using only his five unaided 

 senses. Since no bubbles of any kind are visible in either vessel, no visual 

 difference may be detected. Eventually the onlooker hesitatingly takes a 

 sip from each of the beakers, whereupon a look of certainty crosses his 

 face as he remarks, "This is the beaker that has the high pCOo. It is 

 unmistakable." 



Further experiments can be conducted. For example, does water made 

 equally acid (pH 3-7) with HCl taste the same ? (No.) If a pCOa of 100" „ 

 atm. is unmistakable, can the human tongue detect a pCOg of 50^0 atm.. 



Fig. 3. General view of pilot-plant ior Chlorella production designed and built 

 by Arthur D. Little, Inc., Cambridge, Mass. Note plastic cover to photosyn- 

 thesizing trough and large vertical tower for treating culture medium with CO., gas. 



or of 25",, atm. ? Does bicarbonate with an equal total concentration of 

 dissolved CO.^ taste the same r The answers are quickly and vividly 

 obtained, for onlv gaseous CO., dissolved in water affects the taste buds 

 of the tongue in the manner that is specific to beer and other carbonated 

 waters. 



The respiratory centre of the brain also contains cells that are sensitive 

 to pCO., as a variable different from pH. It is these cells that regulate our 

 breathing in such a way that the percentage of CO., in the base of our lungs 

 is held at S\V,'o CO.,, a physiological mechanism that guarantees that our 



