1020 ADVENTURES IN RADIOISOTOPE RESEARCH 



rate as stated above, namely with a half-time of about 10 minutes only. 

 Interesting observations were made when rats were kept from their 

 conception until they reached an age of 6 months on water containing 

 ^HOo and for the consecutive 10 months on normal water. After that 

 date the various body constituents still retained a significant fraction 

 of their tritium content indicating that a corresponding fraction of the 

 molecules considered were not renewed in the lapse of 10 months. From 

 the saturated fatty acid molecules of the whole body 4% kept their 

 tritium content at that date. It could be followed from these experiments 

 that 60% of the whole body saturated fatty acid molecules have a half- 

 time of 80 days while another fraction has a half-time of 15 days. These 

 data^^^^ though of great interest, do not claim great accuracy as tritium 

 is a somewhat imperfect indicator of hydrogen, the atoms of the former 

 being three times as heavy as those of the latter. 



This great mass difference reflects itself in a corresponding isotope 

 effect. We can best test the accuracy with which the behaviour of deute- 

 rium indicates that of hydrogen by comparing the distribution of deute- 

 rium and tritium in the compound to be investigated. 



If we administer water containing some DgO and some TgO and take 

 the T/D ratio to be 1 in the case of an absence of an isotope effect we 

 should find in the water obtained by combustion of any tissue or tissue 

 constituent a T/D ratio of 1 as well. Thompson and Ballon^^^^ investi- 

 gated the T/D ratio of the combustion water obtained of organs of the 

 mouse 3 days after injecting with deuterium and tritium labehed water 

 and found the following T/D ratios. 



The isotope effect is thus small in the above investigated cases, tritium 

 replacing hydrogen to a slightly minor extent only than deuterium and 

 the difference observed lies within the errors of the experiment. Knowing 



Tablb 4. 

 Retkntion Ratios (T/D) 



the extent to which tritium fails to indicate correctly deuterium we can 

 estimate the error incurred when applying deuterium as an indicator 

 of hydrogen. We arrive at this figure by multiplying by about twice 

 the first mentioned error. 



Diluted heavy water containing almost exclusively DHO molecules 

 is a suitable indicator of the HHO molecule (cf. p. 997), not, however, 

 concentrated heavy water containing to a large extent DDO molecules. 

 Concentrated heavy water is a toxic substance. 



